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The Design of Laterally Loaded Walls | Strength Of Materials | Brick
The Design of Laterally Loaded Walls
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Masonry Structures - Behaviour and Design
BS - Reinforced Masonry Design Guide
Amrhein - Reinforced Masonry Engineering Handbook 6e
Masonry Book
Masonry Structural Design of Buildings
Handbook to BS 5628 Structural Use of Masonry Part 1 Unreinforced Masonry
Masonry Designer's Guide.pdf
Masonry Design Manual, 4th Ed.sec
Handbook to BS5628-2
Building concrete masonry homes
BS 5628 - 1
Design and Performance of Retaining Walls
BS5628 Part 1
Design of Masonry Structures EC6
Masonry Design Examples to BS5628
Finite-element Design of Concrete Structures, 2nd Rombach
PT Masonry Structures
Concrete Masonry - Free Standing Walls
Design of Timber to BS 5268 new
DBE ASIDI - Tembani SDP Water & Sanitation Checkilist Rev A
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BS 5&28: Th. Structural U•• of Ma.
Part 1: Unr. lnforcad Ma.onry
February1986
of laterally loaded
J. Morton BSc PhD CEng MICE MlnstM
BS 5628: The Structural Use of Masonry Part t : Unreinforced Masonry
laterally loaded
This publication is concerned wi th the subject of laterally loaded
walls, with particular reference to uniform lateral pressures. It is
based on visual presentations originally given during a series of
~ semi nars on BS5628: The St ructural Use of Masonry: Part 1 in
~ late 1978and subsequent ly.
~ cope
• The contents cover both the background to the Code provisions
3! aswell asthe provisionsthemselves. Inorder togive thereader
~ an understanding of the Code ·.recommendat ions and the
t reasoning behind them, the subject is dealt wit h in its widest
C1.l sense.
I: Particular attention is paid to Clause 36 of Section 4 of the
Code whi ch gives detai leddesign recommendations for laterally
loadedwalls.
Amendments 2747(October 1978) and 3445(September 1980)
have been taken into account. as hasthe latest amendments
4800, March 1985.
The kind support and guidance of many pecple dur ing the
preparation of both the seminars and this publ ication is
gratefull y acknowledged. Particular thanks are due to BA
Haseltine, current chairman of the Techni cal Drafting Commit-
tee, and to N.J. Tutt who checkedthe design examples.
FOREWORD _ _.............................................. 1
ScOpe _ _ _............................................... 1
Acknowledgements _ _ _ 1
1. MATERIAL PROPERTIES 2
Inlroduction 2
Weak and strong direction of bending 3
Wallette test programme .._ _ _ _ 3
Clay br icks 3
Calcium silicate bricks _.. 4
Concrete bricks 4
Concrete blocks 4
Orthogonal ratio _ _._. .._._ 5
Statistical qui rks .._._ __ 5
Docking or wetting of bricks __._ 5
Review ._ _ _.... 5
2. DESIGN METHOD 5
Introduction _ _..................................................... 5
Cladding panels __ _ __ 5
Panels which arch _ __ _ __._.. 5
Cladding panels _ 6
Wall tests _ __._.__ _ _.._..__ 6
Edgesupports .. _ _ 6
Modes of failure .._ _ _ _ __. _.. 7
Design moment ._. . . _ _._... 7
Designmoment of resistance . _........... 7
Bending moment coeffic ients _.......... .. 8
Part ial load factorfor wind _._._ _ 8
Checking the design _._ _._ _._ _ _..... 8
Shear .._ _ __._._._._ _ _._._._ _._ _ ._._ _ _..... 8
Partial safety factor for materials __ __._._ _._. __.__ 8
Partial safety factor for shear __. __. 9
Li mits to panel size _ . 9
Edge restraints __. ._ 9
Metal ties to columns _. ._ _.___ 9
Bonded to pier s ._._._..__._ _ __._._._ __ _._. 9
Bonded retum wall s .. ._._._ _ _._ _._ _._._ _._.._ _ 10
Unbonded retumwall using metal ties _. ._.__ _ __ 10
Free top edge _. .__._._. .__. . ._._. ._ 10
Wall built up to the struct urewith simple anchorages 10
lnsitu floor slab cast on to wall 10
Effect of damp proof courses 10
Experimental validation 10
Arching walls 11
Freestanding wall s .____ 12
Cavity walls . . . 12
Wall panel edge supports . . . ._. .. 12
Walls with openings 13
3. REFERENCES _ _ 13
4. DESIGN EXAMPLES _ _ 14
Example 1 _ 14
Exampie 2 _ _ _ _ __ 14
Exampie3 _ _ _.......................... 15
Example4 15
Example 5 15
Example6 15
Example 7 16
Example8 16
Example9 17
Example 10 19
Example 11 .. 19
IlATERIALPROFDt IllES
The maj ontyof masonry cladding panel s tend to bend in both the
horizontal and vertical directions simultaneously; they are
essent ial lytwo-way bending plates (figure 1).11 ismoredifficultto
study the material properties in both directions simultaneous ly
than if the panel is spl it into its horizontal and vertical modes and
each of them studied separately. This facilitates any ex-
perimental investigation (frgure 2).
IIei di '!l perpet daABr
to bed joints
IIei di '!l parallel to
Ideally, walls like those shown above should be built and ~
tested in bendi ng to failure. The fail ure mode wi ll be a flexural Q,
crack that will occu r along the bed joint, in the case of simple ~
vertical bending (left) at or near the position of maximum ~
moment. When the wall is subject to simple horizontal bending, 2"
the flexural failure crack will develop through the perpendicular ~
joi nts (perpends) and the bricks, as shown on the right. >
Wh ile ful l scale model tests are perhaps ideal. they are also ;jf
expensive. It is more cost effective to build and test small walls.
knownaswallettes.
Two wallettes and a test rig can be seen above. The wallette.
built to a particular format. in the rig in the background is being
tested in simplevertical bending. The wallette in the foreground,
buil t to a different format. is to be tested in simple horizontal
bending and wil l be tested in adifferent rig.
Weak and strong direction of bending
The wallette in the test rig in figure 4 wi ll fail, at or near
mid-height. when t he interface between the bricks and the
mortar bed is broken. Since the brickwork has relatively weak
tensile properties in both direct and bending tension, it is
reasonable to predict that this direction of bending (simple
vertical) will also be relatively weak. On the other hand, the
wall ette in theforeground, when loadedto failure in the other rig,
wi ll fail when a vertical crack develops across two bricks and two
perpends. Since a brick is much stronger in flexure than a
mortar/br ick interface, it is reasonable to predict that this
direction of bending will be relatively strong.
Experimental evidence confi rms that simple vertical bending
is the weak direct ion, and that simple horizontal bending is the
strong direction. For ease of terminology, the direction of
horizontal bending and t he direction of vertical bending will now
be referred to as the strong and the weak directions of bending
Wallette test programme
The wallette testing programme was carried out by the British
Ceramic Research AssociationI , A wide variety of bricks,
commonly available in Britain, was tested in the four British
Standard mortar designations. The objective was to ascertain
the fiexural strength, in both the weak and strong directions of
bending, for the range of modern brickwork designed and
constructedinthiscountry.
For each clay brick/mortar combi nation, a set of wallettes was
built in both the horizontal and vertical formats, each consisting
of five or ten wallettes. This enabled the mean and standard
~ deviation of each set of result s to be found. With these values for
~ the mean failure stress and the standard deviation, the charac-
~ teristic f lexural st ress could be calculated for the strong and the
~ weak direction of bending. But to what parameters could these
~ results be related?
.. Perhaps the best known propertyof bricksistheir compressive
~ strength. Does the flexural strength of brickwork relate to the
'!r compressivestrengt h of the bricks?The relationship is shown in
~ figure 5. From the limited results available, it was not felt to be a
t-.; relevant approach,
It is reasonable to predict that some relationship would exist
between the st rength of the brick in f lexure and the flexural
st rength of the brickwork - particularly in the strong direction, if
not in the weak. Figure 6 shows that there is some correl ation
between theseparameters.
After examinati on of all the possible relationships, it was
decided that the relat ionship between t he flexural st rength of the
brickwork and the water absorption of the bricks' from which it
was const ructed would be used " The graph above relates the
flexural strength of the brickwork to the wat er absorpt ion of the
brick for wallettes tested in the weak direction and built with
designation (i) mortar. Each point on the graph represents the
mean of five or ten wall ette tests wit h a particular brick. The
number of dots indicates the wide range of bricks that were
tested in order to represent the wi de variety of clay bricks
available in Britainto-day. The widescatter isnot exceptional; it
is to be expected when testing masonry. It reflects the fact that
brickwork hasa widestatistical 'quality' curve,
To comply with limit state philosophy, it was necessary to
establi sh the 95% confidence limit from all the test results. A
statistically basedapproach was used to establish the position of
t he95%confidence limit f rom the mean and standard deviations
associated with each point. This limit is shown in figure 7 as the
curved line, under which lie5%of the results. It can be seen that
the flexural strength tends to decrease With an increase in the
water absorptionof the bricks. Because complex curves make
design less straightforward, it was decided to simplify the curve
with an 'approximation'. This is shown as a straight dotted line 3
Looking at another simi lar graph, figure 8, this t ime in the
stronq directi on - but sti ll wi th mortar designation (i) - the
background to another section of Table 3 of the Code (figu re 13)
can be seen. The approach adopted is identical to that describec
above and the values on this graph correlate With those in Table
The relationship between block bulk density and the flexural
$lrength was also examined to see rt bulk density might be a
useful indicator. Whilecertain trends are apparent infigure 12,
they were not felt to be as relevant as the relationship between
flexural strengt h and the compressive strenqth of the block itself .
For this reason, the flexural strength of blockwork in f igure 13 is ii'
given in termsolthecrushing st rength olthe block. :<
Considering the strong direction of bendi ng, it must be ~
remembered that the fai lure ot the wallelte involves the fai lure of Q.
both the perpend joi nts and the blocks. It i ~ reasonable to expect ~
an increase in flexural stress withan increase inblock strength. i
This is because the stronger the block in compression, the i
denser it will be. The denser the block. the greater the flexural 1l:
stfrtehnglbhl ofkthte
block 'tfseij'IAndI be tthhe grfelater the flexural $lrength
o e ocx. e grea er WI e m uenoe on the strength 0 "
formats, using a range of calciumsilicate bricks representative
of modern production in Britain. They were constructeo using
four different mortar designations. The results of the large
testing programme indicated that only one set of values for the
characteristic flexural stress was needed to cover all calcium
silicate bnckwork. These values, 0.3, 0.2, 0.9 and 0.6 Nlmm' for
the respective mortar designations. are given in flgure 13.
Calcium silicate bricks available in Bntain do not. of course, have
awaterabsorptionvalue associated With theirStanoard",
As withcalcium silicateandconcrete bricks, concrete blocks
do not use the parameter of water absorption in their Standard'.
While the wallelte test programme indicated that there was
some relationship between water absorption and flexural
$lrength (figure 11), water absorption was not considered to be
the most relevant indicator of the flexural strenqth of blockwork.
Concrete br icks
Concrete bricks were tested in an identical manner to the other
brick wallelte tests. The results, see f,gure 13, indicate similar
values to those associated wi th calci um silicate brickwor k.
Concr ete blocks
Concrete blocks are covered in Table 3 (tigure 13) in a somewhat
different manner to clay. calcium silicate and conc rete bricks.
While the cha racteristic flexural stress was ascertained f rom
wallelte tests, a different wallelte format was necessary to
account for the d,fference in urut sizebetween bricks and blocks.
The wanette formats for blocks are given in AppendIX A3of the
waterabsorption .0.
: ~ ...
lUI - ~
Nmm'
The relationship between the characteristic flexural stresses
of brickwork - in the weak and strong directions - and the water
absorption of the clay bri cks from wh ich it was constructed was
established for all the mortar designations. Figures 9 & 10show
the graphs for a designat ion (iii) (1:1:6) mort ar. As before, figure
13contai ns thevalues derived from this work.
Calcium sil icate bricks
The approach for calcium silicate bricks was essentially identical
to that used f or clay bricks. Wallelte sets were buiij, in both
Flexural strength water absorption 10
1:1:6 in strong direction
, _ _ • undocked
l .;: docked
wrth two steps at values of 7% and 12% water absorption. This
conveni ent approach of hav,ng three values of flexural strenqth
for three ranges of water absorption , facilitates the material
properties of brickwork being descnbec in tabular form.
ij is from this research background that the charactenstic
flexural $lressesgiven in TableSot the Code were derived. This rs
reproduced as figure 13on page 5. From both the graph and the
table, these values are 0.7 Nlmm' 0NA less than 7%), 0.5
Nlmm' 0NA greater than 7% and less than 12%), and 0.4
Nlmm' 0NA greater than 12%) when considenng the weak
direction of bending in mortar designation (i).
the walleltes when tested in the strong direct ion. This trend was
found to be signilicant in the strong direction. It is reflected in
figure 13 which bases flexural strengt h on the compressive
strength of the block. The characteristic flexural st ress values
range between 0.9 to 0.4 Nlmm' , and 0.7 to 0.4 Nl mm'
depending on themortar designalion for 100mm blockwork.
In the weak direct ion, failure occurs by breaking the tensile
bond at the interface between the blocks and the mortar bed.
The experimental evidence suggests that there is no significant
difference between blockwork constructed wi th weak or strong
blocks. Consequenttv, for 100 mm block work there are only two
values of 0.25 Nlmm' and 0.2 Nlmm' which are dependent on
the mortar designation but independent of the strength of the
block Itself.
Since the Code was first published, further tests have
indicated that the flexural st rength of thicker walleltes is lower
than that predicted bythe wallelte results for 100mm blockwork.
The Standard has therefore been amended to take account of
For intermediate values of wall thickness, between 100 and
250 mm, the value of characteristic flexural stress can be
obtained byinterpolation.
Orthogonal ratio
The orthogonal ratio is the ratio of the strength in t he weak
direction to thestrength in thestrong direct ion. For clay, calcium
silicate and concrete bricks, the ratio is approximately 1:3 or 0.33
(see figure 13). For design purposes, a value of 0.35 can be
universally adopted for all these bricks.
Unfortunately, with concrete blocks, the rat io varies since the
strongdirectionstressesvary whilst the weakdirectionstresses
remain constant for different strength blocks. There is, there-
fore, no unique value for the orthogonal ratio of blockwork; it is
necessarytoestablish a value for each blockworkstrength.
Therearecertain implicationsinherent ina statistical approach.
The graph shown in figure 10, whilst being statistically correct,
does nonetheless mi sinterpret certain facts. Both the curved line
and the stepped approximation indicate that the strongest
brickwork results when bricks wi th a water absorption of less
than 7% are used, A closer examinat ion of the graph, however,
reveals that brickwork constructed wi th bricks of between 7%
and 12% water absorption can exhibit higher values of flexural
st rength than can be achieved wi th many of the bricks with less
than 7% water absorption. Indeed, the largest value of flexural
st rength is achieved wi th a brick in the water absorpt ion range
7%-12%.
It is because of such stati st ical quirks, t hat the Code permits
wallelte tests to becarried out on particular bricks in accordance
with the test ing procedure set out in Appendix A3. In this way, a
manufacturer's claim of higher characteristic f lexural strengths
than those in the Standard can be object ively assessed by the
designer if, at his di scretion, he decides to askfor wallelteresults
to BS5628: Part 1f rom t he material producer,
Docking or wetting of bricks
The graphs shown in figures 7, 8, 9 <I. 10 make' reference to
'docked' and 'undocked' bricks, This is due to the need to wet
('dock') certain types of brick if their water absorpt ion would
result in water being removed from the mortar at toogreat a rate.
Research has indicated that this high speed removal of water
f rom the mortar can adversely affect both the compressive
strength of the brickwork and the bond at the brick/mortar
The rate at which water is removed from mortar is called the
Initial Suction Rate of the brick (ISR). The ISR of a brick can be
adj usted by partially soaking it. This can be done either by
immersing it in water for a short ti me - typically, a half to t hree
minutes. It can also be partly achieved by sprinkling the stack
with water from a hose. Although this is often done on some
sites, it is not the recommended met hod. When writ ing the
specification, it is normal good practice to suggest t hat 'before
orders for bricks are placed, the contractor shall satisfy the
engineer either that the suction rate of the brick " . does not
exceed 1.5 kg/m' /min or that he is able to adjust it so as not to
exceed thisvalue",
'Docking' or welting of bricks does not mean saturat ing them,
The general rule 'never allow bricks.or brickwork to become
saturated' should always be obeyed,
The foregoing material constitutes the background to the section
of the Code which deals wit h material propert ies. In particular, it
explains the background to Table 3 (figure 13), and provides
more detailed information on the large testing programme on
which it was based. Not all the graphs have been shown since
they would give no more information t han is contained in Table
Turning from the consideranon of material properties to the
design of panels using thesecharacteristic flexural stresses, two
typesof panel are particularly important.
Cladding panel s
The first type is the panel which tails when the tensile stress in
the extreme fibres equals the ult imate stress. It could be a
vertically spanning wall as shown on the left of f igure 14, or a
horizontally spanning panel as shown on the right. This type of
wall is verycommon in Britain, and the vast majority of brickwork
panels used toclad framed buildings fall into this category,
Panelswhich arch
The other type of panel, which behaves different ly f rom a
cladding panel, ist he panel t hat 'arches'. Such a wall (f igure 15),
generates compressive forces wi thin the plane of the wall as it
deflects. These compressive stresses, generated by the deflec-
tion, are superimposed on the tensi le stresses as they develop
and part ly or wholly cancel them out. Consequently, this type of
panel is much st ronger than acladding panel.
A part icular type of wall panel in which in-plane forces are
present is a loadbearing wall in astructural masonry building. In
thi s case, the in·planecompressivest ress present ist hestress in 5
the wall derived from the load rtis supporting.
II is important to distinguish which type of wall is being
designed. and whether it is a wall in which compressive forces
can develop. This will oo« beconsidered in greater depth.
Horizontal arcIli Ig
Vertical arc:tiIlg
The first type of wall to consider is the cladding panel that falls
when the tensile stress developed at the extreme fibres in
bending reaches the ultimate flexural stress for the material.
Having already established the material properties that can
adequately predict the simple vertical and horizontal bending
strengths (figure 16). it is now necessary to combine them in
two-way bending (figure 11). This essentially relates the two
known material properties described in Table3of the Code to the
behaviour of thr eeand four -sided plates.
It isacomplex analysis andinvolves thesummation ofthetwo
stresses generated from two-cirectional bending (figure 18).
Wall tests
To investigate this. a series of f ull scale wall tests were carried
out at the British Ceramic Research Association' (figure 19).
Wallettes awai ting testing can also be seen in t he foreground of
Theobjective was to find the relationship between the strength
in two-way bending to the two known strengths in the simple
vertical andhorizontal directions.
Full scale wall panels were bui lt in test rigs (figure 20) and an
air bag was placed between thewall and a reaction frame behind
it (figure 21). The reaction frame was tied back to the test rig.
Panels of diff erent shapes were then tested using various
brick/mortar combinations. The majority of the panels built were
standard storey height (figure 21) but some taller and shorter :;
panels were also tested, such as the 1'h storey height panel in ::.
f igure 20. The lengt h of the panels was also to be varied - note ~
the two shorter panels awaiting testing in the foreground of ~
~ f f i 2 1 . ;
Edge supports J
Actual failure. when the panel cracks. does not necessarily ~
result in total collapse. The wall in f,gure 22 actually cracked at a il:
much lower deflection. In order to demonstrate the failure ~
pattern more clearly. the air bag was inflated sti ll further until the •.
OorlAa
=fkxZ
=characteristic flexural
strength about vertical axis
Z =section modulus
om =partial safetyfactor
where M: bendingmoment-about
a : bendingmoment co-efflclent
=partial safetyfactor forloads
=characteristic windload
L =panel length 2
In the design of any bending member, the applied moment is
normally WL
divided by a factor, for example 8 for simply
supported one way bending. The design moment for panels is
based onthesameequationbut thedivisorisexpressedasana
coefficient (eg, O. 125for t hecase above). For limit state design V"
the partial safety factor for loads, is necessary to convert the
applied moment into an (applied) design moment. Note that the
formu la is expressed in terms of L, the length, and not h, the
Designmoment of resistance
direct ions, and also for various possible edge conditions (figure
25), a useful design technique would result. The three possible
edge conditionswould be:
1. A freeor unsupport ed edge.
2. A supported but pinned edge where no moment would
3. A supported fully fixed edge where a moment equal to the
panel mid -span moment woulddevelop.
Working on this basis, and taking into account the wall test
result s, the followingdesign procedure was suggested":
Designmoment
The formu la to calculate the moment of resistance is the normal
f x Z term. Because the applied moment uses L (ie, it is
associated wi th horizontal spanning), the moment of resistance
must use f" in the st rong direct ion for compatibility. To be a
design moment of resistance, the f x Z term must be divided by
Failur emodes
wall was grossly deflected (figure 23). This also serves to
illust rate the residual strength of failed panels; although the wall
has st ructurallyfailed and isgrosslydeflected, it is stable. Visible
in f igure 23 are the holes that were cut in the channels forming
the two vertical edges ofthe test rig. Tieswere inserted into these
holes and were used to connect the wall to the f rame, as is done
in practice - ie, the panels were supported in the test frame in a
simi lar way to panels in a real building. It was of course
recognised that this edge support system was more likely to
approximate to simple supports rather than to fully f ixed edge
! Thefailure modes of the panels (figure 24) graduallyemergeda s
the test programme progressed. They were found to be fairly
3; simi lar to those associated wi th concrete slabs although, of
course, brickwork is a brittle material (unless reinforced) while
reinforced concreteexhi bitsplasticyielding. •
It was recognised that, if account could be taken of the two
o!f different bending moments at the centre of the panel in both
BerocAIQ
For themajority of panel s met in practice. failure in bendingwill
be the design criterion. It is nevertheless necessaryto check the
shear. Theaboveformulagivesthisreadily.
Partial safetyfactor materials, Vm
These are normallythe two main ways inwhich the designis
Thevalueof Vmcan vary between four valuesdepending on the ~
category litconstructioncontrol (workmanship andsupervision) a
andthecategoryof manufacturing control (thelikelihoodof weak ,
units being included in theconsignment of structural units). This ~
0·040 0·035
0·050 0-<>65 0·098
0·083 0-081 0·144
0·50·065
1·00·091
1·50·125
Bending moment coefficients are set out in Table 9 of the Code.
They are essentially the coefficients generated from 'yield line'
theory' ·· which. although not fell to bestrictlyapplicableto bnttle
materialssuchas masonry, nevertheless appeartogivereason-
able correlation with experimental results - at least for solid
panels without windows or doors", The a coefficient depends on
the aspect ratio of the panel (hIL). on the orthogonal ratio ~ . and
the nature of the panel's edge support - whether three or
four -side supported. and whether 'simply supported' or 'fully
fixed'. (Notethat a set af values for a has been given in each of
the tables for ~ = 0.35. This value for ~ has been purposely
includedfor all typesof brickwork}
Partial safetytector for load(wind), VI
Vm' thepartial safetyfactor for materials.
Normally, VI for general deSign is taken as 1.4_ However, a
special case is made of cladding panels which do not affect the
stability of the main structure- normally a frame. In this case, VI
can be reduced to 1.2. II must be clearly understood that this is
anexception. basednot ondesign philosophy but on the needto
acknowledge that Wi thout reducing VI to 1.2 many panels
currently known to work well in practice could not bejustified in
the design process. Of course. should the brickwork panel form
part of the structure, and should the removal of the panel
endanger the stability of the remaining structure, VI must be
maintainedat 1.4.
Byequating the design moment of resistance to the design
moment. It is now possible to obiectivelydesign laterally loaded
panels. But It is first necessary to look in detail at certain of the
Bendingmomentcoefficient; a
Checking thedesign
The design moment of resistance must be equal to or greater
thanthedesign moment and, byre-arranging theformulae:
Z is assumed and the f...of the material is checked to see if It is
the f...of the matenal to be used IS fed into the deSign and the Z
8 valueofthesectionto beusedis clhecked.
approach essentially recogni ses that there are bonuses for the
designer If he can be more certain about the structural uni ts, the
mortar and the way they are put together on site. These design
bonusescan beexploited if:
1. There is a small er probability of the structural units (bricks!
blocks) failing below their specified strength.
2. There is a small er. probability of the mortar being below the
strength specified.
3. There is a smaller probabi lity that poor workmanship will be
used in putting the bricks and mortar together on site.
Condi tion 1 is covered by the manufactu ring process, while 2
and 3 are both factors infl uenced by site operations and
As is the casewithcompression andshear loadedwalls, it is
possible to specify bricks to an acceptance limit to BS 3921·
where not more than 5% of the bncks will have a crushi ng
strength below the acceptance limi t specified. This is the
recuirement for special category of manufacturing control. and
the maj ority of BOA member companies will readily be able to
meet this form of producti on quality control.
To achieve speci al category of construction control. mortar
tesling should be carried out to sat isfy condit ion 2. above. and
the work on site should be supervised by a suitably quali fied
person to satisfycondrtion 3.
In most situations. the designer will normally base his design
on Ym = 3.50r 3. 1since. at the design stage. the likelycontractor
and the level of site supervision will not necessari ly be known.
However. lower values of Ym are available, should they be
Ymvfor shear
for shear is taken as 2.5
Limitstopanel size
Twolimitationstopanel sizesaregivenintheCode:
1. An overall limi t on the area of the panel. Detailed restrictions
<: are given. depending on the nature ofthe panel supports.
2. For three and four-sided panels. an overall limit on height or
lengtn'which must not exceed 5Ot.1(the effecli ve thi ckness of the
wall) . Since let for a normal cavity wall is approximately 137 rnrn,
;;; the practical limit for normal cavity walls spanni ng in both
di rections is about 6.5 m in length and height. (I.,. the effect ive
thickness of a cavity wall is + t,) where t , and t. are
!!. the act ual thickness of the twoleaves.)
It is possible to build panels which exceed either or both 1and
2 above, but they should be effectively sub-divided so that. while
they may not appear to be two separate panels. they flex in
bendi ng as though they are. Normally. this sub-division is
achieved using intermediate supports. In thisway, conddions 1
and 2can beeffectively met.
As With any st ructural member. the question of whether the
ends are fully fixed or pinned is not necessanly easy to
determine. The criterion used in the Code, however, is not
diffi cult to grasp. If the edge of a panel is 11l1l%fixed. such that. If
loaded. the panel edge Will fall In flexure before rotating. the
panel is assumed to be 'fully fixed'. Anything less than this and
the panel edge is assumed to be 'simply supported'. even If in
reality it may be partially fixed.
Looking at detailed cases:
Meial tiestocolumns
The non-conti nuous edge. suitably li ed to a column. does not
give I Il1l%fixity against rotati on. It is therefore assumed to be
simplysupported.
The ful ly continuous edge, achi eved by taking the wall past the
column. is 11l1l%fixed. It will break along this vertical edge before
it will rotate.
Bonded t o piers
The above argument will also apply to piers. Wh ile the fIXed
support. where the wall iscontinuous, isreadityunderstood. the
needfor a simple support at the end of the panel is due to the
abi lity of the pier to rotate in torsion. If torsion restraint were
present, the end could beconsidered full y f ixed.
Bondedrelum waifs
This full fixity ISassumed If the pier becomes a bonded long
return wall. Now both edges are fully fixed. and the wall must
crack at the vertical supports before rotation can occur (figure
Unbondedrelum waif using melallies
If the wall is not bonded but merely t ied to t he long return, some
rotation will be possible. This vertical edge should be considered
to bea simple suppo rt.
free lop edge
Irrespect iveof what happens at the vertical edges, the hor izontal
edges can also be considered as either fully fixed or simply
supported. In the above example, the top edge is actuall y free -
no support has been provided. This will be designed as a
t hree-sided panel.
Waif buill up 10Ihe slruclure wilh suilable anchorages
It is more usual to provide some form of support to the top edge.
In the above example, the wall has been effectively pinned to the
floor above. However, the top edge will still be able to rotate
under lateral load and, therefore, it must be assumed to be
simply supported.
In-situ (loor slab casl on 10waif
When an in-Situ floor slab IScast onto a wall. the Code suggests
10 that It can beconsidered fully fIXed.
Effecl ofdamp proofcourses
Irrespective of t he details at the top of the wall, it must be
remembered that most panels have their shear and moment
resistance limited at thei r bases by the need to int roduce dpc
materials. As a result, it is normal to expect the base of most
panels to besimply supported.
Whi lst the above approximations, which give assumed edge
conditions, are useful to the busy design engi neer in practice,
theyare nonethelessover-simpl ifications.
In most of the cases considered where simple supports have
been given, a degree of partial rest raint is actually present. In
many of t he examples, 30%, 50% or 70%fixity may be present.
Besides t he obvious advantage of keeping design simple, this
approach agrees with the published experimental evidence".
Figure 44 compares the predicted failure load, based on the
design method outl ined previously, but using an est imation of ~
the degree of edge restraint, with the experimental results of il
walls tested in the laboratory. Whilst t he correlation is reason- if
able forfailure loads of approximately 5 kNl m
and less, at higher -g'
loads, the design method using partial restraints is found to Q.
predict higher st rengths than are found in the laboratory. The ~
strongest test result shown in f igure 44 is for a wall that failed j
between9-10 kNl m
, yelthe predicted strength was between 14 ~
- 15 kNlm
• It was for this reason that the design method il.
adopted uses eitherfull fixityor simple support condi t ions. ~
Ot part icular interest is the case of the arching action of
wind-loaded external walls in a loadbearing st ructure. In this
case, a modified approach to the arching guidance given in the
Code is suggested. In loadbearing walls, the arching force is
generated from the precompressive stress already present due
tothevert ical loadt hewall is carrying.
Even though a loadbearing wall cracks and forms threehinges
at A, Band C, it does not necessarily fail. Failure wi ll only occur
when t he lateral load reaches a level which will move the point C
to the left ofthe lineAB. Inother words, when 6 = t.
The basic equation q,., = ~ can be established by analysing
the f ree body diagram of one half of the wall (figure 49). This
equation is based on stability considerations and not on t he
flexural strength ofthewall.
The equation was checked against the ulti mate lateral load of
wall panels in a full size 5-storeytest building at the University of
Edinburgh' (figure 50). The walls were removed by jacking and
the failure loads ascertained. Further wall tests, conducted in
laboratory machines under a constant precompression (figure
51 overleaf), confirmed the earlier concl usion that this ultimate
load eouat.on gives good correlation wi th the experimental
results, and is a good failure predict ion equation" . It is felt that
the relatively small scatter associated wit h the body of test
evidence is dueto the stability natureof theequati on.
The equat ion is for verticall y spanning walls. Three and
four-sided loadbearing walls have even greater lateral strength.
To establish the lateral strength of a three or four-sided
loadbearing wall, the strength is first calculated from the basic
equation assuming the wall spans only vertically. The strength
so obtained ist hen multiplied byan enhancement factor k, given
in Table 10of theCode(figure 52overleaf).
The strength enhancement can be signIficant , 3 to 4 for
squareor nearly square aspect ratios. Withlamer aspect ratios.
as large concrete columns or substantial edgebeams, it isnot
able to extend as it deflects. Instead, forces aregenerated wi thi n
the plane of the wall, and t hey effectively induce restoring
moments which stabilise the wall. Walls with arching action
present are inherently much stronger than those - such as
cladding panels - where arching action cannot take place. The
Codegives design guidance on thestrength of thistype of wall.
HorizontaIarchi Ig
This approach", unl ike partial restraint either estimates the
st rength realistical ly or underestimates the strength of the
panels, as can be seen in f igure 45, where all the points lie above
line A - the 45' line. Line Erepresents the design method when
using the part ial safety factors for both t he loads and the
Allowing forprecompressi on
In many cases, justifyi ng a panel using the recommended
design method is quite straightforward. In some cases, how-
ever, the panel' s st rength based on this method may be just
inadequate for the load it is being designed to carry. In this
situation, the characteristic flexural st ress in the weak direction
can be increased to allow for the dead weight of the top haf of
the panel. This enhances the st rengt h ofthe panel by:
1, Increasing f", in the weakdirection.
2. Modifying the a coefficient for 3& 4-sided panels by changing
thevalueof u.
The strength enhancement is given by the formula in figure
46, below.
~ The design method outli ned above has dealt wit h panels only.
~ However, insituations wherearchingaction cantake place, this
~ underestimates t hestrength of walls.
~ To accommodate the deflection induced by loading a wall
-1: panel, the panel must increase in length - either on plan or in
'" section. If t he wall is built solidly between rigid abutments, such
L- -.J 53
DES'GNOlI'BEE
IIDNDIlVGWALLS
pecui" BBslXi
eep. ale IlIo8dan215nm....
ecPi*1l1oedan175nmWllll
4-03-0 1·5 1·2
1-6 1-5 1·1 1-0
Factor k 52
Number Value of k
of returns Lh 0 ·75 ' ·0 2·0 3 ·0
Thus far. only the desig n of solid single-leaf wall s has been
discussed. In the case of loadbeari ng cavity wa lls, it is usually
only the inner leaf wh ich is analysed in arching. since thi s leaf
normally carries the signif icant precompression. In cladding
panel desig n. however. bot h leaves carry significant lateral load,
and this must be taken into account.
Test results" indica te that the lateral st rength of a cavity wall is
equa l to the sum of the st rength of both individual leaves. This is
a fort uitous finding and permits an easy and understandable
desi gn approach.
Wall panel edge suppo rts
when the panel is three times as long as it is high. the stiffening
effect of the vert ical edge support dimini shes to virt ually zero and
k approaches 1. Effectively. this recognises what could reason-
ably be predicted. namely. when L is mu ch greater than h the
panel tends to span purelyvertically.
The Code gives guidance on the design of freestand ing walls.
The normal approach for brickwork f reestanding wall s is based
on flexural strength using the modified flexural stress (for weak
di rect ion bending) mentioned earl ier (page 11). flo< is increased.
as before. by the Ymgd term. In the case of a freestanding wall .
where the flexural failure is at the base, gd is based on the dead
weight of the full height of the wall.
A fulle r treatment of all aspects of the design of freesfanding
walls, including details, materials specification, standard sec-
tions, strength design and the stabili ty of footings is given in BDA
Design Guide 12".
Wh ilst the method used to design laterally loaded wall s against !!.
flexural failure has been covered, it is also important to ensure [
that the panel is adequately tied back to the str uctu re. This is ~
morecritical with claddi ng panels than with loadbearing walls. ~
Claddi ng panels are usuall y t ied back to their supporti ng :l.
structure using metal t ies. and it is useful to have values for the ~
st rength of wall ties used as pane l supports. Characterist ic •.
values for compression. shear and tension are given in Table 8 of
theCode. Whencalculating designstrengths, Ymis3for ties.
Walls with openings
The design methods coveredso far have dealt with sinqle-leaf or
cavity walls of solid construct ion with no door or wind ow
openings. The introduct ion of openings into panels reduces their
ultimate lateral strength. Although some research has been
conducted (figure 56) on perforate walls, no final conclusions
have been reached. Consequently, no firm recommendations
have been incorporated in the Code. Instead. in Appendix D, the
Coderecommendsthat either;
(a) aformof plateanalysis iscarried out:
(b) a superimposition method is used(figure57).
t. H.W.H. West. H.R Hodgkinson & BA Haseltine. The
resistance of brickwork to lateral loading. Part 1: Ex-
perimental methods and results of tests on small speci-
mens and full sized walls. The Structural Engineer, Vol 55, No
10, October 1977.
2. BS 3921: Clay bricks and blocks. British Standards
Institution, 1974
3. BS 187: Specification for calcium sil icate (sandlime &
flintlime) bricks. BritishStandards Institution, 1978
4. BS 6073: Precast concrete masonry units. British Stan-
dards Institution, 1981.
5. Model specification for clay & calcium silicate struc-
tural brickwork. Special Publication 56, British Ceramic
!2 ResearchAssociation, 1980(availablefromBOA).
~ 6. BA Haseltine, H.W.H. West&J. N. Tutt. The resistance of
" brickwork to lateral loading. Part 2: Design of walls to
~ resist lateral loads. The Structural Engineer, Vol 55, No 10,
~ October 1977.
%7. K.W. Johansen. Yield line formula for slabs. Cement &
'0 Concrete Assoc iat ion.
:l' 8. L.L. Jones & RH. Wood. Yield line analysis of slabs.
~ Thames &Hudson, Chatto&Windus, London, 1967.
A full treatment of panelswith openingsis outsidethe scopeof
thispublication. Thereader is referredto References 12&13.
The lateral load design of diaphragm and fin wall structures is
not covered in the Code. Indeed, this subject is under review by
the Code Technical Drafting Committee. While the theoretical
basis of such designs follows normal sound engineering
principles, as outlined above, the reader is referred to BDA
Design Guides: 8 'Design of brick fin walls in tall sinqle-storey
buildings' and 11 , 'Design of brick diaphragm walls', for more
detai led guidance 14 1 ~
9. A.W. Hendry, B.P. Sinha & A. H.P. Maurenbrecher. Full
scale tests on the lateral strength of brick cavity walls
with precompression. Proc. British Ceramic Society21, 1974.
10. H.W.H. West. H.R Hodgkinson &W.F. Webb. The resist-
ance of brick walls to lateral loading. Proc. British Ceramic
Society 21, 1974.
11 . J.OA Korff. Design of free standing walls. Design
GuideNo 12, Brick DevelopmentAssociation, February 1984.
12. BA Haseltine & J.N. Tutt. Exlernal walls: Design for
wind loads. Design Guide No 4, Brick Development Associa-
tion, 1979.
13. BA Haseltine & J.FA Moore. Handbook to BS 5628:
Structural use of masonry: Part 1: Unreinforced masonry.
BrickDevelopment Association, May 1981 .
14. W.G. Curtin, G. Shaw, J.K. Beck & WA Bray. Design of
brick fin walls in tall single-storey buildings. Design Guide
8, BrickDevelopment Association, June 1980.
15. W.G. Curtin, G. Shaw, J.K. Beck & WA Bray. Design of
brick diaphragm walls. Design Guide 11 , Brick Development
Association, March 1982.
16. J. Morton. Accidental damage, robustness & stability.
Brick DevelopmentAssociat ion, May 1985.
4 DESIGN EXAMPLES
A cladding panel 2.6m high is to be designed in cavity bnckworl<. The wall has no returns, and because the vertical edges are
unsupported. It spans vertical ly. Theedge conditionsat the base and the top of the wall are assumed to provide simple supports. The
bricks to be used in both leaves havea water absorption of !f-9'h%and the mortar is 1: 1:6. Workmanship and material s are assumed
to benormal category and the hmiting dimensions are not exceeded.
Estimat ethe design wind pressure. W. , which can be resisted bythis panel.
The two basic equat ions are:
Designmoment =ay, W
Design moment of resistance = f:;;;!'
Forthis panel:
a 0. 125 for a simply supported case.
Y, 1.2 for a claddi ng panel whosefailure does not affect the stability of the remaining st ructure.
L 2.6m span istaken ast he height. h. not the length; since vertical spanni ng. useflu in the weak di rection of bending .
flu 0.4N1mm' for water absorption of 7-12% in 1:1:6mortar.
Ym = 3.5 for normal category construction control and manufacturing control of units.
The strength of a caVltywall isequal to the sumof the strengths of the two individual leaves.
For one leaf:
Z HXXl x - 6-' - = 1.75 x 10" mm
/m run.
0.4 x 1.75 x 10"
= 3.5 Nmmlmrun = 0.2 kNmlm run.
M = 0. 125x 1.2XW
.6' .
Equating M and M
_ 0.2 , _ ,
Wk - 0. 125x 1.2x2.6' kNim -0.2kNlm
For cavitywall :
W. = 0.2 +0.2=0.4kNlm'
This isa low value. W. is unhkelyto be as small asthis in practice. and the panel could not bejustif ied in many areas in Britain.
1.3 x 2.0kNlmrun
0.9 x 1.3 x 2.0 kNim run
2.34 x 10' ,
= 102.5 x H)3 Nlmm
O.023N1mm'
= 0.4 + (3.5 x 0.02) Nlmm'
= 0.47N1mm'
!l.1
0.47 x 1.75
= O.23kN1m'
Basicequationay,W"L
Modif ied f lexural st ress
0.125 x 1.2 x W. x 2.6'
Wi th the above design example try to maximise W
by taking the of the panel into account. Keep Ym= 3.5 and use the
samematerials.
Characteristic dead load G. (assume conservati ve. ie. low. values):
Outer leaf (102.5mm) = 2.0 kNlm'
Inner leaf (102.5mm)
plasteredone side = 2.25kNlm'
The flexural strength in the weak direction can bemodrned to:
flu + Ymgd.
where g"isthe stressduetothe designvertical load. Thefailure crackandthereforethe cntical section, Ie. the pos.tion of maximum
moment. isassumed tobeat rmd-heiqht. Cons.der onlythedeadweight stress duetothe tophalf of the wall.
Outer/eat
Design vertical load
St ressdue to
Basic equat ion aYI W. L'
0. 125x 1.2 x W. x 2.6'
Forthecavltywall . W.
0.9 x 1.3 x 2.25 xl a' NI '
= 102.5 x H)3 mm 'i
= 0.026Nlmm
[0.4+(3.5 x
30S026)J
x 1.75 j
0.24kN1m'
= 0.23 + 0.24kNlm'
= 0.47kNlm'
The increase from 0.4 to 0.47 kNlm' is approximately 20%. This should be remembered when panels hejust outside justifiable
O.4kNl m'
[0.7 + (3.5 x 0.026)) x 1.75
1.75 ,
2 x 3.5 : W. =O.99kNlm
conditions. (Note that the answer could have been achieved much quicker. Since f", was increased by approximately 20%when
modified, W, would increasealsobyapproximafely2O% becauseW, = Kf•• isa linear relationship.)
A panel identical to that in Example1is to beconstructedand1ested in a laboratory. An estimate is needed, for rig designpurposes,
of the likelyultimate failure load. Calculatethis value.
At present, the maximum design value. WI< = 0.47 kNlm
. Thisvalue isderived using design values of Vm. Yt and characteristic
valuesfor flexural stress. Forultimate design Ym= VI = 1.0andthe meanflexural stress valuesshouldbeused.
Assuming noaccuratevaluesfor the mean flexural stressareavailable, these can beestimated from tigure 9. For bricksof 7- 12%
water absorption, themean flexural stresscan rangebetweenapproximately1.0Nlmm' and the0.4Nlmm' f.. value.
Assumef ~ a n = 1.0 Nlmm' (strongest brick/mortar case).
Modifying thisfor selt-weiqht (for simplicity, takeG. = 2.5kNlm' for both leaves. 2.5kNlm' isconservative, ie, high, for rig design):
_ [1 1 x 1.3x2.5 x Hi' ]
- + 102.5 x 10
= 1.03N1mm'
Far one leaf:
O 25 0 W 26
' 1.03x 1.75
. 1 x 1. x rnax X . = 1
W ~ . = 2. 13kN1m'
Forcavilywall: W ~ . = 4.26kN1m'
Thisisstillnot a truemaximumvaluesinceit isbased ona predictionof the maxi mummeanflexural stress. Whilethe meanof, say.
10wall tests should beapproximateIy4.26kNlm' at lateral failure, the resultswill bescatteredaround thismean.
Theexamplecould berecalculated if the standard deviationassociatedWith wallettetest flexural strengthswere known. However,
this isoutsidethe scope of this publication. Instead, the rig should bedesigned using 4.26kNlm' together with an adequatefactor of
Example4 .
A panel identical to that in Examples1&2is to bedesignedusing different bricksand mortar. The material specification isnow:
Outer leaf: water absorption < 7%; mortar 1:%:3
Inner leaf: water absorption > 12%; mortar 1:Y,,3
Calculatethe maximumdesignwind pressure, W" taking account of theself-weiqht (assumegd= 2.25kNlm' for outer leaf, and2.50
for inner leaf).
f", values: outer leaf O.7Nlmm' ; inner leaf 0.4 N/mm' .
0.125x 1.2 x W, x 2.6'
, = [0.4 + (3.5 x 0.029)) x 1.75
0. 125x 1.2x W, x 2.6 3.5
W, O.25kN1m'
For cavilywall: W, 0.4 + 0.25
= O. 65kNlm'
Hence, bychangingthe material specification. thedesignwind pressureWI< can beincreasedfrom0.47to 0.65 kNlm
• Indeed. rf
the inner leaf wereconstructed using a brickof water absorption of lessthan 7%, W, could be further increasedto approximately0.8
kNlm' . Thisis adequatefor manysituations in Bntain.
Comparethe designwind pressureW
= 0.65kNlm' , fromthe previous example, with thedesign wind pressurefor awall, usingthe
samebrickandmortar materials, which is 2.6m long andpurelyhorizontally spanning(ie, assumethe baseofthewall isafreeedge).
f", values: outer leaf = 2.0 Nlmm'; innerleat = 1. 1Nlmm' .
, 1.75 ,
0.125x 1.2x W. x 2.6 = 1.1 x 3.5 : W. = 0.54kNim
For cavily wall: W
= 0.99+0.54kN1m'
l.53kN1m'
This issignrticantlystronger than the0.65kNlm' derivedin Example4and demonstrates theadvantageof arranging panel support
conditions in away whichencouragespanels to bendin their strong horizontal direction. TheIncreasein strength issolelyduetothis.
A design wind pressure of 1.53kNlm' is morethan adequatefor virtuallyall conditions in Britain. (Note: nodead weight was usedto
enhancef ",; this isonlyappropriatefor weakdirection bending.)
~ Repeat Example 5 for the practical case where the wall is supported at its base. Assume the baseof the wall is simply supported,
;: since It Will besitti ng on dpc material. The heightofthe panel is 1.3m. No loads aretakenon the panel other than thewind loading ItS
j surface.
t Length = 2.6m, height = 1.3m.
" h!L= 0.5; ~ = 0.35
~ a = 0.064, Table9 (A), BS5628
~ Outer leaf: , 1.75
~ 0.064x 1.2 x W, x 2.6 2 x 3.5 ' W"" = 1.93
0.064 x 1.2x W... x 2.6
1.1 x 3.5 ' Wk, =1.06
2.99kNlm'
Thisisadequate for anywherein Bntain.
A claddingpanel IS subjectto adesignwind pressure, W
of 0.8kNlm' (suction). It is acorner panel, see below, freeat the topedge
but sitting on dpc material. ~ the wall is to be designed USing 102.5mm bnckworkIn both leaves, are there any urmtations on the
specificationof thebncksor themortar?
Theassumededgecondmons areshown below.
Thecontmuity of theouter leaf over thecolumn and around thecorner
is asumedto provide fiXity to thevertical edges.
FigureB
Limitingdimenslons(BS5628, CI.36.3):
'" = 213(102.5+ 102.5) 137mm
Maximumarea:} 15OOt..' 28m'
AcIualarea = 11.2m'
Maximumdimension:} 50'" 6.85m
AcIuall argestdimension 4m
wall sitson dpc
2·8m
Since neither the maximum area nor the maximum dimension is exceeded, the panel satisfies the limiting dimension
Basicequationfor oneleaf:
oy, WkL' per leaf = !J,d
Y, = 1.2; W
= O.4kNlm'(W"" = W, +W" but W, = W,): Ym= 3.5: Z= 1.75x 10" mm
. . h 2.8
o coe icient: T = "4= O. 7: ~ = 0.35.
o = 0.039forhIL=0.5 }
= 0.045for hIL= 0.75 BS5628, Table9(C)
therefore, o = 0.044for hIL= O. 7, byinterpolation.
M = oy,WkL' = 0.044 x 1.2 xO. 4 x 4' kNmlmrun
= 0.338kNmlmrun
= 0.388x 10"Nmmlm run.
1.75 x 10"
M, = f"" 3 5 Nmmlm run.
EquatingMandM,gives: f"" ~ J ; x 10" = 0.338 x 10".
f"" reqd. =0.68Nlmm' (strongdirection).
Theweakest f""valueinTable3is0.8Nlmm' . Sincethisexceeds therequiredvalue, anyclaybricklmortarcombination canbeused.
Calciumsilicate bricks
The two valuesof f"" givenin Table3, in the strong direction, are0.9 and 0.6 Nlmm'. Onlythe 0.9Nlmm' is acceptable (ie, mortar
designations (i), (ii) & (iii» . It is normal, when using calcium silicatebrickwork, for a mortar no stronger than designation (iii) to be
used. Hence, specification wouldprobablybe'anycalciumsilicatebrickusinga 1:1:6mortar(or equivalent)'.
Becausethe panel just designed in Example 7 is in a sheltered position, the architect would liketo usea calciumsilicatebrick in a
1:2:9mortar(designabon(iv».Canthis bejuslifted?
Theways inwhich thepanel canbestrengthenedandpossiblyjustifiedare:
(i) modifytheflexural strength using thepanel'sself·weight;
(ii) detail thetopedgeolthepanel togiveasimplesupport: or, ii
(iii) usehorizontal bedjoint reinforcement inoneor both leaves. "
Sinceoption (iii) is outside the scope of this publication, option (i)will beexplored- onlya small reduction (approximately 10%) is ;
neededinthef "" required. lithepanel cannot bejustitied, option(ii)will beconsidered. "
Option (i) ~
Deadloadof both leaves = 2.0kNlm' (lowvalueof G
for conservative reason)
16 Vertical load =2
x 2.0 =2.8kNlm
Designverticalload =O.g x 2.8 =2.5 kN/m
Stress due to design 2 5 x 103
vertical load 102.5 x 103 0.025 Nlmm'
Assume a calcium silicate brick using 1:2:9 mortar (designat ion (iv»). Modifying I... (this is done in the weak direct ion):
I (st rong) 0.6 N1mm' ;
I (weak) = 0.2 +3.5 x 0.025
= 0.288N1mm'
For a single leaf:
M = ay, W, L'
a coeff icient, from Table 9(C). u =0.5:
a 0.035; h!L =0.5
a 0.043; h!L =0.75
a 0.041; h!L =0.70. by interpolation.
M 0.041 x 1.2 x W, X4'
O.79W, kNmlm run
!od 0.6 x 1.75 x 10
= Ym = 3.5 x 10
0.3 kNmlm run
Equating Mand M
0.3 ,
W, = 0.79=0.38kN/m .
For cavity wall :
W, = 2 x 0.38 = 0.76 kN/m' .
This is less than W, =0.8 kNl m' .
If calciu m silicate brickwork were to be used in conjunct ion wi th a 1:2;9 (designation (iv) mortar. it woul d be advisable to suppo rt the
top 01thewall in some way. Normally, this is done using either special (sliding)lixings in conjunct ion with special wall t ies. or by using
sections of angle or channel to restrict t he lateral movement 01t he top edge. If this weredone With suitable details the panel would be
four-sided.
Checking the panel. but not taking account of sell -weight, and repeat ing theearlier section for one leaf:
1.75 xl 06N F' C
d = kx 3.5 mmlm run. tqure
M = ay, W, L' .
acoefficient: 2·8m
h!L = O.7; = 0.35;
interpolation lromTable9 (G) gives a =0.031.
M = 0.031 x 1.2 x 0.4 x 4' = 0.238kNmlm run.
gives f... reqd. =O.48N/mm' . 4m
Thi s is less than the f... value 010.6 Nlmm' in Table 3. So, the panel works satisfactoril y as a tour-sided panel in calcium silicate
brickwork in a 1;2;9or equivalent designation (iv) mortar.
__ overhead beam providing
-.....-.........-- support to topof panel
tray (not shown)
A similar panel to Example 7 is to be designed l or two different parts of t he st ructure. The W, value for both posit ions (general wi nd
pressure) is O.68 kNlm' . It is proposed to use a l 00mm 3.5 Nlmm' block and a clay brick (bot h in 1:1:6 mort ar)f or the inner and outer
leaves respectively. 11 the twoarrangementsfor the two positions areasshown below, comment onanyspecification requirements
for the clay bri cks.
....- glazing unit
.............o-- --ground floor panel
(1st floor panel not shown)
overhead beam providing
support to the top of the panel
special column in entrance hall
offset from grid position & not
supporting edge of wall
ground floor panel
_-4=+ - __doc
Assumed edge support conditions:
Side 1simplesupport, dpc tray above
Side 2simplesupport, panel stops - but provide support ties
Side3 simple support, sits on dpc.
Side 4 fully fixed. front leat continues past column.
Inner leaf.'
ay,W
Note: Clause 36.2 permits both leaves to be assumedcontinuous, evenIf only oneleaf is continuous, If (r)cavity wall tiesareused in
accordance with Table6, (ii) thediscontinuous leaf is not tbicke: than theonethat is contmuous. Hence. theassumededgeconditions
shownaboveapplytobothleaves.
tnnerteet:
100mm 3.5 N1mm
blockin 1:1:6 mort ar.
fluweak = 0.025 Nlmm
; flustrong = 0.45 Nlmm
~ = O. 02Ml.045 = 0.55; hIL = 314 = 0.75.
From Table 9(F):
a =0.034; Z = 1000x 6=1.67 x 100mm'/mrun.
Equatin9MandMd,ay, WkL2 - ~
0.45 x 1.67 x 10"
0.034 x 1.2 x Wk x 4
3.5 x 10'
= 0.33 kNlm
Since the caVitywall must carryO.68kNlm
the outer leaf must carry O.68 - 0.33 = 0.35 kNlm
Assume the weakest case in 1: 1:6mortar and check the W
found against theO.35 kNlm
required to be carried:
a = 0 . 0 4 1 ( ~ = 0.35; hIL = 0.75; Table9 (F»
flu = 0.9 Nlmm
EquatingMandM
ay, W
0.9 x 1.75 x 10"
0.041 x 1.2 x Wkx 4
3.5 x Hj6
= 0.57kN1m
This compares l avourablywrth the O.35kNlm
the outer leal will be required tocarry il the inner leaf was l ullywor1<ed.
There are no specification requirements forthe clay bricks of the panel in Posrtion 1. Any clay brick in 1:1:6mortar will be sufficient.
(Design load = 0.68kN1m
Design resistance = 0.9 kNlm
Position2 "
Assumed edge support conditions;
Same as above(Position 1)
exceplthat edge 2 is free. ®
Repeating the above calculations (in large steps) but basing the a coefficients on Table9 (K).
0.057 x 1.2 x W
= 0.20 kNlm
2 0.9xl .75
0.075 x 1.2 x Wk x 4 = 3.5
= 0.31 kNlm
Design wind resistance = 0.20 +0.31= 0.51kNlm
This is not adequate, and a different specification will be required.
recalculate using an llu of 1.5 Nlmm
(to give W
= 0.52 kNlm"l
on the basis thatW
varies Iineartywith lIu,
0.31 x 1.5 2 I 2
Wk 0.9 O.52kN1m when Iu= 1.5N1mm . ii
Thus. design wind resistance is now 0.20 + 0.52 = 0.72 kN/m
. This exceeds the design wi nd pressureof 0.68 kNlm
. Therefore, in :<
posit ion 2, the clay bricks need to be01less than 7%water absorption in a 1; 1;60r equivalent designation (iii) mortar. ~
(Note: It would be Quitepossible to investigate the strength of the outer leal using a brick of 12% water absorption or greater in a g,
stronger mortar. It isunusual, however, todetail a cavity wall with differentmortars ineach leaf; it wouldrequirecareful supervision in ~
prsctice.) ..
rtit is planned to use bricks with a wa. t.er absorption higher than 7%, the strength althe inner leaf will require to be enhanoed . This i
could beachieved by: i!
(i) Increasing the block strength specitied . •
18 (ii) Increasing the thickness olthe inner leaf. This would increase Z. iii
(i ii) Reinf orcing the Olockwork.
Reinforcing the outer or inner leaf (or bot h) may be the only solution if a brickof more than 12%water absorption is to be used.
The above options assume that the simplest solution - namely. providing edge support to the assumed free edge - cannot be
achieved. This solution should. of course. beexplored first.
2·5m
A loadbearing caVitywall is to be checked for lateral strength. It is well supported on its twovertical edges. but is assumed to be simply
supported top and bottom. Both leaves carry the roof equally. W, for thi s exposed site is 1.2kNlm' suction.
0.03 N1mm'
O.02 N1mm'
Stress due to design vertical load
Roof /cads:
Looking for conservative (unfavourable) conditions. and using dead + wind (no imposed) based on 0.9 G, and 1.4 W, . the mi ni mum
design vertical load is 6.2 kNl m of wall . ie, 3.1 kNl m on each leaf. Both leaves have bricks of more than 12% water absorprtion in
1:V,, 3mort ar. f,,(weak) = 0.4 Nlmm': f ,,(strong) = 1. 1Nlmm' ; (assume u = 0.35).
3.1x 1o"
102.5 x 10
0.9 x 1.25 x 2.0 x Hi'
102.5 x Hi'
St ress due to design self ·weight of wall
Total stress for design loads
f,, (weak) = 0.4 + 3.5 x 0.05 = 0.58 Nlmm' ; ~ = 0.5811. 1 = 0.52.
Rrst check whether it wi ll work ascladdi ng (ie, no allowance for vertical load):
hIL = 0.45; a = 0.022(Table 9(G): ~ = 0.35).
W L' h..1
ay, k Ym
, 1.1 x 1.75 ,
0.022 x 1.4 xW, x5.6 3.5 ; W, = 0.57 kNlm .
Cavity wall design strength resistance = 1.14 kNlm' ; design load. W, = 1.2 kNl m'.
Accounting fordesign vertical stress:
a = 0.018; ~ = 0.52; hIL = 0.45
' 1.1x 1.75
0.018 x 1.4 x , x 5.6 = 3.5
W, = 0.70 kNlm'
Cavity wall design strength resi stance = 1.40 kNlm' .
This is adequate far W, = 1.2 kNlm'.
(To be read in conjunction with BOA publ icat ion 'Accidental Damage, Rabustness &Stability"")
A 215mm brickwork wall is to be checked to ascertain whether it can Withstand the 'accidental force' equivalent load of 34 kNlm'
(the equivalent gas explosion lateral pressure). The design load on the wall is 140 kNl m run (derived from usi ng parti al load factors
appropriate for acci dental damage analysi s- Clause 22(d».
.. 1+102·5mm
215mm ~
~ The basic equation q lal = ~ is used for walls with sufficient precompression to develop in-plane (ie, arching) forces. For 19
accidental damageanalysisusing this equation a partial factor of safetyfor materialsrepresented byYm= 1.05can beused(Clause
37. 1.1).
ax 0.215x 140
ql81 = 1.05x 2.5
= 36.7 kNlm
Sincethis is in excess of 34kNlm
, the wall is judgedto remainafter an 'accidental event' and is regarded asa protected member
(Clause37.1.1).
Repeating thedesign, but usingt =170mm,
axO.HOx 140
q lat = 1.05 x 2.5
= 2Q kNlm
(ct. 34kNlm2j.
ij thewall wasconstructedusingthe 170mmCalculonbrick, it would bejudgedto berell10lled byanaccidentalevent.
(Notethat, "there were oneor tworeturnsonthevertical edge(s), thevalues of 36.7and29wwrr? above, shouldbe enhancedby the
appropriate/<factor fromTable 10.)
Assumingthat Itwasdecidednotto usea215mm, onebrickthick, inner loadbearing leaf, thebuildIngwouldneed:
(i) tobechecked tomakesureapartial collapseof unacceptableproportionsdid not ensue;
(ii) tobefullyhorizontallyandverticallytied.
See Clause37.1andTable12.
Forafuller explanabon of thesubject of accidental analysis, see'AccidentalDamage, Robustness<I. Stability'16.
--- -------- - - - - --------------
'0" Readers are expressly advised that whilst the contentsofthis publication arebelievedto be accurate. correct andcomplete. noreliance shouldbe placed uponits contents
e as being applicable to any particular drcumstances. Any advice. opinion or information contained is published only on the footing that the Brick Devefopment
f Assodation. its servants or agents andall contributorsto this publication shall be undernoliability whatsoever in,respect ofits contents.
C1> Designed and produced for the 8IidI 0eveI0pment Assocatoo. INoodsideHouse. WInkfield. WIndsor, Berkshire 51.4 2OX.
~ Tel: Winkfield Row(0344)885651 by TGV Publications. Printed byRoikeepUmited.
the subject is dealt wit h in its w idest Particular attention is paid to Clause 36 of Section 4 of the I: . Amen dments 2747 (October 1978) and 3445(September 1980) have been taken into account. as has the latest amendments 4800. 8 Code which gives detailed design recommendations for laterally loaded walls.J. March 1985.BS 5628: The Structural Us e of Masonry Part t : Unreinforced Masonry The design of laterally loaded walls Prepared by J. ~ S cope ~ • The contents cover both the background to the Code provisions Acknowledgements The kind support and guidance of many pecple dur ing the preparation of both the seminars and this publication is gratefully acknowledged . and to N. 3! as well as the provisions themselves. current chairman of the Techni cal Drafting Comm ittee. Morton BSc PhD CEng MICE MlnstM This publication is concerned with the subject of laterally loaded walls.l reasoning behind them. with particular reference to uniform lateral pressures. Tutt who checked the design examples. Particular than ks are due to BA Haseltine. C1.recommendations and the t se nse. It is based on visual presentations originally given during a series of !!1 ~ seminars on BS 5628: The Structural Use of Masonry: Part 1 in ~ late 197 and subsequently. In order to give the reader ~ an understanding of the Code ·.
.._ _..._ _ 8 Check ing th e design _. REFERENCES 13 _ _ _ _ _ _ _ 13 14 14 14 Cladding panels W all tests _ _ _.__ Edge supports .......... . Mode s of failure .. Orthogonal ratio _ _. _. _ 6 6 6 __...._. 8 Shear ........_..__ _ Free top edge _._ Review ...._......___ __ _. . ..... DESIGN EXAMPLES Example 1 Exam pie 2 _ _ Exam pie 3 _ Example4 Example 5 Example6 Example 7 Example 8 Example9 Example 10 Example 11 _ __ _.....__ _ _ _ _ .. 15 15 15 15 16 16 17 . 8 Partial safety factor for materials __ __...._... wa lls like those show n above shou ld be built and tested in bendi ng to failure..._ _ __..._ _ _. as shown on the right.._.. Bonded to pier s ..._. _. 5 __ _ __ _ __ _ _.._ _ . The fail ure mode wi ll be a flexu ral crack that will occu r along the bed joint.... they are also ii ~ Q._._.11 ismoredifficu ltto study the material properties in both directions sim ultaneous ly than if the panel is spl it into its horizontal and vertical modes and each of them studied sepa rately........_ ......._... 7 Design moment of resistance ...._. MATERIAL PROPERTIES Inlroduction Weak and strong direction of bending Wallette test programme .. ~ ~ ~ ~ 2" > 2 ..__. When the wall is subject to simple horizontal bending....._._ 5 5 5 _._.. 5 5 Walls with openings 3.. 19 19 1 IlATERIALPROFDt IllES Introduction The maj ontyof ma sonry cladding panel s tend to bend in both the horizontal and vertical direction s simultaneously.__ 8 Partial safety factor for shea r __.. Wh ile ful l scale model tests are perhaps ideal......_ _....._..._.._...._.._.. 7 _ _..._ _..._._ Bonded retum wall s . __._ Metal ties to columns _ ._. ._ _ _.. Concrete bricks Concrete blocks ......jf . ._ Unbonded retum wall using metal ties _. Li mits to pane l size _ Edge restra ints __......_._.._ __ Docking or wetting of bricks __.. ......FOREWORD _ ScOpe _ Acknowledgements _ _ _ _.. Wall built up to the structu re with simple anchorages lnsitu floor slab cast on to w all Effect of damp proof cou rses Experimental validation _..... IIei di '!l parallel to bed joints to bed joints IIei di '!l perpet daABr 3 Ideally.._ _ 10 _ _ 10 .._ _._ _ Clay br icks Calcium silicate bricks _. 9 4._ _._ _ __.........._.._. . 9 _.. in the case of simple vertical bending (left) at or near the position of maximum moment....._......_ . 12 ......_..... . Statistical qui rks ... .__...... ... ..._ _ Design moment .. ..._. _.__.._._..... they are essent ially two-way bend ing plates (figure 1).._ _... 1 _......... 8 Partial load factorfor wind _. the flexural failu re crack will develop through the perpendicu lar joi nts (perpends) and the bricks..... 5 2._ _ .... This facilitates any experimental investigation (frgure 2). .._._ 10 10 10 10 10 11 11 12 . 12 9 9 9 _ _ Arching walls Introduction Freestanding wall s .. ..._._ _.._.._ _ _..._....... 1 _ 1 2 2 3 3 3 4 4 4 1..._ _ _...____ Cav ity walls W all panel edge su pports .. 7 Bend ing mom ent coeffic ients _. DESIGN METHOD Introduction _ Cladding panels Panels which arch _ _ 5 _.
The wa llette. the wallette in the foreground . and that simple horizontal bending is the strong direction. Each point on the graph represents the mean of five or ten wallette tests wit h a particular brick. The objective was to ascertain the fiexural strength. But to what parameters could these results be related? Perhaps the best known property of bricks is their compressive strength. Wallette test programme The wallette testing programme was carried out by the British Ceramic Research AssociationI . For ease of termin ology. A statistically based approach was used to establish the position of the 95% confidence limit from the mean and standa rd deviations associated w ith each point. for the range of modern brickwork designed and constructedin thiscountry.. Two wa llettes and a test rig can be seen above. will fail when a vertical crack develops across two bricks and two perpends. Because complex curves make design less straightforward. it is reasonable to predict that this direction of bending w ill be relatively strong. This limit is show n in figure 7 as the curved line. It reflects the fact that brickwork has a wide statistical 'quality' curve. To comply with limit state philosophy. in both the weak and strong directions of bending.expensive. knownaswallettes. commonly available in Britain. It can be seen that the flexural strength tends to decrease With an increase in the water absorption of the bricks. Does the flexural strength of brickwork relate to the compressive strength of the bricks? The relationship is shown in figure 5. Since the brickwork has relatively weak tensile properties in both direct and bending tension. Weak and strong direction of bending The wa llette in the test rig in figu re 4 will fail. After examination of all the possible relationships. '!r ~ . it was decided to simp lify the curve with an 'approximation'. It is more cost effective to build and test small wa lls. ~ ~ ~ ~ ~ ~ t-. the characteristic flexural stress could be calculated for the strong and the weak direction of bending. it was necessary to establish the 95% confidence lim it from all the test results. built to a different format. From the limited results available. A wide variety of bricks. This enabled the mean and standard deviation of each set of results to be found . Experimental evidence confirms that simple vertical bending is the weak direction. it is reasonable to predict that this direction of bending (simple vertical) will also be relatively weak. at or near mid-height. wh en loaded to failure in the other rig. the direction of horizontal bending and the direction of vertical bend ing will now be referred to as the strong and the weak directions of bending respectively. It is reasonable to predict that some relationship would exist between the strength of the brick in flexure and the flexural strength of the brickwork . it was decided that the relationship between the flexural strength of the brickwork and the water absorption of the bricks' from wh ich it was constructed would be used " The graph above relates the flexural strength of the brickwork to the water absorption of the brick for wallettes tested in the weak direction and built w ith designation (i) mortar. With these values for the mean failure stress and the standard deviation. it was not felt to be a relevant approach. Figure 6 shows that there is some correlation between these parameters.particula rly in the strong direction. Clay bricks For each clay brick/mortar combi nation. Since a brick is much stronger in flexure than a mortar/br ick interface.. in the rig in the background is being tested in simple vertical bending. a set of wallettes was built in both the horizontal and vertical formats. The wide scatter is not exceptional. The wallette in the foreground . w hen the interface between the bricks and the mortar bed is broken. On the other hand. is to be tested in simple horizontal bending and wil l be tested in a different rig. built to a particular format. under w hich lie 5% of the results. The number of dots indicates the wide range of bricks that were tested in order to represent the wi de variety of clay bricks available in Britain to-day. each consisting of five or ten wallettes. it is to be expected wh en testing masonry. if not in the weak. was tested in the four British Standard mortar designations. This is shown as a straight dotted line 3 .
Calcium sil icate bricks The approach for calcium silicate bricks was essentially identical to that used f or clay bricks. Co nc rete br icks Concrete bricks were tested in an identical manner to the other brick wallelte tests.but sti ll wi th mo rtar design ation (i) . The denser the block. Figures 9 & 10 show the graphs for a designat ion (iii) (1:1:6) mort ar. using a range of calcium silicate bricks representative of modern production in Britain. :< Cons idering the strong directio n of bending. From both the graph and the table. ind icate similar values to those associated wi th calci um silicate brickwor k. These values . For this reason. see f. both the perpend joi nts and the blocks. calcium silica te and conc rete bricks .: 10 _ • undocked . As before. in both ii' an increase in flexural stress with an increase in block strength.. concrete blocks do not use the parameter of wa ter absorption in their Standard'. They were constructeo using four different mortar designations. 0. It i~ reasonab le to expect ~ waterabsorption .. 0. facilitates the material prope rties of brickwork being descnbec in tabular form . figure 13contains the values derived from this work. ~ I .fference in urut size between bricks and blocks.6 Nlmm' for the respective mortar designations. it must be ~ remembered th at the failure ot the walle lte involves the fai lu re of Q.4 Nlmm' 0NA greater than 12%) when considenng the weak direction of bending in mortar designation (i). docked Flexural strength Nmm' l -I U :~ .the background to another sectio n of Tab le 3 of the Code (figu re 13) can be seen. water absorption was not considered to be the most relevant indica tor of the flexu ral strenqth of blockwork. The results.ng three values of flexural strenqth for three ranges of water absorption .gure 13._ l . have a waterabsorptionvalue associated With theirStanoard". Calc ium silicate bricks available in Bntain do not. the grea ter the flexural i1l: 4 stfrtehnglbhl ofkthte block 'tfseij'IAndI t h e grfelater the flexural $lrength ~ be o e ocx. The approach adopted is identical to that describec above and the values on th is graph correlate With those in Table 3.3. this t ime in the stronq direction . formats. The results of the large testing programme indicated that only one set of values for the characteristic flexural stress was needed to cover all calcium silicate bnckwork. The wanette formats for blocks are given in AppendIX A3 of the Code.7 Nlmm' 0NA less than 7%). This conveni ent approach of hav..5 Nlmm' 0NA greater than 7% and less than 12%). ------ The relationship between block bulk density and the flexural $lreng th was also examined to see rt bulk density might be a useful indicator. i This is because the stronger the block in compression.2. the denser it will be. he grea er WI e m uenoe on the streng th 0 f " . 0. these values are 0. a different wallelte format was necessary to account for the d.and the water absorption of the clay bricks from wh ich it was constructed was established for all the mo rta r desig natio ns. are given in flgure 13. Flexural strength water absorption 1:1:6 in strong direction . the flexural strength of blockwork in f igure 13 is given in termsolthe crushing st rength olthe bloc k.0. While certain trends are apparent in figure 12.. While the cha racteristic flexural stress was ascertained f rom wallelte tests. Looking at another simi lar graph. of course. The relation ship between the cha racteristic flexural stresses of brickwork .wrth two steps at values of 7% and 12% water absorption.9 and 0. ij is from this research background that the charactenstic flexural $lressesgiven in TableSot the Code were derived . and 0. they we re not felt to be as relevant as the relationsh ip between flexural strengt h and the compressive strenqth of the block itself. While the wallelte test programme indicated that there was some relationship between water absorption and flexural $lrength (figure 11).in the weak and strong directions . This rs reproduced as figure 13on page 5. Con cr ete blo cks Concrete blocks are covered in Table 3 (tigure 13) in a somewhat diffe rent man ner to clay. figure 8. Wallelte sets were buiij. As withcalcium silicate and concrete bricks. 0.
This trend was found to be signilicant in the strong direction. Althou gh this is often done on some sites. a manufacture r's claim of higher characteristic flexural strengths than those in the Standa rd can be object ively assessed by the designer if. Consequenttv. it is necessary to establish a value for each blockwork strength. The general rule 'never allow bricks . In the weak direction. Not all the graphs have been shown since they would give no more information than is contained in Table Statistical quirks Ther are certain implications inherent in a statistical approach. two types of panel are particularly important. it is not the recommended method. generates compressive forces within the plane of the wall as it deflects. In this way. and provides more detailed information on the large testing programme on wh ich it was based. fu rther tests have indicated that the flexural strength of thicker walleltes is lower than that predicted by the wallelte results for 100 mm blockwork. that the Code permits wa llelte tests to be carried out on particular bricks in accordance w ith the testing procedure set out in Appendix A3. For intermediate values of wall thickness. with concrete blocks.5 kg/m ' /min or that he is able to adjust it so as not to exceed this value". a half to three minutes. the ratio varies since the strongdirection stresses vary whilst the weak direction stresses remain constant for different strength blocks. at his discretion . does not exceed 1. Docking or wetting of bricks The graphs shown in figures 7. the largest value of flexural strength is achieved with a brick in the water absorption range 7%-1 2%.2 Nlm m' wh ich are dependent on the mortar designation but independent of the strength of the block Itself. 9 <I. and 0.25 Nlmm' and 0. This can be done eithe r by immersing it in wate r for a short time . There is. A particular type of wall panel in wh ich in-plane forces are present is a loadbearing wall in a structural masonry building. The Standard has therefore been amended to take account of this. A closer examination of the graph. for 100 mm block work there are only two values of 0. 'Docking' or welting of bricks does not mean saturating them. it explains the backgrou nd to Table 3 (figu re 13). a value of 0. and the vast majority of brickwork panels used to clad framed buildings fall into this category. e 3. The characteristic flexural stress values range between 0.or brickwork to become saturated' should always be obeyed. This is due to the need to wet ('dock') certain types of brick if their water absorption would result in water being removed from the mortar at too great a rate. no un ique value for the orthogonal ratio of blockwork. In particular. is the panel that 'arches'. however.33 (see figure 13).typically. It could be a vertically spanning wa ll as shown on the left of figure 14.4 Nlmm' depending on the mortar designalio n for 100 mm blockwork. the in·plane compressive stress present is the stress in 5 . Orthogonal ratio The orthogonal ratio is the ratio of the strength in the weak direction to the strength in the strong direction. the contractor shall satisfy the eng ineer either that the suction rate of the brick " .7 to 0.9 to 0. Research has indicated that th is high speed removal of water from the mortar can adversely affect both the compress ive strength of the brickwork and the bond at the brick/mortar interface. The ISR of a brick can be adjusted by partially soaking it.35 can be universally adopted for all these bricks.the walleltes wh en tested in the strong direct ion. For clay. or a horizontally spanning panel as shown on the right. are superimposed on the tensi le stresses as they develop and partly or wholly cancel them out. 8. The graph show n in figure 10. This type of wall is very comm on in Britain. 10 make' reference to 'docked' and 'undocked' bricks. the ratio is approximately 1:3 or 0. Cladd ing panel s The first type is the panel wh ich tails when the tensile stress in the extreme fibres equals the ultimate stress. does nonetheless misinterpret certain facts. reveals that brickwork constructed with bricks of between 7% and 12% water absorption can exhibit higher values of flexural strength than can be achieved with man y of the bricks w ith less than 7% water absorption. Since the Code was first published. Consequently. it is normal good practice to suggest that 'before orders for bricks are placed. Indeed. th is type of panel is mu ch stronger than a cladding panel. Both the curved line and the stepped approximation indicate that the strongest brickwork results when bricks with a water absorption of less than 7% are used. the value of characteristic flexural stress can be obtained by interpolation. wh ich behaves different ly from a cladding panel. The rate at wh ich water is removed from mortar is called the Initial Suction Rate of the brick (ISR). generated by the deflection. he decides to ask for wallelte results to BS 5628: Part 1from the material producer. calciu m silicate and concrete bricks. Such a wall (figure 15). It can also be partly achieved by sprinkling the stack w ith water from a hose. w hilst being statistically correct. In this case. failure occurs by breaking the tensile bond at the interface between the blocks and the mortar bed. between 100 and 250 mm. It is reflected in figure 13 w hich bases flexural strength on the compressive strength of the block. For design purposes. The experimental evidence suggests that there is no significant difference between blockwork constructed with weak or strong blocks. These compressive stresses. therefore. DESIGN METHOD Introduction Turning from the consideranon of material properties to the design of panels using these characteristic flexural stresses. Unfortun ately. When w riting the specification. Summary The foregoing materia l constitutes the background to the section of the Code wh ich deals with material propert ies. Panels wh ich arch The other type of panel.4 Nlmm' . It is because of such statistical qu irks.
W all tests To investigate this. The majority of th e panels bu ilt were standard storey height (figure 21) but some taller and sho rter panels were also tested. Wallettes awaiting testing can also be seen in the foreground of the picture. ~ ~ Edge supports vertical andhorizontal directions. :. such as the 1'h sto rey height pane l in figure 20. The reactio n frame was tied back to the test rig. This essentially relates the two know n material properties described in Table 3 of the Code to the behaviour of thr ee and four -sided plates.t5 Horizontal arcIli Ig Vertical arc:tiIlg the wall derived from the load rt is suppo rting. The objective was to find the relationship between the strength in two-way bending to the two known strengths in the simple it (figure 21). It isa complex analysis andinvolves thesummation ofthe two stresses generated from two-cirectional bending (figure 18). The lengt h of the panels was also to be varied . it is now necessary to combine them in two-way bending (figure 11). ::. J il: . a series of full scale wall tests were carried out at the British Ceramic Research Associa tion' (figure 19). when the panel cracks. . In order to demonstrate the failure ~ pattern more clearly.gure 22 actually cracked at a much lower deflection. The wall in f. 6 Full scale wall panels were built in test rigs (figu re 20) and an air bag was placed between thewall and a reaction frame behind Actual failure. the air bag was inflated still fu rther until the •. Cladding panels The first type of wall to consider is the cladding panel that falls when the tensile stress developed at the extreme fibres in bending reaches the ultima te flexural stress for the material. This will oo« be considered in greater depth. Panels of different shapes were then tested using various brick/mortar com binatio ns. does not necessarily ~ result in total collapse.note the two shorter panels awa iting testing in the foreground of ~ffi2 1 . II is important to distinguish which type of wall is being designed. and whether it is a wall in which compressive forces can develop. Having already established the material properties that can adequately predict the simple vertica l and horizontal bending strengths (figu re 16).
and taking into account the wall test results. A supported fully fixed edge where a moment equal to the panel mid -span moment wo uld develop. For lim it state design V" the partial safety factor for loads. To be a design moment of resistance. =partial safety factor forloads Wk =characteristic wind load L =panel length 2 In the design of any bending membe r. A free or unsupported edge. 3. the length. A supported but pinned edge w here no moment would develop. the panels were supported in the test frame in a similar way to panels in a real building. They were found to be fairly 3. and not h. M=a~. Working on this basis. it is stable. Ties we re inserted into these holes and were used to connect the wall to the frame. the height. 2.OorlAa o or a or a o directions. Because the applied mome nt uses L (ie. the follow ing design procedure was suggested": Designmoment 26 wa ll was grossly deflected (figure 23). the f x Z term mu st be divided by ~ the test programme progressed. and also for various possible edge conditions (figure 25).125 for the case above). of ~ course. if account could be taken of the two o!f different bending mome nts at the centre of the panel in both 7 . the moment of resistance mu st use f" in the strong direction for compatibility. is necessary to convert the applied moment into an (applied) design moment.wkl! where M : bending moment-about vertical axis a : bending moment co-efflclent ~. • ~ It was recogn ised that. it is associated with horizontal spanning). The design moment for panels is based on the same equationbut the divisor isexpressed as an a coefficient (eg.ie. the applied moment is norm ally WL2 divided by a factor. This also serves to illustrate the residual strength of failed panels. for example 8 for simply supported one way bending. Note that the formu la is expressed in terms of L. although the wall has structu rally failed and is grossly deflected. similar to those associated with concrete slabs although. It was of course recogn ised that this edge support system was more likely to approximate to simple supports rather than to fully fixed edge conditions. Failuremodes Designmoment of resistance 27 i ! ~ Md =fkxZ Om fkx = characteristic flexural strength about vertical axis Z =section modulus om =partial safetyfactor on materials The failure modes of the panels (figure 24) gradually emergeda s The formu la to calculate the momen t of resistance is the normal f x Z term. The three possible edge conditions wou ld be: 1. O. Visible in figure 23 are the holes that were cut in the channels forming the two vertical edges ofthe test rig. a useful design techniq ue wou ld result. brickwork is a brittle material (unless reinforced) while ~ reinforced concreteexhibitsplasticyield ing. as is done in practice .
VI 32 BerocAIQ Normally. a 0·50·065 0·040 0·035 1·00·091 0·050 0-<>65 0 ·098 1·50·125 0·083 0-081 0 ·144 31 Bending moment coefficients are set out in Table 9 of the Code.4_ However. It is nevertheless necessary to check the shear. II must be clearly understood that this is an exception. should the brickwork panel form part of the structure. VI can be reduced to 1. Z is assumed and the f. This value for ~ has been purposely includedfor all typesof brickwork} or These are normally the two main ways in which the design is carried out. Partial safetyfactor materials. ~ The value of Vm can vary between four values depending on the ~ category litcons truction control (workman ship and supervision) and the categoryof manufacturing control (the likelihoodof weak .whether three or four -side supported. units being included in the consignment of structural units). Bendingmoment coefficient.. Shear Partial safety tector for load(wind). They are essentially the coefficients generated from 'yield line' theory' ·· which. Vm 8 Checking the design The design moment of resistance must be equal to or greater than the design moment and. and whether 'simply supported' or 'fully fixed'. (Note that a set af values for a has been given in each of the tables for ~ = 0. The a coefficient depends on the aspect ratio of the panel (hIL)..4. The above formu la givesthis readily. nevertheless appeartogivereasonable correlation with experimental results . and should the removal of the panel endanger the stability of the remaining structure. Of course.of the material is checked to see if It is adequate. In this case. on the orthogonal ratio ~ .normally a frame. It is now possible to obiectivelydesign laterally loaded panels. VI for general deSign is taken as 1. For the majority of panel s met in practice. failure in bending will be the design criterion. basednot on design philosophy but on the need to acknowledge that Without reducing VI to 1.of the matenal to be used IS fed into the deSign and the Z valueofthe section to be usedis clhecked. by re-arranging the formulae: either.35. This ~ ->' a .V the partial safety factor for materials. ii ~ a. the f.2 many panels currently known to work well in practice could not be justified in the design process. But It is first necessary to look in detail at certain of the terms used. or. although not fell to be strictly applicableto bnttle materials such as masonry..2. m' By equating the design moment of resistance to the design moment. VI must be maintained at 1.. ~ . and the nature of the panel's edge support . a special case is made of cladding panels which do not affect the stability of the main structu re .at least for solid panels without windows or doors".
is not difficult to grasp. the mo rtar and the way they are put together on site. Bonded to p iers ~:2·5 Y~ for shear is taken as 2. depend ing on the nature ofthe panel supports. ~ 2.. Since let for a normal cavity wall is approximate ly 137 rnrn. this sub-division is achieved using intermediate supports. achi eved by taking the wall past the colum n. does not give I Il1l% fixity against rotation. Normally. These design bonuses can be exploited if: 1. To achieve special category of construction control. If the edge of a panel is 11l1l% fixed. . but they should be effectively su b-divided so that.) where t .5 Limitsto panel size The above argu men t will also apply to piers. the need for a sim ple suppo rt at the end of the panel is due to the Two limitationsto panel sizesare given in theCode: ~ 1. should they be req uired. wh ile they may not appea r to be two separate panels. The fully continuous edge.) . while 2 and 3 are both factors influenced by site operations and It is possib le to bu ild panels w hich exceed either or both 1 and 2 above. However. they flex in bending as though they are.. Looking at detailed cases: supervision. suitab ly li ed to a co lumn. conddions 1 and 2 can be effectively met. is 11l1l% fixed. the panel edge Will fall In flexure before rotating. above. and the majority of BOA member companies w ill readily be able to meet this form of producti on quality control. In this way. where the wall is continuous. There is a sma ller probabi lity that poor workmanship w ill be used in putting the bricks and mortar together on site. The criterion used in the Code. 3. Anything less than this and the pane l edge is assumed to be 'simply suppo rted'. M eial tiesto columns Ym for shear v 34 The non -conti nuous edge. Edge restraint As With any structural member. Wh ile the fIXed support. are 9 the actual th ickness of the two leaves. is readityunderstood. Condi tion 1 is covered by the manufactu ring process. the panel is assumed to be 'fu lly fixed'. and the work on site shou ld be supervised by a suitably quali fied person to satisfy condrtion 3.approach essentia lly recogni ses that there are bonuses for the designer If he can be more certain abou t the structural uni ts. An overall limit on the area of the panel. the question of wh ether the ends are fully fixed or pinned is not necessanly easy to determine.. an overall lim it on height or ~ lengtn'w hich mu st not exceed 5Ot. It w ill break along th is vertical edge before it w ill rotate. it is possible to specify bricks to an accept nce lim it to BS 3921 · a whe re not more than 5% of the bncks will have a crushi ng strength below the acceptance limi t specified. Detailed restrictions <: are given. In most situatio ns. the effect ive ~ !!. As is the casewith compression and shear loadedwalls. such that.. There is a small er. even If in reality it may be pa rtially fixed. the likely contractor and the level of site supervision will not necessarily be known. If loaded. probab ility of the mo rtar being below the strength specified.50r 3. 1 since. the practical lim it for normal cavity wa lls spanning in both ~ di rections is about 6. mo rtar tesling should be carried out to satisfy condition 2. however. For three and four-sided panels. 2.5 m in length and height. (I. thickness of a cavity wall is normally~ (tl + t. and t. There is a small er probability of the structural un its (bricks! blocks) failing below their specified strength. This is the recuirement for special category of manufacturing control. the designer w ill normally base his design on Ym = 3. It is therefore assumed to be sim ply supported. lower values of Ym are available.1 effecli ve thi ckness of the (the ~ wall) . at the desig n stage.
theyare none thelessover-simplifications. some rotation will be possib le. Besides the obvious advantage of keeping design simple. and the wall mus t crack at the vertical supports before rotation can occur (figu re 38). ~ if " . Bonded relum waifs This full fixity IS assumed If the pier becomes a bonded long retu rn wall. In the above example.abi lity of the pier to rotate in torsion. This will be designed as a three-sided panel. it must be assumed to be simply supported. In most of the cases conside red where simple supports have been given. with the experim ental results of il walls tested in the laboratory. The ~ strongest test result shown in figure 44 is for a wa ll that failed j between9 -10 kNl m 2. the hor izontal edges can also be considere d as eithe r fully fixed or simply supported . This vertica l edge should be considered to be a sim ple suppo rt. therefore. Experimental validation Whilst the above app roximations. are useful to the busy design engineer in practice. adopted uses eithe rfull fixity or simple suppo rt condi tions . In-situ (loor slab casl on 10waif 10 When an in-Situ floo r slab IScast onto a wall. at higher -g' loads. If torsion restraint were present. the design method using partial restraints is found to Q. which give assumed edge conditio ns. it is normal to expect the base of most panels to be simply suppo rted. the wall has been effectively pinned to the floor above. this approach agrees with the pub lished experimen tal evidence". 50% or 70% fixity may be present. the end could be considered full y fixed. However. In many of the examples. the Code sugges ts that It can be considered fully fIXed. Unbonded relum waif using melallies Effecl ofdamp proofcourses If the wall is not bonded but merely tied to the long return. yelthe predicted streng th was between 14 ~ . based on the design method outlined previously. Whilst the correlatio n is reason able forfailure loads of approximately 5 kNlm 2 and less. the top edge will still be able to rotate under lateral load and. Irrespect ive of what happens at the vertica l edges. a degree of partial restraint is actua lly present. As a result. Figure 44 com pares the predicted failu re load.15 kNlm 2 • It was for this reason that the design method il. the top edge is actuall y free no suppo rt has been provided. but using an estimation of ~ the degree of edge restraint. 30%. Waif buill up 10Ihe slruclure wilh suilable anchorages It is more usual to provide some form of support to the top edge. it must be remembered that most panels have their shea r and moment resistance lim ited at their bases by the need to introduce dpc materials. In the above examp le. predict higher strengths than are found in the laboratory. Now both edges are fully fixed. free lop edge Irrespective of the details at the top of the wall.
Band C. when 6 = t. The strength enhancement can be signIficant . Line E represents the design method when using the partial safety factors for both the loads and the materials. where all the points lie above line A . the arching force is generated from the precompressive stress already present due tothevert ical load the wall is carrying. as can be seen in figure 45. The basic equation q. in situations where archingaction cantake place.. given in Table 10 of the Code (figure 52 overleaf). this ~ underestimates the strength of walls. confirmed the earlier concl usion that this ultimate load eouat. In other wo rds. Increasing f". This equation is based on stability consideratio ns and not on the flexural strength ofthewall. and is a good failure prediction equation " . ~ To accom modate the deflection induced by loading a wa ll -1: panel. the characteristic flexural stress in the weak direction can be increased to allow for the dead weight of the top haf of the panel.either on plan or in • '" section.on gives good correlation with the experimental results. unlike partial restraint either estimates the strength realistically or underestimates the strength of the panels. To establish the lateral strength of a three or four-sided loadbearing wa ll. justifyi ng a panel using the recommended design method is quite straightforward. the strength is first calculated from the basic equation assuming the wall spans only vertically. In some cases. the panel must increase in length . a modified approach to the arching guidance given in the Code is suggested.the 45' line. Even though a loadbearing wa ll cracks and forms three hinges at A. This approach". Walls w ith arching action present are inherently muc h stronger than those .as large concrete columns or substantial edge beams.where arching action cannot take place. The equation was checked against the ulti mate lateral load of wall panels in a full size 5-storey test building at the University of Edinburgh' (figure 50).such as cladding panels . Three and four-sided loadbearing walls have even greater lateral strength . it does not necessarily fail. This enhances the strength ofthe panel by: 1. 2. the panel's strength based on this method may be just inadequate for the load it is being designed to carry. Failure will only occur when the lateral load reaches a level which will move the point C to the left ofthe lineAB. in the weak direction. The Code gives design gu idance on the strength of this type of wall. Modifying the a coefficient for 3 & 4-sided panels by changing the value of u. With lamer aspect ratios. below. 3 to 4 for square or nearly square aspect ratios. The strength enhancement is given by the formula in figure 46. It is felt that the relatively small scatter associated wit h the body of test evidence is dueto the stability natu reof the equation. 11 . The walls were removed by jacking and the failure loads ascertained. Instead. Allowing forprecompression In many cases. ~ However. In loadbearing wa lls. The strength so obtained is then mu ltiplied by an enhanc ement factor k. forces are generated withi n the plane of the wall. and they effectively induce restoring mome nts which stabilise the wall. The equation is for vertically spann ing walls . such the free body diagram of one half of the wall (figure 49). it is not able to extend as it deflects.. In this situation. conducted in laboratory machines under a constant precompression (figu re 51 overleaf). however. If the wall is built solidly between rigid abutm ents. Further wa ll tests. = ~ can be established by analysing HorizontaIarchi Ig ->- ~ The design method outli ned above has dealt wit h panels only. Ot particular interest is the case of the arching action of wind-loaded external walls in a loadbearing structure. In th is case.
DES'GNOlI'BEE IIDNDIlVGWALLS LCav ity walls -. . only the desig n of solid sing le-leaf wall s has been d iscussed . In the case of load beari ng cavity wa lls. when L is mu ch g reater tha n h the panel tends to span pu rely vertically . Wall pan el edge suppo rts when the panel is three tim es as long as it is h igh .. as before. since this leaf normally carries the sig nif icant precompression . this recogn ises what could reasonably be pred icted . including details. materials specification.. Cha racte rist ic ~ !!. Claddi ng pane ls are usually t ied back to their supporting structu re using meta l t ies. bot h leaves ca rry significa nt lateral load. by the Ymgd term . The normal app roach for brickwork f reestand ing wall s is based on flexu ral strength using the modified flexural stress (fo r weak di rect ion bend ing) men tioned earlier (page 11). and th is m ust be taken into account. Freestand ing walls The Code gives guidance on the des ign of freestand ing wa lls. In cladding panel desig n. it is also importan t to ensure tha t the panel is adequately tied back to the str uctu re. BDA 12 Wh ilst the method used to design laterally loaded wall s against flexu ral failure has been cove red. gd is based on the dead weight of the full heigh t of the wall.. it is usua lly only the inner leaf wh ich is analysed in arc hing . namely. strength design and the stability of footin gs is given in Design Guide 12". however. and it is useful to have values for the st rength of wall ties used as pane l suppo rts . flo< is increased. In the ca se of a freestand ing wall . standard sections.J 53 pecui" BBsl Xi eep. Test results" indica te that the lateral st rength of a cavity wa ll is equa l to the su m of the st reng th of both individual leaves. This is a fortuitous finding and permits an easy and understan dable design approa ch . [ ~ ~ s- :l. ale IlIo8d an 215 nm . Effectively. w here the flexura l failure is at the base. A fulle r treatment of all aspec ts of the design of freesfanding 3' o walls. This is more c ritical with claddi ng panels than with load bearing walls. the stiffening effect of the vertical edge support dim inishes to virt ually zero and k approaches 1. 52 2 ·0 3 ·0 • 1-6 1-5 1·1 1-0 4-03-0 1·5 1·2 Thus far. ecPi*1l1oed an 175nmWllll Factor k Number of retu rn s Value of k Lh 0 ·75 ' ·0 . ~ •.
~ 9. Yield line formula for slabs. Brick DevelopmentAssociat ion. H. Proc.OA Korff. H. Shaw. Curtin. Jones & RH.values for compression. 5. Sinha & A. ~ Thames & Hudson. 1980(available from BOA). K. Design of brick diaphragm walls. shear and tension are given in Table 8 of the Code. BA Haseltine & J. Although some research has been conducted (figure 56) on perforate walls. BA Haseltine. N. February 1984. Model specification for clay & calcium silicate structural brickwork. West. in Appendix D. Cement & '0 Con crete Assoc iat ion. 1981. Morton. H. 11 .W. the reader is referred to BDA Design Guides: 8 'Design of brick fin walls in tall sinqle-storey buildings' and 11 . 2. J. G. Johansen.H. Design Guide 8. 10. H. Proc.W. June 1980. Exlernal walls: Design for wind loads. H. 1967.W. Brick DevelopmentAssoc iation. (a) a form of plate analysis is carried out: or. Brick Development Association. ~ October 1977. Wood. 1979. 13. for more detailed gu idance 14 1 ~ The design methods covered so far have dealt with sinq le-leaf or cavity wa lls of solid construct ion with no door or wind ow openings. (b) a superimposition method is used(figure 57). Design Guide 11 . Curtin. W. Chatto & Windus. Design of brick fin walls in tall single-storey buildings. Brick Development Association. REFERENCES t.P. J. British Ceramic Society21. BS 3921: Clay bricks and blocks. West. The Structural Engineer. Vol 55.P. British Standards Institution. 1974 3. BS 6073: Precast concrete masonry units. 1974. BS 187: Specification for calcium sil icate (sandlime & flintlime) bricks. Indeed.H. The resistance of " brickwork to lateral loading. B. 1974. W. British Standards Institution.R Hodgkinson & BA Haseltine. Webb.R Hodgkinson & W. L. 1978 4.F. Tutt. No 10. J.K. 13 . %7. March 1982. Part 2: Design of walls to ~ resist lateral loads. Accidental damage.W. West&J.G. Design Guide No 4. this subject is under review by the Code Technical Drafting Committee. G. The introd uction of openings into pane ls redu ces their ultimate lateral strength. Hendry. Instead. Special Publication 56. London. 14. October 1977. the Code recommends that either. no final conclusions have been reached. Full scale tests on the lateral strength of brick cavity walls with precompression. British Standards Institution.W. No 10.N. Brick Development Association.K. The resistance of brick walls to lateral loading.H. Consequently . no firm recommendations have been incorporated in the Code. Walls with openings A full treatment of panelswith openings is outside the scopeof this publication. British Ceramic Society 21. Part 1: Experimental methods and results of tests on small specimens and full sized walls. H. The Structural Engineer. Vol 55. c :l' 8. May 1985. 12.FA Moore. The reader is referred to References 12& 13. When calculating design strengths. Design of free standing walls. Beck & WA Bray. 15. Brick Development Association. May 1981 . as outlined above. BA Haseltine & J. British Ceramic !2 ResearchAssociation. The lateral load design of diaphragm and fin wall structures is not covered in the Code. Shaw. While the theoretical basis of such designs follows normal sound engineering principles. robustness & stability. 'Design of brick diaphragm walls'. Handbook to BS 5628: Structural use of masonry: Part 1: Unreinforced masonry. J. Tutt. A. • ~ 6. Maurenbrecher. Ymis 3 for ties. Yield line analysis of slabs.G. 16. The resistance of brickwork to lateral loading. Design Guide No 12. Beck & WA Bray.L.
23 + 0.2kNlm For cavity wall : W.125 x 1.2 for a claddi ng panel whose failure does not affect the stability of the remaining structure . ie.tion of maximum moment.. is unhkelyto be as small as this in practice .5mm) plastered one side = 2. f:.W" L 0.1 Ym 0.3 x 2. flu 0.47 x 1.75 3. This should be remembered when panels he just outside justifiable j ~ ._ .0 kNlm ' Inner leaf (102. L' ~ ~ 14 [0.9 x 1.24 kNlm ' ~ = 0.75 x 10" 3. and the panel could not be justified in ma ny areas in Britain .2 x W.6' kNim -0. The edge conditions at the base and the top of the wall are assumed to provide simple supports.!' Forth is panel: 0. Workmansh ip and material s are assumed to be normal category and the hmiting dimen sions are not exceeded. O.23 kN1m' W.023N1mm' = 0.3 x 2. -' Md = 0.0kNlmrun 0. W k .9 x 1.2 +0.24 kN1m' ~ Forthecavltywall. Estimate the design wind pressure. M = 0.2 kNmlm run.47 kNlm' is approximately 20%.6m high is to be designed in cavity bnckworl<.4 x 1. 125 x 1. The bricks to be used in both leaves have a water absorption of !f-9'h% and the mortar is 1: 1:6.2x2. x 2.. Cons.75 x 10" mm 3/m run . W. low. values): Outer leaf (102. The two basic equat ions are: Designmoment = ay.2 x W. It spans vertically..6' 2 !l.= 1. not the length. Ie.6m span is taken as the height.0 kNim run 2. . The strength of a caVltywall is equal to the sum of the strengths of the two individual leaves.34 x 10' = 102. wh ich can be resisted by this panel.4kNlm' This is a low value. The failure crack andthereforethe cntical section. h.5 for norma l category construction control and manufactu ring control of un its. W. Keep Ym = 3.4N1mm' for water absorption of 7-12% in 1:1:6mortar.2=0. Equating M and M d gives: _ 0.3 x 2. x 2.4+(3. Inner leaf: Stress due to design vertical load 0. The wa ll has no returns.75 0.25 kNlm ' The flexural strength in the weak direction can be modrned to: flu + Ymgd.0. Example 2 With the above design example try to maximise W k by taking the se~·weight of the panel into account. W .der onlythe dead weight stress dueto the tophalf of the wall.6' .47N1mm' Basicequation ay. For one leaf: 102 5' Z HXXl x .5 Nmmlm run = 0.125 for a simply supported case.125x 1.02) Nlmm' = 0. isassumed to beat rmd -heiqht. = 0. Ym = 3. Basic equat ion aYI W.6' 30S026)J W.4 to 0. (assume conservative.2 .5 x H)3 mm = 0. Outer/ea t Vertical load Design vertical load Stress due to design vertical load Mod ified flexural stress 1.5 and use the samematerials.4 DESIGN EXAMPLES Example 1 A cladding panel 2. the pos.026Nlmm2 'i ~ a :.5 x 0. Characteristic dead load G.25 xl a' NI ' = 102. = 0.5 x x 1.5 x H)3 Nlmm .4 + (3.47kNlm' The increase from 0. L 2. use flu in the weak direction of bending .5 = O. 0. 125x 1. and because the vertical edges are unsu pported. 1. Wh L2 Design moment of resistance = a Y. where g" isthe stressdue to the design vertical load.6 .2XWkX2 .5mm) = 2. since vertical spanning.
using the same brick and mortar materials. mortar 1: %:3 Inner leaf: water absorption > 12%. W.50 for inner leaf). For ultima te desig n Y = VI = 1. x 2. However. Y and characteristic t valuesfor flexural stress. Whilethe meanof.0 Nlmm' (strongest brick/mortar case). = [0. =O . rf the inner leaf were constructed using a brick of water absorption of less than 7%. A design wind pressure of 1. by changing the material specification.93 15 .5 x 0.026)) x 1.: since It Will besitting on dpc material.5 kNlm' is conservative. ExampleS Compare the design wind pressure W k = 0.125 x 1. = 2. inner leaf 0.4 Inner leaf: .75 .1 x 3. height = 1. m Assuming no accurate valuesfor the mean flexural stress are available. take G.5 : W. values: outer leaf = 2. 0. ~ = 0.65 kNlm2 • Indeed. for rig design): - _ [1 + 1 x 1.029)) x 1. 1 O 125 x 1.6' 3. Assume f~an = 1. x 2.25kNlm' for outer leaf. with the design wind pressure for a wall.0 and the mean flexural stress valuesshould be used.4 + (3. the results will bescattered around this mean. . Since f".5 W. An estimate is needed.6 ~ 1. ie.0 Nlmm ' and the 0. mortar 1: Y.4 + 0.5 : W. x 2.2 x W. 1. x 2.54 kNim W k = 0.5 kNlm' for both leaves.conditions.75 0. from the previous example. The example could be recalculated if the standard deviation associated With wallette test flexural strengths were known.3 Calculate the maximum design wind pressure.4 Nlmm ' f. W" taking account of the self-weiqht (assume gd= 2.125 x 1.75 0. 65kNlm' Hence. Table9 (A).W. high.7 + (3. .6m long and purely horizontally spanning (ie.. these can be estimated from tigure 9. 10wall tests should beapproximateIy4.25kN1m' 0.6 = 1.5 ' W"" = 1.03 N1mm ' Far one leaf: ' 1. At present. For bricksof 7. 2.064 x 1. f". Outer leaf: 1..2 x W. f".7 Outer leaf: [0.25 = O. " h!L= 0.. = K f•• is a linear relationship. = 0.13kN1m' Forcavilywall: W~. x 2. WI< = 0. values: outer leaf O Nlmm' . The Increasein strength is solelydue to this. the design wind pressureWI< can be increasedfrom 0. This is adequate for many situations in Bntain. the mean flexural stresscan range betweenapproximately1.4 N/mm' . Thisvalue isderived using design values of Vm.99 +0. this is outside the scope of this publication.0 Nlm m'.53kN1m' This is signrticantly stronger than the 0. which is 2.47 to 0.3m . this is only appropriate for weakdirection bending.47 kNlm2 . 0. could be further increased to approximately 0.5 O .125x 1. innerleat = 1.8 kNlm' . Calculate this value.54 kN1m' l.75 . the maximum design value.75 .5. for rig design purposes. assume the baseofthe wall is a free edge). would increasealso by approximafely2O% becauseW. 0 x Wrnax X 26 = . (Note that the answer could have been achieved much quicker. Modifying this for selt-weiqht (for sim plicity.6' 2 x 3.3x2.3m.65 kNlm' .75 2 x 3. No loads are taken on the panel other than the wind loading ItS j surface.53 kNlm' is more than adequate for virtually all conditions in Britain. ~ 0.064.65 kNlm' derived in Example4 and demonstrates the advantage of arranging panel support conditions in a way which encourages panels to bend in their strong horizontal direction.26 kNlm' together with an adequate factor of safely.1 Nlmm' .26 kNlm' at lateralfailure. W.99kNlm Inner leaf: .2 x W. = 2.6m. Instead. The material specification is now: Outer leaf: water absorption < 7%.26kN1m' This isstillnot a truemaximum value sinceit isbased ona predictionof the maximum mean flexural stress.) Example 3 A panel identical to that in Example 1 is to be constructed and 1 ested in a laboratory.03x 1.125x 1. = 4. BS5628 ~ Outer leaf: .6 3. say.5 x 0. was increased by approximately 20% when modified.5 x Hi' ] 102. value. W~.5 x 103 = 1. the rig should be designed using 4. A panel identical to that in Examples 1 & 2 is to be designed using different bricksand mortar. Example4 . (Note: no dead weight was used to enhancef ".12% water absorption.2 x W. t Length = 2. Assume the base of the wall is simply supported. For cavilywall: W. The heightofthe panel is 1. O kNlm' . of the likelyultimate failure load.35 ~ a = 0.) For cavily wall: ~ Example 6 Repeat Example 5 for the practical case where the wall is supported at its base.2 x W. and 2.
1 x 3. = W.338kNmlm run = 0. x 10" = 0.): Y = 3. (ii) detail the top edgeolthepanel to give a simple support: or.62 = 1. Wk = O .5) 137mm Maximum area:} 15OOt.' 28m' AcIuala rea = 11.Inner leaf: 1. 7. in the strong direction.045for hIL = 0.388x 10"Nmmlm run. 1.5 } M M. or. Since option (iii) is outside the scope of this publication. option (ii)will beconsidered. o coe fl icient: T = oy. + W" but W. of 0.5mm bnckwork In both leaves. lithe panel cannot bejustitied.044for hIL = O.gives: f"" ~J. and f"" reqd. It is normal. WkL' per leaf = !J.. Option (i) Assume: Dead load of both leaves " . It is a corner panel. mortar designations (i).338 x 10".75 x 10" M. Only the 0.only a small reduction (approximately 10%) is neededin thef "" required. M = oy.5 ' W k.2 x W. Hence. ~ the wall is to be designed USing 102.WkL' = 0.0 = 2.9 Nlmm' is acceptable (ie. = 1. h "4 = O.. anyclaybricklmortarcombination can beused. byinterpolation.6 Nlmm'.35.2 xO. the architect would like to use a calcium silicate brick in a 1:2:9 mortar(designabon (iv». see below.0 kNlm' (lowvalueof Gkfor conservative reason) 16 Vertical load = 228 x 2.75 0.99kNlm' Example 7 A cladding panel IS subjectto a designwind pressure.85m AcIuall argestdimension 4m columns wall sits on dpc : 4m / 2·8m Since neither the maximum area nor the maximum dimension is exceeded. (ii) & (iii» . The contmuity of the outer leaf over the column and around the corner is asumed to provide fiXity to the vertical edges. Equating o = 0. = 0. Thisisadequate for anywherein Bntain.Can this bejuslifted? Theways in which the panel can bestrengthened and possiblyjustified are: (i) modifythe flexural strength using the panel'sself·weight. m Y = 1. ExampleS Because the panel just designed in Ex ample 7 is in a sheltered position. 2.044 x 1.2. are there any urmtations on the specification of the bncksor the mortar? FigureA The assumededgecondmons areshown below. Table9 (C) therefore. the panel satisfies the limiting dimension requ irements. are 0.8Nlmm' .36. for a mortar no stronger than designation (iii) to be used.68Nlmm' (strong direction). 4 x 4' kNmlm run = 0.8 = 0.9 and 0. 7: ~ = 0. = f"" 35 Nmmlm run. x 2.. o = 0.4kNlm'(W"" = W.3): '" = 213(102.75 x 10" mm3/m. " i< ii ~ ~ = 2.039forhIL=0.06 2. when using calcium silicate brickwork.2 m' Maximum dimension :} 50'" 6.8 kNlm' (suction).064 x 1. (iii) usehorizontal bedjoint reinforcement in oneor both leaves. Sincethis exceeds the requiredvalue. option (i)will beexplored. FigureB Limiting dimenslons(BS 5628. specification would probably be'anycalcium silicatebrickusinga 1:1:6 mortar(or equivalent)'. Clay bricks Theweakest f""value in Table3 is 0.75 BS5628.d .5 + 102. Basic equation for one leaf: Y m .5: Z = 1. Calcium silicate bricks The two values of f"" given in Table3. Wk.8 kNlm . freeat the top edge but sitting on dpc material. CI.
(this is done in the weak direct ion): I (strong) I (weak) 0.38k N/m . For cavity wall : W.2 + 3.reqd. W.7.. and repeating the earlier section for one leaf: F' C 1.035. by interpolation . h!L = 0..041.. = 0.~~§§§§§§~~ =-+-+---I-dPC tray (not shown) .79 W..8 kNl m' .238 kNmlm run ...041 x 1. = O....48 N/mm' . 2·8m Example 9 A sim ilar panel to Exam ple 7 is to be designed l or two different parts of the structu re.. O ption (ii) If calciu m silicate brickwork were to be used in conjunction with a 1:2. columns ~~_ _ overhead beam providing -.6 N1m m' .9 (designation (iv) mo rtar.76 kN/m' .Designverticalload Stress due to design vertical load = O.5 kN/m 2 5 x 103 102.9 or equ ivalent designatio n (iv) mortar. Checking the panel. this is done using either special (sliding)lixings in conju nct ion with special wall ties. This is less than W. So.6 Nlmm' in Table 3.288N1mm' For a single leaf: M = ay.2. This is less than the f.3 . If this we re done With suitable details the panel would be four-sided...031 x 1.68 kNlm' ..-g ro und floor panel (1st floor panel not shown) dpc position 1 overhead beam providing support to the top of the panel (not shown) special column in entrance hall offset from grid position & not supporting edge of wall ground floor panel (1st floor panel not shown) _-4=+ .g x 2.3 kNmlm run Equating M and M d 0. = 0.. h!L = 0. The W.. value 01 0. u = 0.75 x 106 M d = Ym = 3. W. L' . = 0.o-3m . it woul d be advisable to suppo rt the top 01the wall in some way .5 x 0.5 x 106 0.70.2 x W. but not taking accoun t of sell -weight..031.. kNmlm run !od 0.... from Table 9 (C).043.75 a 0.-- support to top of panel (not shown) -!.5 Nlm m' block and a clay brick (both in 1:1:6 mortar)f or th e inner and outer leaves respectively.4 x 4' = 0. = ay..025 Nlmm' Assume a calcium silicate brick using 1:2:9 mortar (designat ion (iv»).. comment on anyspecification req uirements for the clay bricks. value for both positions (general wi nd pressure) is O. Normally. It is proposed to use a l 00mm 3.38 = 0.. tqure M interpo lation lrom Table 9 (G) gives a = 0.79=0.. 4m Equ ating M and M d gives f. a coefficient: h!L = O ~ = 0.5 x 103 0.35. or by using sections of ang le or channel to restrict the lateral movement 01the top edge. -..glazing unit ..5: a 0. X4' O..6 x 1.025 = 0. M 0. h!L = 0. 11 the two arrangements for the two positions areas shown below. ....8 = 2. Modifying I.... = 2 x 0. ~ M = 0. the panel wo rks satisfactoril y as a tour-sided panel in calcium silicate brickwork in a 1.75 xl 06N M d = f kx 3. W.2 x 0. L' a coeff icient..5 mmlm run.5 a 0.__ doc position 2 17 .
35 kNlm 2 required to be carried: a = 0. the assumededgeconditions shown above applyto both leaves.but provide support ties Side 3 simple su pport.67 x 100mm'/ mrun.5 W k = 0.75.45 X 3 51.5 x 10' W k = 0.034.2 x W k x 42 = 0.20 + 0. Equatin9M andMd.60r equivalent designation (iii) mortar. 02Ml.72 kN/m 2.45 x 1.. rt it is planned to use bricks with a wa t. Side 4 fu lly fixed.041 x 1. 1. front leat continues past column .9 x 1.5 Nlmm 2(to give W k = 0.0. m flu weak = 0.68 .5 2 I 2 Wk 0. and a different specification will be required .57kN1m 2.33 kNlm 2.68 kNlm 2.75 0.68 Outer leaf: Assume the weakest case in 1: 1:6mortar and check the W kfound against theO. W k L2 - ~ Y m 0.20 + 0.075 x 1. todetail a cavity wall with differentmortars ineach leaf. Z = 1000 x 6 = 1. Since the caVitywall must ca rry O kNlm 2. .034 x 1. " ® Inner leaf. This is not adequate.55. hIL = 0. hIL = 314 = 0. flustrong = 0.5 x Hj6 W k = 0. Outer leaf: 2 0. There are no specification requirements forthe clay bricks of the panel in Posrtion 1. Table9 (F» flu = 0. Thus. ay .ay. • • 18 (ii) Increasing the thickness olthe inner leaf. EquatingM andM d.er absorption higher than 7%. recalcu late using an llu of 1.20 kNlm 2. design w ind resistance is now 0. From Table 9 (F): a =0.5 1 kNlm 2.35.35 kNlm 2.9 Nlmm 2. on the basis thatWkvaries Iineartywith lIu. in position 2. This would increase Z.75 x 10" 3. This .2 x Wk x 4 Position 2 Assumed edge support conditions.67 x 10" 3. Either.52 = 0. however. Cavity wall: ii ~ g. dpc tray above Side 2 simple support.31 kNlm 2. could be achieved by: i! (i) Increasing the block strength specitied .041(~ = 0.9xl . . the clay bricks need to be 01less than 7% water absorption in a 1. ~ :< (Note: It would be Q uite possible to investigate the strength of the outer leal using a brick of 1 water absorption or greater in a 2% stronger mortar. evenIf only one leaf is continuous.) . This compares lavourably w rth the O.33 = 0. WkL2 2 0. (ii) the discontinuous leaf is not tbicke: than the one that is contmuous. tnnerteet: 100mm 3. panel stops .2 permits both leaves to be assumedcontinuous.Position 1 Assumed edge support conditions: Side 1simple su pport. Therefore. ~ = O.31 x 1. (Design load = 0. Hence. iii i .68kN1m 2.31 = 0. Design resistance = 0.) 2 0. ® Note: Clause 36.45 Nlmm 2. the strength althe inner leaf will require to be enhanoed . This exceeds the desig n wi nd pressu re of 0. Same as above (Positio n 1) exceplthat edge 2 is free.9 kNlm 2. Repeating the above calculations (in large steps) but basing the a coefficients on Table 9 (K).67 W k = 0. Any clay brick in 1:1:6 mortar will be sufficient.52 kNlm"l o r. Design wind resistance = 0. 0. the outer leaf must carry O. it would requirecareful supervision in prsctice.045 = 0.025 Nlmm 2. ' ay.52kN1m when Iu = 1.2 x Wk x 4 = 3.75.5 N1 m 2 block in 1:1:6 mortar.2 x Wk x 4 - 100> ~ Y m 0.5N1mm .35 kNlm 2the outer leal will be required to carry il the inner leaf was l ully wo r1<ed. If (r)cavity wall ties are used in accordance with Table6.9 O.057 x 1. It isunusual. sits on dpc .WkL2 - -~ Y m 0.
14 kNlm' .1 x 1.Clau se 22 (d». 1 Nlmm' . This is adequate far W.(iii) Reinf orcing the O lockwork. This solution should. be explo red firs t.75 0. Example 10 2·5m A loadbearing caVity wall is to be checked for lateral strength. It is well su pported on its twovertical edges . = 1.35).1 = 0. Both leaves carry the roof equally.75 3.03 N1mm' Stress due to desig n vertica l load 102. (weak) = 0.018 x 1.4 + 3. . ~ = 0.5811.4 W.05 = 0.52. = 1.(strong) = 1. providing edge support to the assu med free edge .1 Ym . I t ".cannot be achieved. Cavity w all design strength resistance = 1. 3.018.(weak) = 0. hIL = 0.J 1 . arching) forces. .2 kNl m of w all .57 kNlm . Example 11 (To be read in conjunction with BOA publ icat ion 'Accidenta l Damage. W..9 G.45 ' 1..35). no allowa nce for vertical load): hIL = 0.5 W.5 x 106 i' 0. ay. 1. (assume u = 0. = 0.022 x 1. For 19 .9 x 1.. for thi s exposed site is 1. f. of course.58 Nlmm' .6 h.6 = 3. ~ 1+1 02·5mm The basic equation q lal = ~ is used for walls with sufficient precompression to develop in-plane (ie. a = 0.2 kNlm ' suction. design load.022 (Table 9(G): ~ = 0.2 kNlm'.. the mi nimum design vertica l load is 6. Rrst check whether it wi ll wo rk ascladding (ie. Roof /cads: Looking for conserva tive (unfavourable) conditions. Wk L' 0.4 xW. but is assumed to be sim ply su pported top and bottom.5 .4 x W. The above option s assume that the simplest solution .nam ely. W. The design load on the wall is 140 kNl m run (derived from using parti al load factors app ropriate for acci dental da ma ge ana lysis . ~ = 0.40 kNlm ' .. = 0. 3mortar. ie.70 kNlm' Cavity wall design strength resistance = 1.!l ~ 215mm .1x 1. and using dead + wind (no imposed) based on 0.5 x H i' Total stress for design loads f..5 x 0.1 kNlm on each leaf. W. Reinforcing the outer or inner leaf (or bot h) may be the only solution if a brick of mo re than 12% water abso rption is to be used .52.0 x H O. Both leaves have bricks of more than 12% water absorprtio n in 1:V. x5. 3. and 1.25 x 2. " s . Rabustness & Stab ility"") A 215mm bric kwork wall is to be checked to asce rtain whe ther it can Withstand the 'acc idental force' equivalent load of 34 kNlm ' (the equ ivalent gas explosion lateral pressure).45.4 Nlmm': f. Accounting fordesign vertical stress: a = 0.1x 1o" 0. x 5.02 N1 St ress due to design self·weight of w all mm' 102.2 kNlm '.
) Assuming that Itwas decidednotto usea 215mm.. ij thewall wasconstructedusing the 170mmCalculon brick..... (ii) to befully horizontallyandverticallytied..05x 2. inner loadbearing leaf. ax 0.215 x 140 ql81 = 1.HOx 140 q lat = 1.. the wall is judged to remain after an 'accidental event'and is regarded as a protected member (Clause37.1) . "there were oneor two returns on the verticaledge(s). See Clause37..1. one brick thick...52 = 2Q kNlm 2 (ct..05 can be used (Clause 37. (i) to bechecked to makesurea partialcollapseof unacceptable proportionsdid notensue.. axO.. the values of 36. ~ ~ I . or. Fora fuller explanabon of the subject of accidental analysis. Robustness <I.1and Table12. but using t = 170mm.05 x 2. the buildIngwould need : either. t !t ~ ~ .1)..7 kNlm 2• Sincethis is in excess of 34 kNlm 2... 34kNlm2j. Stability'16..- I .52 = 36. (Notethat.. see'AccidentalDamage..7 and29 wwrr? above.-.. Repeating thedesign.. 1. should be enhanced by the appropriate/< factor from Table 10.-.accidental damage analysisusing this equation a partialfactor of safetyfor materials represented by Ym = 1.. it would bejudgedto berell10lled by an accidentalevent...
WInkfield. . its servants or agents andall contributors to this publication shallbe under no liabilty whatsoever in . INoodside House . correct and complete. ~ f C1> Designed and produced for the 8IidI 0eveI0pment Assocatoo. Printed byRoikeepUmited.. opinion or information contained is published only on the footing that the Brick Devefopment Assodation. ~ Tel: Winkfield Row (0344)885651 by TGV Publications.i ) '0" Readers are expressly advised that whilst the contentsof this publication are believed to be accurate.respect of its contents. WIndsor.4 2OX. Berkshire 51. no reliance shouldbe placed upon its contents e as being applicable to any particular drcumstances. i ~ . Any advice.
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