No. 60 December 2020

STEEL CONSTRUCTIONTODAY & TOMORROW

https://www.jisf.or.jp/en/activity/sctt/index.html

Feature Article: Fire Resistant Design of Steel Structures

Membrane Actions ofReinforced Concrete Floor
Slabs Exposed to Fire
1 Part 1:

Application to Fire Resistant Design of Steel Structures

3 Part 2:

Load-bearing Fire Tests for Composite Flooring Systems

9 Part 3:

Numerical Analysis of Load-bearing Fire Tests for Composite
Flooring Systems

13 Part 4:

Fire Resistant Design of Steel Structures in Japan and Future
Development

Serial Article: Latest Design of Steel Buildings in Japan (5)Miki Park Gymnasium
15 Sharply-designed Steel Truss
Framing Lightly Floats Up
into a Large Arena Space

Published Jointly by

The Japan Iron and Steel Federation
Japanese Society of Steel Construction

Feature Article: Fire Resistant Design of Steel Structures (1)Membrane Actions of Reinforced
Concrete Floor Slabs Exposed to Fire

by Takeo Hirashima, Professor, Chiba University

Takeo Hirashima: After completing the courseof graduate school, Chiba University, he entered
Fujita Corporation as a researcher in 1996. He
became Research Associate, Faculty of Engineering of Chiba University in 2001 and Associate Professor in 2006. He assumed his current
position as Professor, Department of Architecture, Graduate School of Engineering, Chiba
University in 2015.

Part 1: Application to Fire Resistant Design of Steel Structures
Fire Resistance Required in
Building Construction
Buildings are required to possess fire resistance that conforms to the application
purpose and scale of the building. Specifically, in an event of fire, the wall andfloor slab applied as compartment elements are required to possess thermal
insulation and fire integrity, and further
the column, beam, floor slab and bearing wall are required to possess appropriate load-bearing capacity. Accordingly, it
is general practice in steel-structure construction to provide fire protection withcolumns and beams and to ensure fire resistance. In Japan, fire protection is provided with the secondary beam that supports the floor slab.
In fire resistance tests for deck composite slabs designed as 1-way slabs, because of the precondition that the steel
beam to support the slab is to be appliedin a sound condition, a secondary beam
is provided with fire protection. Meanwhile, in the floor slabs that support theload as 2-way slabs, as the deflection increases, the membrane stress increases, and as a result, the ultimate strength
greatly surpasses the bending strength
due to this membrane action. It is reported in past experimental studies1)-4) thateven when the floor slab was exposed tofire for 75~180 minutes, it continued tosupport the load.
Accordingly, in cases when the load
bearing capacity of composite floor slabs
during fire is assessed taking into account the membrane action, it is feasible
to eliminate the application of fire protection for secondary beams. If an unprotected secondary beam (beam with no fire
protection covering) can be applied insteel-structure construction, great effects
will be brought about such as reduced
construction cost, enhanced freedom indesign and reduced waste disposal at the
1 Steel Construction Today & Tomorrow December 2020stage of dismantling buildings.
In steel-structure construction inearthquake-prone Japan, moment frame
structures are being increasingly appliedin which the column is rigidly joinedwith the primary beam. Accordingly, inmaking designs that take into accountthe membrane action of the floor slab,
it is required that the peripheral beams

to support the floor slab, including thejoint, are applied in a sound condition.
In steel structures employing moment
frames in Japan, because the rigid joint
is applied for primary beam-end connection, it is considered that designs caneasily be made that take into account themembrane action of the floor slab.
Fig. 1 shows an assumed framing plan

Fig. 1 An Example of Framing Plans in the Case in Which Secondary
Beams (b1) Are Not Covered with Fire Protection

unit: mm

for a standard floor in an example of thedesign of a steel-structure office building5). If the 11×6 m floor slab can support the load during fire by utilizing themembrane action, the secondary beam(fine dual lines; symbol: b1) shown in theframing plan is likely to be made unprotected. In the figure, the primary beam
located in the painted thick line serves as
the vertical-direction supporting member for the floor slab, and the floor slab
surrounded by these supporting members and the unprotected secondary beamwork as one composite flooring system.
Targeting such composite flooring
systems, load-bearing fire test and numerical analysis were conducted to examine the membrane action of reinforced
concrete floor slabs during fire. In the
succeeding Part 2 and Part 3 of the current issue, part of the results of the examinations of membrane actions obtainedin the above-mentioned test and analysis
are introduced.
In Part 4, the fire resistant design ofsteel structures in Japan and its future development are introduced as a final topic
of this serial article.
Purpose of the Study of Fire Resistance of RC Flooring Systems
In spaces where the fire range is restricted and the steel product temperature becomes low (for example, open-type mobile parking lots, atrium spaces, spaceinside elevator shafts), there are cases
in which unprotected steel members canbe applied by conducting the fire resistant design of the steel structure. Further,even in spaces where fully-developed
fires may occur, if the use of fire protection for the secondary beams of steel
structures can be eliminated, more rational design becomes possible, and as a result the predominance of steel structures
over other structural systems will grow.
To attain this goal, it is necessary to
conduct fire resistant design for steel
structures that takes into account themembrane action of floor slabs duringfire, and at the same time, it is also necessary to accumulate basic data relating
to such fire resistant design.
With regard to past experimental studies of the membrane action of RC floorslabs in an event of fire, the compartment
fire test conducted at a full-scale steelframe building of the Cardington Laboratory in the UK in the 1990s is wellknown1). In the test, a composite flooring system employing unprotected secondary beams and composite slabs was
exposed to a fully-developed fire in an
office space, but the system continued
to support the load while being accompanied with large deformation. Further,
it was explained by means of numericalanalysis that this phenomenon is causedby the tensile membrane action of thefloor slab6), based on which fire resistantdesign method for the composite flooring system was proposed that takes into
account this tensile membrane action7).
Since then, many researchers have tackled this appealing research theme.
Meanwhile, it cannot be said thatthorough experimental studies on the fire
resistance of full-scale flooring systems
have ever been made, as shown in Table 1. This was due primarily to both theconstraints on relevant experimental facilities and the cost.
In addition, any experimental study
on the membrane action of floor slabs
during fire had not been conducted until
recently in Japan, and currently it is impossible to perform fire resistant designthat takes into account the membrane action during fire.
Given such a situation, in order toclarify behavior of composite flooring
systems composed of flat RC slabs anddeck composite slabs in an event of afire, load-bearing fire test and numericalanalysis were conducted for the composite flooring system. Because the test andanalysis results particularly pertaining to
the composite flooring system employing flat concrete floor slab and unprotected steel beam (beam with no fire protection covering) are less available in pastexperimental studies, the test and analysis were made for this type of composite flooring system in this experimental
study, the results of which are introduced

Table 1 Outline of Previous Experimental Studies and This Study on Composite Flooring Systems
Ref.

Floor slab

Specimen size(mm)

Unprotected
beam

Total load
(kN/m2)

Heating duration(ISO 834)

Result of the maximum
vertical displacement

1)

Compositeslab

6000×9000
Depth 200 (130+70)356x171–

Under real firecondition

428 mm (Test 3)

Flat RC slab

3300×4300
Depth 100

2)

3)

4)

This
study

Compositeslab

3300×4300
Depth 130 (75+55)

Compositeslab

6660×8735
Depth 155 (97+58)

Compositeslab

Flat RC slab
Compositeslab

3720×5232
Depth 146 (70+76)

4200×5800
Depth 1204200×5800
Depth 130 (80+50)

210 mm
155 mm

5.31
5.51
None

5.32

180 min

5.43
300x150(Two beams)
250x118None
350x175–

630 – 750°C

271 mm
253 mm

500 – 700°C

3.87

120 min

About 450 mm

280 – 350°C

18.38

75 min

175 mmAve. 17°C

17.71

90 min140 mmAve. 240°C

8.75

100 min

135 mmAve. 370°C

100 min

150 mmAve. 360°C

132 min

190 mm

500 – 550°C

9.47

202 min

269 mm670 – 710°C

217 min

280 mm740 – 780°C

8.04

210 min

350 mm

500 – 610°C

10.72

156 min

370 mm420 – 550°C

11.23

None
350x175

640 mm (Test 6)

Reinforcement
temperature

December 2020 Steel Construction Today & Tomorrow 2

Feature Article: Fire Resistant Design of Steel Structures (2)in Part 2 and Part 3. The load-bearing fire
test and numerical analysis were subsidized by the Japan Iron and Steel Federation.
References
1) British Steel plc, Swinden Technology
Centre, A European Joint Research Programme, The Behaviour of Multi-Storey
Steel Framed Buildings in Fire, 1999
2) Linus C. S. Lim, Membrane Action inFire Exposed Concrete Floor Systems,
Ph D thesis, University of Canterbury,
20033) Bin Zhao, Mohsen Roosefid, Olivier
Vassart, Full Scale Test of a Steel andConcrete Composite Floor Exposed to
ISO Fire, Proceedings of the Fifth International Conference on Structures inFire, pp.539-550, 2008
4) Na-Si Zhang, Guo-Qiang Li, Guo-Biao
Lou, Shou-Chao Jiang, Kun-Chao Hao,
Experimental Study on Full Scale Composite Floor Slabs under Fire Condition,
Proceedings of Application of Structural Fire Engineering, Prague, Czech Republic, pp.502-511, 20095) Architectural Institute of Japan, Rec-ommendation for Fire Resistant Designof Steel Structures, 2nd edition, 2008
(in Japanese)
6) Z. Huang, I. W. Burgess, R. J. Plank:
Three-dimensional modelling of two fullscale fire tests on a composite building,Proc. Instn Civ. Engrs Structs & Bldgs,
134, pp.243-255, 1999
7) C. G. Bailey, D. B. Moore: The structural
behaviour of steel frames with composite floorslabs subject to fire: Part 1: Theory, The Structural Engineer 78, No.11,
pp.19-27, 2000

Part 2: Load-bearing Fire Tests for Composite Flooring Systems
Aim of Load-bearing Fire Tests
In Part 2, the results of load-bearing fire
test are introduced that targets a composite flooring system composed of flat RC
slabs and unprotected beams.
When the unprotected steel beam applied as a secondary beam is exposed tofire, it loses strength at an early stage offire. On the other hand, for flat RC slabs
supported in 2 ways, when deflection increases, membrane stress occurs within
the cross section of the slab, and as a result the load bearing capacity of the slab
is improved due to this membrane action.
In RC slabs subjected to heating from thelower side during fire, because of large
deflection caused by reduction in the rigidity of the material and thermal deflection due to the temperature difference between the upper and lower sides of slab,
the membrane action becomes morestriking at that stage of fire than at theroom temperature stage, and as a result
the slab can withstand fire heating for along time.
The ultimate load of an RC slab during fire surpasses by a great margin thebending collapse load based on yield-line
theory, and the ratio of the ultimate loadto the bending collapse load depends
on the amount of deflection1). Further, acharacteristic feature of the membraneaction of RC slab lies in that compression stress occurs to the circumferential
direction of the peripheral section of the
RC slab and this compression stress bal-ances the tensile stress widely occurringat the center section of slab, and as a result it is unnecessary to restrict the horizontal-direction displacement at the four
peripheral sections of the slab2).
In RC slabs in which two-tier reinforcements are arranged, when subjected to fire heating, the temperature at theupper-side reinforcement becomes lower
than that at the lower-side reinforcement,
and thus the tensile strength at the upperside reinforcement is kept to a high level
for a long time. As a result, it is forecast
that the retention of strength by the slab
due to the membrane action during fire
will become more conspicuous.
In order to understand these behaviors, a load-bearing fire test was carriedout by means of standard fire heating inaccordance with ISO 834, targeting acomposite flooring system composed of
two-tier reinforcement-arranged flat RC
slabs and unprotected steel beams.
Outline of Testing• Test Conditions

Table 1 shows the conditions of the loadbearing fire test. The first aim was to understand the effect of the headed stud arranged at the periphery of the slab on thedeflection behavior of the RC slab during fire by comparing specimens No. 1
and No. 2. The second aim was to understand the effect of applying unprotected
secondary beams on the fire resistance ofthe composite flooring system by com-paring specimens No. 2 and No. 3.
In the test, a load of 11.23 kN/m2, including the dead load of the floor slab,
was given as the target value. The live
load was the collapse load based onyield-line theory in the case when the RC
slab was simply supported at the four peripheral sections of the slab. The collapse
load was calculated using the following
equation (1) paying no attention to upper-side reinforcement.
Wp =24 0.9 d Asl2

un
1 + 3n 2 1

No. 1
No. 2
No. 3

Headed stud at the endof the slab
Diameter:15.4 mm
Distance: 150 mm
None

3 Steel Construction Today & Tomorrow December 2020

Unprotected
secondary beam

Load including deadload of the slab

Heating condition

H-300×150×6.5×9(SN400)11.23 kN/m2

Standard fire curvein accordance with
ISO 834-1

None

(1)

where,
Wp: Collapse load based on yield-line
theory (=11.23 kN/m2)
l: Short-side span (=4.2 m)
n: Aspect ratio (=1.38)
d: Effective depth (=0.09 m)
As: Cross section of lower-side reinforcement per unit width (=366.6 m2/m)
σu: Yield stress of lower-side reinforcement shown in mill sheet (=0.380
kN/m2)The live load was set at 8.35 kN/m2,
assuming that a unit weight of concrete
applied to the floor slab is 24 kN/m3. The
test specimen was heated based on thestandard time/temperature curve in accordance with ISO 834-1.

Table 1 Parameters in Load-bearing Fire Tests
Specimen2• Test Specimens

Fig. 1 Outline of Specimen (Example: Specimen No. 1)
Overhang beam

RC slab: 120 mm in thickness (20 mm in covering thickness)

Temperature measurement position
at general section
Secondary beam H-300×150×6.5×9
Vertical displacement measurement
position in the center

Temperature measurement
positon in the center

Headed stud φ16@150 single

Primary beam H-350×175×7×11

5,800 mm (long-side span)

Photo 1 Specimen No. 1 before placement of concrete

Photo 2 Specimen No. 1 to which weights are arranged

4,200 mm (short-side span)

Reinforcement 2-D10@200 (SD295A)

Fig. 1 shows an outline of specimen No.
1. Photo 1 shows the specimen before
the placement of concrete. The span of
the flooring system specimen was set at
5.8×4.2 m in a distance between primary beam cores. The thickness of the flat
RC slab was set at 120 mm, and a 1 mmthick flat deck was applied as a concrete
permanent form.
Regarding specimen No. 1, the headed stud was arranged in a primary beamthat supported the four peripheral sections of the RC slab so as to restrict rotational- and horizontal-direction displacement at the slab edge under conditions
roughly similar to those in a practicalsteel structure. The unprotected secondary beam was structured to a complete
composite beam together with the RC
slab. The cross section of the secondary
beam was H-300×150×6.5×9, and thesteel grade adopted was SN400B (JIS G
3136). The lower-limit yield point of the
SN400B was 235 N/mm2, and the lower-limit tensile strength was 400 N/mm2.
Three high-strength bolts (diameter: 20mm) were arranged at the end of the secondary beam web to form a pin joint.
The cross section of the primary beam
applied for the peripheral section of theslab was H-350×175×7×11. The orthogonal primary beams were joined bymeans of full-penetration flange welding.
Regarding specimen No. 2, the same
specifications as those of specimen No. 1
were adopted except for the headed studarranged in the primary beam that supported the four peripheral sections of thefloor slab.
Regarding specimen No. 3, the same
specifications as those of specimen No.
2 were adopted except for the secondary beam arranged in the center sectionof the floor slab. That is, specimen No. 3
had a structure in which the RC slab was
only arranged on the primary beam.
Deformed steel bars (nominal diameter: 10 mm) were applied for RC slab reinforcement and arranged at 200 mm intervals between reinforcing bars for boththe upper- and lower-side reinforcements
and with a covering thickness of 20 mm.
According to the mill sheet for the deformed steel bar applied, its yield stresswas 380 N/mm2. The water to cement
ratio of the concrete was set at 51.9%,
and polypropylene fiber was mixed withthe concrete to prevent explosions in thecase of fire. The strength of the concrete
after 28-days aging was 36.1 N/mm2. A
December 2020 Steel Construction Today & Tomorrow 4

The test was conducted using a heating
furnace for horizontal elements operated
by the Building Research Institute. The
heating area at the lower surface of the
RC slab was set at 5.6×4.0 m. A total of
24 weights (6×4 weights) were placed
at equal intervals on the upper surface of
the RC slab, and the slab was then heated from its lower surface. The weight perweight was set at 8.62 kN. Photo 2 shows
the specimen on which the weight wasplaced.
The vertical displacement of the slab
was measured at 18 positions at the center section of the short and long sides
and on the diagonal line. The temperature of the unprotected secondary beam
was measured at the center section andat the end of the bolt. The temperature
within the cross section of the RC slab
was measured at the center section andat a position at a distance 1/4 of the diagonal line, as shown in Fig. 1. The temperature of reinforcement within the RC
slab was measured at positions where thelong- and short-side reinforcement bars
crossed or contacted each other.

• High-temperature Mechanical

Properties of Steel Products
AppliedThe high-temperature tension test basedon JIS G 0567 was conducted for reinforcing bars and secondary beams used
for the specimens in the current loadbearing fire test. Fig. 2 (a)-(b) show the
relationship between the 0.2% proof
stress, stress at 1% strain and tensile
strength and the temperature, which was
obtained from the test results. The effective yield stress shown in the figure is thevalue for the stress at 1% strain.
As shown in Fig. 2 (a), the strength of
the reinforcing steel bars was similar tothat at room temperature before the temperature reached 400°C, but the strength
rapidly lowered when the temperaturesurpassed 400°C, and when the temperature reached 700°C, the strength lowered
to a level of 1/4 to 1/5 that at room temperature. The trend of strength lowering
was nearly close to the high-temperature

5 Steel Construction Today & Tomorrow December 2020(a) Reinforcing Steel Bars
800

Tensile strength

700

Effective yield stress
0.2% proof stress

600
Strength (N/mm2)• Test Methods

Fig. 2 High-temperature Strength of Steel Products Used for Specimens

500
400
300
200
100
0

0

100

200

300

400

500

600

700

800

Temperature (°C)(b) Steel Product Used for Secondary Beam
600

Tensile strength
Effective yield stress

500

Strength (N/mm2)

ceramic-wool blanket with a thicknessof 100 mm for use as fire protection wasprovided to the heating side of the peripheral primary beams, and the samefire-protection blanket with a thicknessof 25 mm was provided to the highstrength bolt joints at the end of the secondary beam.

0.2% proof stress

400
300
200
100
0

0

200

400

600

800

1000

Temperature (°C)

strength retention rate of hot-rolled reinforcing bars in Eurocode 23).
As shown in Fig. 2 (b), the strengthof the steel member used as the unprotected secondary beam lowered to a levelof nearly 1/10 that at room temperaturewhen the temperature surpassed 800°C.
It is forecast from these experimental test
results that unprotected secondary beam
subjected to standard fire heating would
lose its strength in an earlier stage of afire.
Test Results• Temperature Measurement
Results
Fig. 3 shows the temperature measurement results for specimen No. 2 as an ex-

ample of the temperature measurements
results for the composite RC flooring system. In the test of specimen No. 2, heating stopped at the stage when the temperature of the lower-side reinforcement in
the slab reached 700°C. The heating duration was 202 minutes. The temperatureinside the heating furnace was controlled
following the standard fire curve in accordance with ISO 834-1.
The temperature of the lower flange ofthe unprotected beam surpassed 700°C
after 30 minutes of heating, and 1,000°C
after 120 minutes of heating. The temperature of the web reached a level similar to that of the upper flange. The temperature of the upper flange came closer
to that of the lower flange as the heat-• Deflection at the Center Sectionof RC Slabs
Fig. 4 shows changes in deflection occurring in the center section of the RC
slab.
The heating duration was 132 minutes for specimen No. 1, 202 minutes
for specimen No. 2 and 217 minutes for
specimen No. 3. Because specimen No.
1 was first subjected to the test, heating was stopped when the temperatureof the lower-side reinforcement reached
500°C. When the damage condition of
specimen No. 1 was confirmed, it was
found that the RC slab concrete caused
no explosion and that the damage to thehigh-strength bolt joint at the secondary
beam end was lessened. Consequently,
heating was sustained for specimen No.
2 until the temperature of lower-side reinforcement reached 700°C, and in thetest for specimen No. 3 heating was sustained until the heating duration reachedFig. 3 Temperature Measurement Results (Specimen No. 2)
1200
1000
900
Temperature (°C)

Furnace

Lower flangeof the beam

1100

Upper flangeof the beam
Bottomof the slab

800
700
600

Reinforcement(lower side)

500
400

Reinforcement (upper side)

300
200

Top of the slab

100
0

0

60

120

180
Time (min)210 minutes.
In the case of specimens No. 1 andNo. 2, large deflection occurred for 30
minutes from the start of heating. Thislarge deflection was greatly affected by
the thermal deflection that occurred dueto the difference of temperatures between the upper and lower flanges of theunprotected secondary beams. In specimen No. 3, deflection rapidly increased
for about 2 minutes after the start of heating to reach about 50 mm. The reason forthis rapid increase was considered to be
attributable to the cracking that occurredin the lower-side concrete due to the thermal expansion of the flat deck plate andthe movement of the neutral axis in the
cross section of the RC slab to the uppersurface side of the slab. Meanwhile, in

240

300

360

specimen No. 3, which did not use a secondary beam, there was deflection for 30
minutes after the start of heating, which
was smaller than that of specimens No.
1 and No. 2.
After 40 minutes from the start ofheating, deflection increased in every
specimen nearly in proportion to theincrease in heating duration, and they
showed nearly identical deflections. In
spite of whether or not the headed stud
was arranged at the peripheral area ofthe slab, both specimens No. 1 and No.
2 showed nearly identical deflection behaviors, and therefore it was found thatthe effect of the restraint of rotationaland horizontal displacement at the slab
end on the deflection behavior of composite flooring systems during fire wasFig. 4 Deflection in the Center of Beam
350

Specimen
No.3

300
Central deflection (mm)ing duration increased, and it surpassed
950°C after 120 minutes of heating. The
temperature of the RC slab lower surface
was low in comparison with the temperature inside the furnace, and it reachedabout 800°C after 120 minutes of heating. For the test specimens in this test,
a flat deck was applied as a permanent
form. Therefore, the lower surface of
the RC slab was not exposed directly toflames, and a gap occurred between theflat deck and the RC slab lower surface
during heating, and as a result it was considered that the temperature of the RC
slab lower surface was lower than that
inside the furnace.
While the temperature of the lowerside reinforcement in the RC slab waslow, or about 250°C after 60 minutes ofheating, due to stagnation from the temperature rising in the vicinity of 100°C
accompanied by the evaporation of water contained in the concrete, the temperature then increased as the heating duration lapsed, reaching about 650°C after
180 minutes of heating. On the other
hand, the temperature of the upper-side
reinforcement in the RC slab showeda longer stagnation time from the rising temperature at 100°C compared to
the lower-side reinforcement, and it was
200°C even after 180 minutes of heating.
The temperature of the RC slab uppersurface was about 120°C even after 180
minutes of heating, and the difference intemperatures between the upper and lower-surfaces after 180 minutes of heating
was about 700°C.

Specimen
No.2

250
200
150

Specimen
No.1
Stop
heating

100
50
0

0

60

120

180

240

300

360

Time (min)December 2020 Steel Construction Today & Tomorrow 6

Photo 3 Cracking conditions on upper surfaces ofRC slabs in specimens No. 1, No. 2 andNo. 3(a) Specimen No. 1(b) Specimen No. 2(c) Specimen No. 3
7 Steel Construction Today & Tomorrow December 2020

small. Further, in spite of whether or not
the unprotected secondary beam was applied, both specimens No. 2 and No. 3
showed nearly identical deflection behaviors after 40 minutes from the start ofheating, and therefore it was consideredthat the live load of the composite flooring system was supported only by the
RC slab after 40 minutes from the startof heating. Further, these test results canclearly be understood from that the lower
flange temperature surpassed 800°C after 40 minutes from the start of heatingin the temperature measurement resultsin Fig. 3 and that the strength of the secondary beam at 800°C lowered to nearly
1/10 that at room temperature as shownin Fig. 2 (b).
The maximum deflection of specimens No.2 and No. 3 after stopping heating was 1/15 the deflection of the shortside span of the RC slab. Because large
deflection occurred in the slab, the membrane stress occurred within the slab, andit was considered that the slab could sustain the load due to this membrane action. In addition, because the temperature of the upper-side reinforcement waslow, it was considered that the tension resistance of the upper-side reinforcement
worked effectively in terms of membraneaction. Further, these test results can beunderstood by comparing the temperature of the upper and lower-side reinforcements after 3 hours from the startof heating shown in Fig. 3 and the hightemperature strength of reinforcing steel
bars shown in Fig. 2 (a).
While heating was stopped for specimen No. 1 after 132 minutes, when taking into account the test results for specimens No. 2 and No. 3, it was consideredthat specimen No. 1 sustained the loadbearing capacity for 3 hours or more aswith the case of specimens No. 2 and No.
3.

• Crack Occurrence Condition atUpper Surface of RC SlabPhoto 3 shows the occurrence of cracking on the upper surface of the RC slab.
In specimen No. 1 shown in Photo
3 (a), because the heating duration was
short, the cracking width in the center
section of the RC slab was narrow, but
because the end of the RC slab was restricted, it showed remarkable cracking
in the vicinity of the end.
In specimen No. 2 shown in Photo 3(b), large cracks vertical to the long-side
direction occurred in the center sectionof the RC slab. In the center section of

the RC slab, it was considered that thefull cross section experienced a tension
region in the long-side direction area.
Meanwhile, no cracking vertical to theshort-side direction was observed. Because the curvature in the short-side direction area was large, it was consideredthat the compression due to bending andthe tension due to membrane stress wereoffset at the upper surface of the slab.
In specimen No. 3 shown in Photo 3(c), the cracking condition was similar tothat of specimen No. 2.
In specimen No. 1, oval-shaped cracking was observed in the vicinity of theperipheral section. In specimens No. 2
and No. 3, diagonal cracking like diamond-shaped cracking was observed.
The reason for such cracking was considered to be the suppression of floatingof the RC slab at the corner section by theweight arranged in the corner section. At
the stage where the membrane action of
the RC slab worked, the tension force occurred to the radial direction in the center
section of the RC slab, and the compression force occurred to the circumferential direction in the peripheral section of
the RC slab.
The trends in cracking in the RC slabs
under the condition of the membrane action were confirmed from the cracking
conditions in these three specimens usedin this load-bearing fire test for the composite flooring system.
Potential for Unprotected
Application of Secondary Beams
The following will summarize the resultsof the load-bearing fire test that was conducted for the composite flooring system
employing the two-tier reinforcementarranged flat RC slabs and unprotected
steel beams:• The RC slab to which the bending collapse load based on the yield-line theory was given sustained load-bearing capacity even after being subjected to 3.5
hours of standard fire heating while involving deflection surpassing 1/20 thatof the short-side span.
• In the RC slab in which two-tier reinforcements were arranged, even when
the temperature of the lower-side reinforcement reached 700°C, the temperature of the upper-side reinforcement was
about 300°C, thereby allowing for theslab to retain its load-bearing capacity.
The reason for this successful retention
was considered to be that the tension resistance of the upper-side reinforcement
worked effectively due to the membraneaction of the RC slab.

• In the composite flooring system usedin this test, the effect of the applicationof unprotected steel beam on fire resistance was small.
• The effect of the headed studs arrangedin the four peripheral sections of the RC
slab on the deflection behavior of the RC
slab during fire was small. Due to thesmall effect, even when the rotationaland horizontal displacement at the endof the slab was not restricted, the membrane action of the RC slab worked effectively, and it was thus shown that theload-bearing capacity of the composite
flooring system was retained during fire.
• It was shown from these test results thatthe application of unprotected secondary
beam in the composite flooring system
is made possible by means of the membrane action of the RC slab.
References
1) C. G. Bailey, D. B. Moore: The structural behaviour of steel frames with composite floor slabs subject to fire: Part
1: Theory, The Structural Engineer 78,
No.11, pp.19-27 (2000)
2) Zhaohui Huang, Ian W. Burgess, Roger
J. Plank: Modeling Membrane Actionof Concrete Slabs in Composite Buildings in Fire. II: Validations, Journal of
Structural Engineering, ASCE, 129 (8),pp.1103-1112 (2003)
3) EN 1992-1-2, Eurocode 2: Design ofconcrete structures – Part 1-2 Generalrules – Structural fire design, 2004

December 2020 Steel Construction Today & Tomorrow 8

Feature Article: Fire Resistant Design of Steel Structures (3)Membrane Actions of Reinforced
Concrete Floor Slabs Exposed to Fire

by Kei Kimura, Steel Research Laboratories, Nippon Steel Corporation

Kei Kimura: After completing the course at theGraduate School of Engineering, Kyoto University,
he entered Nippon Steel Corporation in 2009. Thenhe was assigned to the Steel Research Laboratories,
Research & Development, and in 2014 became Researcher of Steel Structures Research Laboratory. He
serves as a member of Sub-committee on Fire Resistant Design for Steel Structures, AIJ, and assumes his
current position as Senior Researcher since 2019.

Part 3: Numerical Analysis of the Load-bearing Fire Tests for
Composite Flooring Systems
Purpose of Numerical Analysis
A numerical analysis was made targeting the load-bearing fire test of composite flooring systems composed offlat RC slabs and unprotected beams(beams with no fire protection covering), and the test results and numerical analysis results were compared.
The main aim in numerical analysis
was to verify the accuracy of the numerical analysis approach, and at thesame time to study the stress distribution within the composite floor surface
at the stage where the membrane action worked, the data for which is difficult to obtain from the load-bearing
fire test.
Analysis Targets
The numerical analysis targeted theload-bearing fire test for the composite flooring system introduced in thepreceding Part 2. The plane size ofthe floor slab in the composite flooring system measured 5,800×4,200mm with a thickness of 120 mm, and
10 mm-diameter reinforcing steel bars
were arranged in the slab in two tiers
at a pitch of 200 mm. Four floor perimeters were supported with protectedprimary beams, and unprotected secondary beams were arranged as they
connected the center of the short sidesof slab.
A uniformly distributed loadamounting to 11.23 kN/m2 (including
the dead load) was loaded on the floorslab. The slab was heated from its lower surface by means of the standard fire
curve in accordance with ISO 834.
Then the numerical analysis was
made, targeting test specimen No. 1 in
9 Steel Construction Today & Tomorrow December 2020

which headed studs were arranged at
the perimeter beams.
Outline of Numerical Analysis
Approach
For the numerical analysis, the finite
element analysis software “SAFIR”
developed at the University of Liege
in Belgium was adopted. This software
is composed of a thermal analysis program and a structural analysis program.
First, the distribution and hysteresisof the temperature in the cross sectionof the structural members were calculated using the thermal analysis program, and then the stress and deformation conditions of these members were
calculated using the structural analysis
program that employs the results thus
obtained in the above calculation. (Formore details about SAFIR, refer to reference 1).
Thermal Analysis• Analysis Models
Fig. 1 shows the thermal analysis mod-el. Fig. 1 (a) shows the RC slab, (b) the
secondary beam and (c) the primarybeam. For the thermal analysis model inFig. 1, the thickness of the RC slab wasset at 120 mm, the cross section of thesecondary beam H-300×150×6.5×9,
and the cross section of the primarybeam H-350×175×7×11. In Fig. 1 (c),ceramic wool was coated on 3 surfaces of the H beam, excluding its upper
flange, and the heat transfer caused byconvection and radiation in the cavity
was taken into account.
The thermal properties of the concrete and steel followed that prescribedin Eurocode 42), and the moisture content of the concrete was set at 5.8%
based on the material test results. Further, the thermal properties of the ceramic wool applied the room-temperature value of calcium silicate board,and the moisture content was set at
10%.
In order to simulate the condition inwhich heating at the flashover stage offire is applied to respective structur-

Fig. 1 Thermal Analysis Model
20°C (h=4W/(m2K))

ISO834 (h=23W/(m2K))(a) RC slab(b) Secondary beam(c) Primary beam

al members from their lower side, theatmospheric temperature at the heated surface followed the standard heating temperature prescribed in ISO 834,
and thus the atmospheric temperature
at the non-heated surface was set at
20°C. The convection heat transfer coefficient at the heated surface was set
at 23 W/(m2・K), and that at the nonheated surface at 4 W/(m2・K). Incidentally, reinforcing steel bars andheaded studs in the RC slab, and steel
decks were not modelled, and the effect of these materials on the member
temperature was disregarded.

• Results of Analysis

Fig. 2 shows the temperature hysteresis of the RC slab as an example of thethermal analysis results. In this figure,the solid line shows the analysis values, and the test value of specimen No.
2, which was heated for 202 minutes, isplotted. Regarding the temperature of
the lower-side reinforcement, a major
factor affecting membrane action during fire, it was understood from the figure that the difference in temperature
between the numerical analysis and thetest was approximately 100°C at maximum.
On the other hand, while the temperature at the lower surface of thecomposite floor obtained by numerical analysis was higher by as much as
300°C than that obtained in the test, thereason for the higher temperature wasattributable to that the thermal insulation effect of the steel deck removed
from concrete and the air layer on thetemperature was disregarded. Further,while the temperature at the upper-side
reinforcement and the upper surface ofthe composite floor, obtained by numerical analysis, was about 50°C higher than that obtained by the test, thereason for this was attributable to thefact that the movement of moisture inside the composite floor could not betaken into account.
While the temperatures of the structural members obtained by numericalanalysis showed a trend of being higher than those obtained by the test as
seen above, it was considered possible
to qualitatively assess these temperatures, and these numerical analysis results were thus adopted as the input data for use in structural analysis.

Structural Analysis• Analysis Models

Fig. 3 shows the structural analysis model.
The RC slab was modelled using theshell element, and both the unprotected
secondary beam and the primary beam
was modelled using the beam element.
The vertical-direction displacement at
the floor edge was to be restricted, and a
vertical load of 11.2 kN/m2 was uniformly given to the entire surface of the composite floor.
The stress-strain relationship of con-crete and steel at high temperature followed those prescribed in Eurocode 42),and the strength of structural materials at room temperature was set basedon the material test results. Specifically,
the compression strength of the concrete
was set at 36.1 N/mm2, and its tensile
strength at 0 N/mm2; and the yield pointof the reinforcing bars was set at 380 N/
mm2, and the yield point of the secondary beam and primary beam respectively
at 349 N/mm2 and 298 N/mm2 (refer toTables 1 and 2).

Fig. 2 Comparison of Temperature Hysteresis inside RC Slab
1200

Temperature (°C)

1000

ISO 834

Lower surface ofcomposite floor

Analysis value

800
600

Lower-side
reinforcement

Test value

400
Upper-side reinforcement

2000Upper surface ofcomposite floor
0

60

120

180

Fig. 3 Structural Analysis Model

Time (minutes)

ux, uy, uz
Restriction

Primary beam

Vertical load:
11.2 kN/m2
ux, uzRestriction
uy,uz
Restriction
Secondary beamz

5800 mm

4200 mmx

uz : Restriction

Table 1 Material Properties of Steel
Young’s modulus
Material(N/mm2)210,000
SN400B
210,000
SS400
210,000
SD295A

Yield strength(N/mm2)
349
298
380

Table 2 Material Properties of ConcreteCompressive strength
Material(N/mm2)
Fc24
36.1

Tensile strength(N/mm2)0yPoisson ratio
0.3
0.3
0.3

Poisson ratio
0.2

December 2020 Steel Construction Today & Tomorrow 10

• Results of Analysis

Fig. 4 shows the floor deformation(magnification: 5 times) as an example
of structural analysis. It can be seen in
the figure that with the lapse of heating
duration, the deflection of the composite floor increases.
Fig. 5 shows the deflection hysteresis at the center of the composite floor.
It was understood from the figure that,while the analysis value is smaller than
the test value up to heating for about
45 minutes, the trend of the increase in
deflection after 45 minutes of heating
was nearly identical for the analysis
and test values. The deflection at thetime of 132nd minute (stop heating)was 191 mm in the test and 197 mm in
the analysis, which showed favorable
consistency between them.
Fig. 6 shows the distribution of mainstresses occurring within the composite floor surface by dividing the stresses into compression and tension. From
the figure, while it was confirmed thatthe floor long side-direction tension
stress due to the thermal expansion of
the unprotected beam occurred after
5 minutes of heating, it can be understood that the compression stress occurred to the circumferential directionin the peripheral area of the composite
floor after 30 minutes of heating, andat the same time a tension sphere wasformed in the center of the composite
floor. Further, with the lapse of heating duration, the temperature of lower-side reinforcement rose to cause thereduction of the strength of the composite floor, and as a result, the tension
sphere expanded.

Fig. 4 Floor Deformation(Magnification: 5 Times)(a) After 60 minutes of heating(b) After 120 minutes of heating

Fig. 5 Comparison of Deflection Hysteresis in theCenter of Composite Floor
0

0

60

120

Time (minutes)
180

Test value

100
Specimen No. 1

200

Analysis value

Specimen No. 2
300

Deflection in floor center (mm)Fig. 6 Distribution of Main Stresses within Composite Floor Surface (Blue: Compression; Red: Tension)(a) After 5 minutes of heating

11 Steel Construction Today & Tomorrow December 2020(b) After 30 minutes of heating(c) After 60 minutes of heatingFig. 7 shows the transition of thetemperature and axial force of the unprotected secondary beam. Incidentally, the tensile force was treated as apositive value. It is understood fromthe figure that the temperature of theunprotected secondary beam reached
450°C after 7 minutes of heating
and that the compression axial force
reached the maximum level at 930 kN.
Then, as the temperature of the secondary beam increased, the axial force approached the level of 0.
It is forecast from these analysis results that in specimen No. 1 the verticalload can be continually supported due
to membrane action.

♦♦♦♦♦

In Part 3, examinations by means of
numerical analysis were conducted
targeting the load-bearing fire test for
full-scale RC slabs to confirm the accuracy of thermal and structural analysis.
Further, the stress distribution within
the composite floor surface when themembrane action works was shown.

Fig. 7 Transition of Temperature and Axial Force ofUnprotected Secondary Beam
1500

Temperature (°C) and axial force (kN) of secondary beam(Tension)

Temperature T

1000
500

Time (minutes)
0

0

60

120

180

-500
-1000
-1500
-2000

Axial force N
Yield axial force at high temperature Ny(T)=A・σy(T)(Compression)-2500

References
1) Gerna, T., Franssen, J.M.: Modelingstructures in fire with SAFIR®: Theoretical background and capabilities,
Journal of Structural Fire Engineering,Vol. 8, issue 3, pp. 300-323, 2017.
2) Eurocode 4: Design of composite steel
and concrete structures, Part 1.2: General rules - Structural fire design, EN
1994-1-2 (2005), 2005.

December 2020 Steel Construction Today & Tomorrow 12

Feature Article: Fire Resistant Design of Steel Structures (4)Membrane Actions of Reinforced
Concrete Floor Slabs Exposed to Fire

by Fuminobu Ozaki, Associate Professor, Nagoya University

Fuminobu Ozaki: Graduating from the
School of Architecture and Building Engineering, Tsukuba University, he finished thedoctoral course at the School of Engineering,
Tsukuba University and obtained Dr. Eng. in
2003. He assumed his current position as
Associate Professor, the Graduate School ofEnvironmental Studies, Nagoya University in
2013.

Part 4: Fire Resistant Design of Steel Structures in Japan and
Future Development
Fire Resistant Design of Steel
Structures in Japan
It is the AIJ Recommendations for Fire
Resistant Design of Steel Structures, prepared by the Architectural Institute of Japan (AIJ), that is adopted as common
guidelines for fire resistant design methods for steel structures in Japan.
Following the revision of the Building Standard Law of Japan in 2000, twodesign methods were introduced for thefire resistant design of buildings: design
based on specification route (Route A)
and design based on performance route(Routes B and C). With the introductionof Routes B and C, it has legally become
possible to apply performance-based fire
resistant designs for steel structures.
In the verification of the fire resistance of steel structures by means ofsimple calculations in Route B, the basic concept stated in the AIJ Recommendations has been applied, and the basic
concept has also been applied in the verification of the fire resistance of steel
structures that requires advanced calculations in Route C. To that end, the AIJ
Recommendations has been accepted as
the guidelines that play an important rolein the practical fire resistant design ofsteel structures in Japan.
AIJ Recommendations for Fire
Resistant Design of Steel
Structures
The first edition of the AIJ Recommendations was published in 1999, and thethird edition was published in 2017. In
the latest 2017 edition, a new fire resistance assessment method was proposed
and the existing assessment method was
improved by incorporating into the AIJ
Recommendations many attainments of
13 Steel Construction Today & Tomorrow December 2020the latest research on the fire resistanceof steel structures. Noting the ultimate
strength of steel framing, the AIJ Recommendations provides fire-resistant design methods for overall and partial steel
framing in an event of fire. Fig. 1 shows
the procedure of fire resistant design
shown in the AIJ Recommendations.
A notable feature of the AIJ Recommendations lies in that the fire resistant design of steel structures is able tobe completed using just the AIJ Recommendations. The left side in the figure shows the design route that assesses
fire actions, and the right side shows theroute that assesses structural strength. As
a result, the simple fire resistant design
method can be put into practical use dueto the complete division of fire resistantdesign elements into these two routes asshown in the figure, and further examination by means of advanced numericalcalculation is not entirely required.
The AIJ Recommendations is prepared based on existing research in which
the thermal stress during fires does not inmost cases affect the ultimate strength ofsteel structures, which thus allows for therealization of performance assessment
based on a comparison between the maximum steel member temperature owing
to the fire action and the collapse temperature when the steel member collapses (see Fig. 1).
In Japan, a certain level of or higher member strength and plastic deformation capacity are imparted with steel
structures through seismic design, and insuch steel structures, because high-temperature strength and plastic deformation
capacity can be expected to demonstrateduring fire, the effect of thermal stress on
steel structures at the ultimate state has

become extremely small. Meanwhile,regarding the flexural or local bucklingof columns and the high-temperaturestrength of high-strength bolt connections and other structural members insufficient in high-temperature strengthor plastic deformation capacity during
fires, the collapse temperature is reduced
by providing specific assessment methods for each.
In the third edition, a design method was provided that prevents the entire
collapse of a building during fire by assessing the load re-distribution capacity (redundancy) of the overall framing,and it has thus become possible to improve the collapse temperature of framings through the optimization of redundancy.
Future Development of Fire
Resistant Design
Steady efforts are being directed towards
the publication of the fourth edition of
the AIJ Recommendations, into whichplans call for the incorporation of research results on the reinforced-concrete
flooring system introduced in the preceding Parts 1~3. Further, the scope of steel
grades subjected to fire resistant design
is being expanded and it is examined toincorporate high-strength steel productsin the future edition of the AIJ Recommendations.
On the other hand, in settling the steel
effective strength for use in fire resistantdesign, the traditional approach has been
adopted since the first edition. Specifically, the design strength has been settled at zero at fire temperatures of 750°C
or higher, and therefore fire resistant design has been required to be made sothat the collapse temperature becomesFig. 1 Fire Resistant Design Procedure in AIJ Recommendations for Fire Resistant Design of Steel Structures

START
Design condition:
Fire separation, structural
material applied, fire protection,member dimension, etc.

Settlement of subassemlage
Sec. 6.2

Settlement of fire compartment andselection of fire property
Sec. 4.1

High-temperature strength ofsteel member Chap. 2
Working load
Sec. 3.3

Fire load
Sec. 3.2
Calculation of fire temperatureof compartment room
Local fire
Sec. 4.2
Moving fire
Sec. 4.3
Fully developed compartment fire Sec. 4.4
Ejected fire from opening
Sec. 4.5

Calculation of collapse
temperature of framing

Calculation of steel member
temperature
Unprotected steel member
Protected steel member

Calculation of basic collapse
temperature of framing
Moment frameSec. 6.3
Bracing frameSec. 6.4

Sec. 5.2
Sec. 5.3

Sec. 6.5
Effect of high-temperature creep
Sec. 6.6
Flexural buckling of column
Sec. 6.7
Local buckling of column
Sec. 6.8
Load re-distribution in frame
Lateral-torsional buckling of beam Sec. 6.9Sec. 6.10
Composite beamSec. 6.11
High-strength bolt connection
Sec. 6.12
Weld connection

Steel member temperature＜
Collapse temperature of framing

END

750°C or lower. At the time of publication of the first edition of the AIJ Recommendations, because of the lack of available test data for steel effective strength,a comparatively large safety factor wasprovided for steel effective strength. But
relevant test data has steadily been accumulated since, and reliable theories havealso been developed for the fire resistantdesign of steel structures. As a result, it
is considered that it will be possible in
the future to introduce limit state fire resistant designs represented by the LRFD
that will be carried out without counting
on the excess safety factor.
When the concerns involved in an upper-limit design temperature of 750°C or
higher are solved and the results of re-search on reinforced-concrete flooring
systems introduced in Parts 1~3 are reflected in future editions of the AIJ Recommendations, building design employing unprotected steel structural members(members with no fire protection covering) will become easier. To that end, theattractiveness of steel structures is expected to further improve.
December 2020 Steel Construction Today & Tomorrow 14

Serial Article: Latest Design of Steel Buildings in Japan (5)Mikiyama Park Gymnasium-Sharply-designed Steel Truss Framing Lightly Floats Up
into a Large Arena SpaceIshimoto Architectural & Engineering Firm, Inc.
Nippon Steel Engineering Co., Ltd.
A notable feature of Miki City in Hyogo
Prefecture is the hardware industry thathas experienced sustained development
since old times, and currently the city is
known as a hardware town that produces carpenter tools that are ranked as thebest in Japan.
In the project of the Mikiyama Park
Gymnasium, we intended that this arena will be loved by Miki citizens by expressing “Hardware Town Miki.” A remarkable feature is that the truss framewith small curves gives the inner space
an image of sharp hardware (Photo 1).
“Hardware Eagle,” a Symbol of
Miki City
The “hardware eagle” is an eagle-shaped
art object assembled employing a totalof 3,329 saws, kitchen knives and otherhardware products, all of which are Miki-made hardware products (see Photo2). The art object with a total wing length
extending to 5 m is manually assembled,

consuming as much as 8 hours, whenever the Miki hardware festival is held every year. It is a precious symbol for Miki citizens.

Photo 2 Hardware eagle, a symbol of the
hardware town of Miki

Photo 1 Finely-designed space truss framing
15 Steel Construction Today & Tomorrow December 2020

Image of Gymnasium ArenaSpaceAt the initial stage of design of the Mikiyama Park Gymnasium, the architect
involved in the project had an image ofan arena space that is covered with treestate framings and surrounded with agrove of trees growing in the forest.
As the design proceeded, the annual
Miki hardware festival was held. Whenthe design team joined the Miki hardware festival, the team was fascinatedby the power and beauty of the hardware
eagle. Since then, the team was eager topursue and express an image of hardware
rather than tree-like structure.
In a frame composed of a lot of structural members, the impression and atmosphere are determined by the detail ofjoints and the shape of steel members.
In order to save the construction cost, weaimed to achieve an elegant structure using usual and low-price steel members.
Then we added an idea of applying a
hardware image to the detail design anddeveloped an idea that strengthens thebeauty of the edge and surface of truss
framing.

Outline of Structural Plan
For the main structure of the gymnasium,a reinforced-concrete structure was adopted for the lower structure, and a steel
structure for the roof. The arch truss basesection of the roof was settled at the levelof the second-floor spectator seating, andthe base was joined with the seat floorusing pivotal bearings. (Refer to Figs. 3
and 4, Photo 3)

2650

2050

Instrument warehouse

4500

Fig. 1 1st-floor Floor Plan

7200

Outdoorfacility
yard
Plannedbuilding

7200

Porch
45050

950

7200

Arena

±0

+400

Training
room

2700
7775

7775

Study
room

Porch

Conference
room
6600

7200

Wind
shieldingroom
Footwear
room

Lobby

Lounge

6600

6600

6600

4400

5000
4100
9100

Porch

Porch

4575

7275

Physicalstrengthconsultation
room

1700

器具庫

Kids
terrace Dressing Dressing
room for room for
men
women
Kids
room
Staff
room

3250

Existing
swimming
pool building

Training
hall

5500

Outdoorfacility
yard

53800

Existingbuilding

35020

Building Plan
The newly-built gymnasium was connected to an existing swimming pool building
via a passage to form an integrated facility. The building plan called for the arrangement of an arena and exercise rooms on thefirst floor and the arrangement of spectator seating and terraces on the second floor,
and at the same time the linkage of an entire facility using a passages-like corridor
that runs through the entire facility fromsouth to north. (Refer to Figs. 1 and 2)
A variety of exercises can be taken at
the gymnasium. In this connection, somedevices to promote motivation for exercising were provided. For example, windows were arranged in various places of
the corridor so that visitors can see enjoying exercises of others too.

6600

6600

6400

73225

Fig. 2 2nd-floor Floor Plan

Existing
swimming
poolbuilding

Terrace

Arena upper section
Training
Outdoor hallfacility upper
yard section
Terrace

Outline of Arch Trusses
An arch truss configuration was formed
employing the lower chord member of
the three circles-triangles type and theupper chord member of the circle typesimilar to that of the lower chord member. H-beams were applied for the upper
chord member, built-up steel angles wereapplied for the lower chord member, andcovering plates were added to the lower
chord nearby the base subjected to large
axial force, thereby structuring the archtruss with a triangular cross section.
As the inner space with “hardware
town MIKI”-specific structure, the designteam tried to design structural details that
express not only the clear and straight edge
but the shining surface of sharpened steel.

Spectator seating
Running gallery
Lobby
Terrace Terrace• Expression of “Refined Lines”

Photo 3 Full view of Mikiyama Park Gymnasium
Ventilation window
named “Eco Feather”

Eaves Maximum
height height
▽

958.9

2480
16550
12225
16845

12225

▽

6175

Training
hall
7775

7775

Kids
room
7275

▽2FL

45100

1775

Arena

2700

Outdoorfacility
yard

12500

1450

6600

6600

6600
73225

4400

6600

6600

6600

6400

20 4600

Fig. 3 Cross Section

5000 2160

of Hardware
We took the lower chord member as themain item that imparts deep impression
with the arena. The bottom edge of lower chord member curves from the lower
level through the top so that visitors cansee the elegant curved line floating in thearena. T-shaped steel members with twosteel angles, used as the lattice member,make the edge clear. The gap between
two angles seems like the line that divides the flat surface.

△

Designgroundlevel▽ GL＝102.50＝1FL

Groundsurface＝102.48＝GL−20

December 2020 Steel Construction Today & Tomorrow 16

• Expression of “Fine Surfaces

Fig. 4 Outline of Structural Plan
Elevation in conformity with an existing indoor pool building→Erection of 5 arch trusses to longitudinal direction of arena

Training hall

Arena

To e
pool xisting sw
build
ing imming

46 m

Lobby

2nd fl.
RC structure
Entrance

Lower structure: Seismic strength type
Application of outer and partition walls as seismic-resistant wallFig. 5 (1) Configuration of Truss

Fig. 5 (2) Detail of Truss

4,000
2,000

2,000

P.P.

40
45

45

（SS440）PL−25×80×100
1−M16
@500

Lower side

80

10
0

（SN490B）
PL−16

45
45
40

5×
7

×7

30

7
5×

×7

TR
1：
2L
−1
25

35

30

（SA440B）
PL−25
5−M16

95

25
−1
2L
1：
TR

PL−16
（SN490B）

30

P.P.

PL−25
（SA440B）
5−M16

Gap 5 mm

SB1

TL1：BL−200×200×16（SN490B）

26
0

SB1：H−148×100×6×9

SB1

PL−25
（SS400）1−M16 ＠500

275

80

350

90

2,000

Surfacealignment

and Brilliance” of Hardware
We created the details in which the lower chord member and the lattice member
are aligned to realize an identical structural plane (Fig. 5).
Fig. 6 shows the details of the truss
framing. The structural members gather
at many joints in various angles. Therefore, we verified the detail in terms of theappearance from various viewpoints andthe construction efficiency using mockups and 3D simulation.
For the column base (A in the figure), cast steel was applied. The dimension of the steel-frame members adopted were H-beam-H-300×150×6.5×9(SN490B) for the upper chord member; built-up angle-BL-250×250×19+ PL-19 (SN490B) for the lower chordmember and angle-2L-125×75×7 forthe lattice member.
Steel-frame Fabrication
The design team and the steel-frame fabrication team discussed repeatedly to share
design concepts and intentions. During thisprocess, the steel-frame fabrication teamsteadily understood that how to attractively assemble the framing was an importantfactor in creating an attractive arena spaceand how to secure high finishing accuracy
was a demanding element for the interior.
To meet these requirements and as a member of one team of the project, the steelframe fabrication team resolved to maketheir work unique to the hardware town.

Fig. 6 Detail of Steel Truss Framing
Lattice member:Cut tees-Expression of surface↓
Built-up angle-Expression of edge
Slimming of joints by means of
2-face shearing

Purlin b:

Purlin a:
965

Lower chord member:
Angle
A:
Column base: Cast steel
pivotal bearing• Gathering of 3 triangle-shaped
structural members at sharp angle• Clear expression of conditions for joiningcolumn base and lower RC framing bymeans of pin support

Cast steel(NCN490)

Cast steel(NCN490)
Lower chord member(in the vicinity of column base):
Angle+Shielding plate
Triangle closed surface
Cast steel (SCW480)
Pivotal bearing

17 Steel Construction Today & Tomorrow December 2020

Cast steel joint:
Gathering of
6 structural members

Many skillful engineers joined theteam to find out the best solution. We
thought this timing as the most importantmilestone and raised two keynotes:
One was to show the “clear edge of steel
members.” The edge of the lower chordmember was required to go straight fromthe side to side like the Japanese sword.
Another was to align the surface ofstructural members applied. Each surface of lower chord and lattice members was required to be aligned in one
surface. The clear finish of aligned surface depends on surface finish of welds.
We chose to sharpen the welding finish
although it required high cost and time
consuming process.
Tasks and Responses in
Steel-frame Fabrication
In cases when we adopt the angle and it
is bent as the lower chord member, theessential member, there was a possibility that a large strain would occur at theend of flange, To solve this problem, theteam drew unfolded drawings of the angles by using 3D-CAD. The fan shape
was chosen as the shape of unfolded angles. Two fan-shaped plates were weldjoined to manufacture the built-up angle with a curved configuration. (Refer
to Photo 4 and Fig. 7)
On-site Erection of Steel Framing
In order to secure the erection accuracy of the lower chord members, the construction team planned to build the truss
by single unit instead of assemblingunits on the ground so that truss assembly could be done only with slight ad-

Photo 4 The fan-shape plate for manufacturing
the built-up angle and confirmation of
the configuration of the lower chordmember using mockups

justments. Assembly work was managed
not to cause the errors during building oftruss framing.
The on-site welding of lower chordmembers was unusual in terms of welding-line angles and welding directions.
The team built mockups in order to confirm the appearance and quality, and decided to grind the welding line alignedwith each other (Photo 5). For highstrength bolt joint of the chord and latticemembers, how to insert each bolt wasplanned in advance considering bolttightening efficiency and appearance
from the spectator seating (Photo 6).
(Continued overleaf)
Photo 5-1 Confirmation of the configuration of
joints using mockups

Fig. 7 Fabrication Method for
Structural Members

Assembly of fan-shaped plate

Photo 5-2 Checking of angle grinding condition
to make the angle edge conspicuousPrimary assembly:
Assembly of cast steel and built-up angle

Final assembly:
Final assembly of respective structuralmembers on surface plate

Photo 5-3 Examination of the configuration of
the column base cast steel using a
Styrofoam mockup

Photo 6 Checking of bolting work by means of 3D-CAD
December 2020 Steel Construction Today & Tomorrow 18

Before entering into the jacking down
phase, the team simulated the procedure
and analyzed the degree of displacement. Jacking up was carried out in 2
steps. The result was almost same as preconstruction analysis.
♦♦♦♦♦

Construction of the Mikiyama Park
Gymnasium was successfully completed, and when looking up at the completed arena of the gymnasium, the spacetruss framing structured with great cares
stands in overwhelming beauty. When
we see the great completion of the Mikiyama Park Gymnasium, the difficult
time that we had has gone away. And
we’d like to share our feeling with Miki hardware craftsmen and to say “We
did IT.”

Outline of Mikiyama Park Gymnasium
Location: Miki City, Hyogo Prefecture
Project owner: Miki City Office
Main application: Gymnasium
Area• Site area: 239,922.00 m2 2
• Building area: 3,538.33 m 2• Total floor area: 4,203.47 m
Structure• Aboveground: RC structure, partly steel
structure• Gymnasium roof: One-way steel-frame
truss• Pile, foundation: Spread foundation (partly
shallow-layer improvement)
No. of stories: 2 aboveground
Maximum height: 16.84 m

Architectural design: Ishimoto Architectural& Engineering Firm, Inc.
Structural design: Ishimoto Architectural &Engineering Firm, Inc.
Construction• Building: Takashina Corporation• Gymnasium roof: Nippon Steel Engineering
Co., Ltd.
Design period: June 2015~March 2016
Construction period: June 2016~July 2017
Photo: Koichi Torimura(Part of photos: Ishimoto Architectural &Engineering Firm, Inc., Nippon SteelEngineering Co., Ltd.)

Fine Arena Space Worthy of the
Hardware Town of MikiIn previous architectural designs, steel
pipes have commonly been used for the
space trusses. In this project, the teampursued “Miki-specific structure” andreached the unique structure that featured “LINE” and “SURFACE” by theuse of steel angles. (See Photo 7)
The truss frame “flying” in the arena is painted carefully and these sharpened edges remind the hardware andcarpenter tools. We believe that we haveachieved “Miki-specific space” with thisunique structure detail.

Photo 7 New space truss structure stressing the “structural ‘LINE and SURFACE’of framing” with the use of steel angles

STEEL CONSTRUCTION TODAY & TOMORROW
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