AlllOO

R.I.C.

NBS BUILDING SCIENCE SERIES 51

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Structural Evaluation of

Steel Faced Sandwich Panels
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structural Evaluation of

Steel Faced Sandwich Panels

n65 %Adim our. National Bureau of Stajidsrjt?

I ' ' ^

MAY 3 1374

J. H. Pielert, T. W. Reichard, and

L. W. Masters 1

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington. D.C. 20234

Prepared for the

Office of Policy Development and Research Department of Housing and Urban Development

Washington, D.C. 20410

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secreiary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, D/recfor

Issued April 1974
Library of Congress Cataloging in Publication Data

Pielert, J. H.

Structural evaluation of steel faced sandwich panels.

(National Bureau of Standards Building Science Series, 51)

Supt. of Docs. No.: C13. 29/2:51.

1. Sandwich construction—Testing. I. Reichard, Thomas W.

,

joint author. II. Masters, Larry W. , joint author. III.

United States. Institute for Applied Technology. Center for

Building Technology. IV. Title. V. Series: United States.

National Bureau of Standards. Building Science Series, 51.

TA435.U58 no. 51 [TA492.S25] 690'. 08s [624'. 1779] 74-5085

National Bureau of Standards Building Science Series 51

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 51, 43.pages (Apr. 1974) CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1974

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.29/2:51). Price 90 cents.
SI Conversion Units

In view of the present accepted practice in this country for building

technology, common US units of measurement have been used throughout this

paper. In recognition of the position of the United States as a signatory

to the General Conference on Weights and Measures, which gave official

status to the metric SI system of units in 1960, assistance is given to

the reader interested in making use of the coherent system of SI units

by giving conversion factors applicable to US units used in this paper.

Length

1 in = 0.0254 meter (exactly)

1 ft = 0.3048 meter (exactly)

Force

1 lb (Ibf) - 4.448 Newtons (N)

Pressure

1 Ibf/ft^ = 47.88 N/m^

1 Ibf/in^ = 6894 N/m^

Temperature °C = 5/9 (Temperature °F - 32)

iii

Structural Evaluation of Steel Faced Sandwich Panels

J. H. Pielert

T. W. Reichard

L. W. Masters

Center for Building Technology

Institute for Applied Technology

A series of structural evaluation tests performed on components and materials

intended for use in one of the Operation BREAKTHROUGH housing systems

is described. Four samples of steel faced, paper honeycomb, sandwich panel

material and four full size prototype roof panels were evaluated.

The samples of sandwich panel material were used to evaluate the variability

of panel material properties and the effect of aging on tensile and shear

strength. The roof panels were used to determine the behavior in service

considering the effects of adverse environmental conditions on ultimate

strength and mode of failure. In addition, the performance of one panel

under sustained loading was evaluated.

Key Words : Accelerated aging; adhesive bond; ductility; flexural shear;

housing systems; local buckling; material variability; moisture conditioning;

Operation BREAKTHROUGH; paper honeycomb; structural sandwich; sustained load.

1.0 Introduction

1 . 1 Description of System . A housing system (figure 1) proposed for the "Operation BREAKTHROUGH"

program of the Department of Housing and Urban Development (sponsor) utilizes

factory-produced sandwich panels for roof, floor and wall members. The

wall and floor panels are attached to a steel grade-beam system as shown

in figure 2. The floor panels are supported along the edges by the coldformed steel grade beams and by joists spanning between the grade beams.

All panels are 3-inches thick; the roof and wall panels consist of a 26-

gage (.0179 in.) steel sheet bonded to each side of a resin-impregnated

paper honeycomb core as shown in figure 3. The floor panels are similar

except that the upper surface is 3/8 in. plywood. Urethane foam is pressed

into the honeycomb prior to final assembly to improve thermal properties.

Wood edge members are fastened around the perimeter of the panels.

1
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Figure 3. Section of Panel Material

4
1 . 2 Scope of Evaluation . One phase of Operation BREAKTHROUGH is an evaluation of the structural

adequacy of each proposed housing system. Two basic questions regarding

structural performance must always be considered when evaluating each

system. First, is it structurally safe; and second, will the structure

perform adequately in service? This paper covers only the structural

evaluation; however, other performance factors such as fire, acoustic,

and thermal resistance have been evaluated.

An evaluation of the housing system showed the roof panel to be one of the critical elements in maintaining structural integrity during the

life of the structure. The panels used on the roof are subjected to larger

stresses than those used in the floors and walls. Also, the roof panels

are more susceptible to weathering damage because no roofing material

is applied to the roof, other than flashing and sealants at the panel

joints and at the intersections with the wall panels.

The adhesive bond between the honeycomb core and the steel skins

is important in maintaining both the load capacity (safety) and stiffness

of the structural system (serviceability). Many adhesives that are exposed

to environmental conditions, such as high humidities and elevated temperatures,

are known to undergo deterioration resulting in decreased bond strength.

This process is referred to as aging. The speed of the aging process

is frequently dependent upon the severity of exposure conditions. To

evaluate the effect of environmental conditions on the performance of

the panel materials, small specimens were exposed to conditions more severe

than those actually encountered in service on the premise that the more

severe conditions would serve to accelerate the aging process. A description

of these test procedures and the results are included in Chapter 2. The flexural behavior of the roof panels was evaluated by testing

to determine the effect of moisture conditioning on the structural performance

of the panel. A description of these test procedures and the results

are included in Chapter 3.

2.0 Panel Material Evaluation

2 . 1 Scope . In evaluating the sandwich panel material it was assumed that, in

service, moisture would accumulate within the panels and under certain

conditions the temperature of the exterior facing of the roof panels would

approach 180*"? [1] .-^

— Numbers in brackets indicate references listed at the end of this paper.

5
High humidities, in combination with high temperatures, are especiallydetrimental to adhesive bonds used in sandwich panels [2]. Therefore,

evaluation of the panel material included a determination of the effect

of moisture and high temperature on strength.

Four samples of panel material were evaluated. All samples were fabricated by the producer and were similar except for the adhesive and

bonding procedure used in making the panels.

Sample A was a preliminary design bonded with a poly (vinyl acetate) adhesive. Test specimens were fabricated to the required size. Testing on this sample

was not completed (See 2.3.1.1)

S amp 1 e B was a preliminary design bonded with a neoprene -phenol ic contact adhesive. Test specimens

were fabricated to the required size.

Sample C was cut with a fine-tooth band saw from a prototype roof panel which was bonded with an epoxy

adhesive.

Sample D was a wall panel taken at random from the

production line which was bonded Avith an epoxy

adhesive. Test specimens were cut to size with a fine-tooth band saw.

The shear (parallel to the plane of the sandwich) and flat-wise tensile

(normal to the plane of the sandwich) strengths of specimens taken from

each sample were determined before and after accelerated aging.

2 . 2 Test Procedures

2.2.1 Flexural Shear Test . The shear strength of the honeycomb cores was determined by ASTM

C 393-62 [3] using quarter point loading. The test span (18-in) was chosen so that failure would be either by shear buckling of the core or by shear along the core-facing interface. The specimens for samples B,

C, and D were 6 in wide x 23 in long. Figure 4 shows a typical test setup

and a Sample A specimen following the test. The rate of loading was approximately one-third the expected maximum load per minute.

The shear strength values determined by this test method will be

approximately those determined by other methods (for example, ASTM C 272).

These values vary with test span, thickness of core and facings and other

test variables. One such variable is the size and thickness of the plates

used to distribute the loads into the specimen and to prevent local crushing

6
7
under the load points. The load distribution plates used in this test were 6 x 2 x 1/4 inches and the reaction plates were 6 x 2 x 1/2 inches

and oriented as shown in figure 4.

2.2.2 Flatwise Tensile Test . The test method o£ ASTM C 297-61 [3] was used to determine the flatwise

tensile strength of the panel material. The 4 in x 4 in specimens were

tested as shown in figure 5. The steel loading blocks (2 in thick) were

2/ bonded to the specimen with a hot-melt adhesive,—' The pull rods were connected to the hydraulic testing machine through spherical seats. The

loading rate was approximately one-third of the expected maximum load

per minute.

The purpose of this test is to determine the tensile strength of

the weakest link in the sandwich. Thus, the core will fail if the tensile

strength of the core is less than the tensile strength of the adhesive

bond.

The purpose of this study was to determine the mode of failure as well as the strength values before and after accelerated aging.

2.2.3 C onditioning and Aging of Specimens . All specimens were tested after being brought to equilibrium with

laboratory conditions (73° ± 3°F and 50 ± 3% rh) . Half of the specimens

from each sample were stored in the laboratory until tested while the

other half were artificially aged using the standard procedure of ASTM

C 481-62, Cycle A [3] before testing.

This procedure consists of 6 repetitive cycles of warm water soaking,

steam spraying, freezing and dry-heating. Although, this arbitra;ry procedure

is widely used in evaluating sandwich materials there is as yet no acceptable

correlation with natural aging.

2,3 Test Results

2.3.1 Accelerated Aging (ASTM C 481) . Specimen appearance after the aging procedure was noted with significant

observations as follows:

— This adhesive had enough fluidity at approximately 325°F for this bonding

operation. This temperature did not degrade the adhesive used in

producing the panels.

8
Figure 5. Flatwise Tensile Test

9
2.3.1.1 Sample A . The first step of the aging procedure was to immerse the specimens

in warm water (120°F) for one hour. At the end of this step all specimens

from Sample A were showing signs of delamination . Most specimens had

at least one facing completely delaminated from the core. Figure 6 is a picture of a f lexural-shear specimen (6 in x 23 in) after water soaking.

The stresses set up in the core from the moisture expansion were sufficient

to completely separate the facings from the core. Note, in this photograph,

that the core had expanded until it was about 1 in longer than the facing.

Sample A material was rejected and testing terminated because the adhesive

was water soluble,

2.3.1.2 Samples B, and D . The accelerated aging procedure darkened the color of the paper honeycomb,

but no other significant changes were observed.

2.3.2 Flexural Shear Test Results (ASTM C 393) . The results of the shear tests are presented in table 1. The core in

the sandwich panels is oriented (see figure 7) so that the core shear strength

in the "W" direction is the critical parameter. [The paper ribbons of the core run perpendicular to the span of the panel]

The shear strengths and the relationship between the strengths in the

"W" direction and the "L" direction depend to a large extent on the shape

of the honeycomb cells. If the core is 100 percent expanded in the sandwich,

the cell shape is a true hexagon and the "W"/"L" shear strength ratio would

be approximately 0.6. Most paper honeycomb sandwich cores are either over

or under expanded and the expansion usually varies within a panel. Thus,

some variability of core strength values should be expected when testing

small specimens cut from the same panel.

The primary interest in this evaluation was in the "W" direction shear

values and in the effect of the aging on these values. It can be seen from

the data in table 1 that the shear strength of the unaged core in the "W"

direction varied from 20.9 to 28.9 psi and that the aging reduces this

strength 18 to 20 percent.

All the flexural shear specimens failed by shear buckling of the core indicating that the strength of the adhesive bond was sufficient to develop

the shear capacity of the core. (See figure 4)

2.3.3 Tensile Test Results (ASTM C 297) . The tensile test results presented in table 2 show a wide variance in

the tensile strength values between samples and, in the case of Sample C,

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Figure 7. Hexagonal Honeycomb Orientation

13
Table 2. Tensile Test Results

Aged Specimens Unaged Specimens

S amp 1

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Spe c imen Failure Mode^ Tensile Strength Failure Mode

p s i ps i

B 1 32. 2 100% Bond 31

.

5 100% Bond

2 ?fi i. u • nu 1001 Bond 33

,

8 100% Bond

32 0 z. . u 1001 Bond 33

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5 100% Bond

42 100"^ Bond 28

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0 100% Bond

52 100% Bond 2 7 0 100% Bond

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C 1 9. 13 90% Bond nu 5% Bond

2 in n 3 75% Bond 74 7 5% Bond

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42 10% Bond

52 7 1 1% Bond

A Ifia T" Q fT <a rV V C 1 cl)i 22 .

4

55. 6

D 1 22.6 35% Bond 36. 3 0% Bond

2 23.0 50% Bond 26. 3 0% Bond

3 28.3 65% Bond 38. 8 0% Bond

4 23.1 50% Bond 46. 3 0% Bond

5 39. 7 60% Bond 31. 6 01 Bond

Average 27.3 35. 5

^Failures were classified as either bond or core failures. Bond

failure generally was at the primer-bonderized steel interface

except for Sample B specimens where the adhesive failed cohesively.

^Specimen cut from flexural shear specimen.

^Specimen was cut from an area of the panel where there had been

relative movement between the core and one facing during fabrication. This movement had "squeegeed" the adhesive so that

certain areas were starved for adhesive. (See Fig. 8)

14
within the sample. Observations made following each test helped in explaining

this variance.

These observations as well as the strength data indicated that the

specimens of Sample C were different from those of Sample D. Essentially,

failure occurred in the core of all unaged specimens from both samples.

However, observations indicated that the adhesive for Sample D was bonded

mostly to fibers on the edges of the paper core (resulting from surface

sanding of core) rather than to the solid portion of the paper as it was

for Sample C. It can be concluded that the actual tensile strength of the

dry, unaged core is probably at least 50 psi. HoAvever, because of the

fabrication practice the tensile strength should be considered to be no more

than 35 psi.

There is insufficient data to properly judge the effect of the aging

on the tensile strength of the core. However, from a comparison of the

Sample C data for aged specimens 4 and 5 with the unaged specimens, it

appears that the aging may reduce the tensile strength by as much as 40

percent. A comparison of the Sample D data for the aged with the unaged

specimens indicate a reduction of only 24 percent in bond strength.

The data indicate that the aging had little if any effect on the

tensile strength of the adhesive (neoprene-phenolic) of Sample B. It is

also noted that the bond strength of the adhesive of Samples C and D is

greater than that for Sample B. However, the strength of the adhesive bond

after aging was less for Sample C and D than for B. Observations made on the Sample C specimens after the tensile tests

emphasize the effect that fabrication techniques can have on the test results.

Aged specimens No. 1, 2 and 3 of Sample C had been taken from an area of the

panel where there had been a relative movement between one facing and the core during mating. This movement, which can be seen in figure 8, "squeegeed" the

adhesive in front of the cell edges so that the areas behind the edges were starved for adhesive. The data for these three specimens indicate a greatly

reduced bond strength as a result of this movement during fabrication.

Most bond failures for Samples C and D occurred at the interface of the

steel facing and a primer which had been applied prior to application of the

adhesive. Since significant bond failures occurred only on the aged specimens

it seems reasonable to assume that the durability of the primer bond to the

steel is the weak link in the system and not the adhesive. However, it would

appear from the low results for the "squeegeed" aged specimens 1, 2 and 3 of Sample C that the adhesive does furnish some protection for the primer.

3.0 Structural Testing of Full Size Roof Panels

The reliability of the procedure for predicting the structural behavior

for the type of construction in the roof panel was not known. Consequently,

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a testing program was undertaken to verify structural performance. Various

moisture conditioning procedures were used to simulate in-service conditions

since the panel may be susceptible to water penetration during service life.

Four full-size, prototype production panels were obtained from the producer

and three were subiected to short-term load testing and one to a 24-hour

sustained load test.

3. 1 Description of Moisture Conditioning Procedures . Each of the three short-term test panels was moisture conditioned by

a different procedure before being subjected to the flexural tests. The three

procedures were:

1. 50? relative humidity at 73 ± 3°F for five days.

2. 95?i relative humidity at 73 ± 3°F for five days by storage in

a fogroom with the panel draped with a plastic film to prevent

the deposition of liquid water.

3. Complete submersion in a water bath at 73 ± 3°F for 7 days and a subsequent 9 day drying period, under procedure 1 conditions and

without forced air.

The fourth roof panel for the 24-hour sustained load test was conditioned

using procedure 1.

3 . 2 Description of Test Setups .

3.2.1 Short-Term Flexural Te st.

The roof panel was tested in the horizontal inverted— position with an air bag sandwiched between it and a wood support placed on the laboratory

floor as shown in figures 9 and 10. The support members were 3 in x 3 1/2 in

wood blocks to simulate the actual wall support condition. The overall panel

length was 16 ft 0 in and inside - to -inside dimension of the supports was

13 ft 0 in. The specimen supports Avere square tubular tie down beams with a roller at one end and a knife edge at the other. Four steel bracing members

were placed between supports in an attempt to simulate lateral restraint

provided by adjacent panels to the edge members in a completed roof structure.

— Normal position refers to panel orientation in service. Inverted position

refers to panel turned upside do^^^n.

17

19
Three linear variable differential transformers (LVDT) were placed at

midspan to record vertical movement with one over each edge beam and one at

the centerline of the specimen. X-Y recorders plotted air bag pressure

versus midspan deflection.

3.2.2 Twenty-Four Hour Sustained Load Test . The roof panel was tested in the normal position with sand bags applied

between supports as shown in figures 11 and 12. The overall panel length

was 16 ft 0 in and inside- to-inside dimension of the support was 13 ft 0 in.

A single def lectometer was placed at centerline of midspan to record vertical

movement

.

3. 3 Description of Test Procedure . All loads discussed under testing are equivalent applied loads. For the

short term tests where the panels are tested in the inverted position, the

equivalent applied load is determined by taking the applied air bag load minus

twice the panel weight minus the weight of the loading apparatus. The

equivalent applied load for the 24-hour sustained load test is the actual load

since the panel was tested in the normal position.

3.3.1 Short-Term Flexural Tests . For each of the three short-term tests a preload of 20 psf was applied

to the panel and then removed in order to seat the specimen in the test

fixture. Load was then applied in 5 psf increments to 30 psf and then

removed. Load was then applied in 5 psf increments to failure. Deflection

readings were taken at each increment.

3.3.2 Twenty-Four Hour Sustained Load Test . Sand bags were distributed uniformly between panel supports to provide

a load of 45.7 psf (0.2 DEAD + 1.5 LIVE). Load was maintained for 24 hours

and removed. Deflections were periodically recorded during the 24 hour period,

immediately after load removal, and 24 hours thereafter.

3. 4 Test Results - Short Term Flexural Tests .

3.4.1 Panel Conditioned by Procedure 1 . This panel failed at an equivalent applied load of 135 psf with elastic

load-deflection behavior up to failure. The specimen was then unable to

sustain any significant load. Figure 13 shows the load -def lection history

of the midspan LVDT's located at a side beam and at the centerline. Failure

occurred by local buckling (wrinkling) of the compressive facing in a straight

line transversely across the panel approximately six inches from, and parallel

to, a splice in the honeycomb core (see figure 14).

20
21
Figure 12. Sustained Load on Full-Size Roof Panel

22
23
7

Figure 14. Portion of Short-Term Flexural Specimen Conditioned byProcedure 1 Showing Buckling of Compressive Facing

24
3.4.2 Panel Conditioned by Procedure 2 . This panel failed at an equivalent applied load of 155 psf with elastic

load-deflection behavior up to failure. The specimen was then unable to

sustain any significant load. Figure 15 shows the load -deflection history

of the midspan LVDT ' s located at the side beam and at the csnterline.

Comparison of figures 13 and 15 indicate an inconsistency in relative

movements of the north side beams and the panel centerlines. However, this

is not unexpected since the panels are not exactly symmetrical (see difference

in edge members in figure 9). Because they are not symmetrical, the panels

will warp slightly under uniform load. Failure occurred by local buckling

of the compressive facing in a straight line transversely across the panel

at, and parallel to, a splice in the honeycomb core (see figure 16).

3.4.3 Panel Conditioned by Procedure 3 (Soaked) . The panel behaved elastically up to an equivalent applied load of 80 psf

when a sudden drop in load occurred (see figure 17). This was believed to be

caused by readjustment of the air bag and with a slight increase in effective

loaded area. The panel was again able to take load linearly with respect

to deflection.

Failure occurred at an equivalent applied load of 104 psf and was apparently initiated by fracture at a knot in the edge member as is shown

in figure 18. This was followed almost immediately by a local buckling of

the compressive facing and a complete loss of load carrying capability. The

buckling occurred in a straight line transversely across the panel at, and

parallel to, a splice in the honeycomb core as shown in figure 19.

Water had entered the panel during the soaking period at the wood edge

members and traveled along the honeycomb sheet edges and splices. The panel

weighed 207 pounds before placing in the water and 240 pounds on the. day of

test. The panel was taken apart after the test and the moisture content was determined on portions of the material taken from an area which appeared to

be the dampest portion of the panel. This area was at the intersection of

the edge member and core splice. The moisture contents, listed below, are based on oven-dry (220°F) weights.

Foam Insulation 1291

Honeycomb Core 41%

White Fir Edge Beam 24%

Inadequate adhesive bonding between the honeycomb and steel facings to the

right of the core splice is evident in figure 19. It apparently was caused

by a difference in the thickness of the two adjacent core pieces.

25
26
J

Figure 16. Portion of Short-Term Flexural Specimen Conditioned byProcedure 2 Showing Buckling of Compressive Facing

27
no -

0 0 5 10 15 2 0 25 3.0 3.5 MIDSPAN DEFLECTION, IN

Figure 17. Short-Term Flexural Test Results on S-necimen Conditioned

by Procedure 3,

28
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ro

d)

<DUo

Cm

>N0o

4-* oo

•H o u c

(D

C/} ci

+-* d) o 3

•H

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<D

f-H •H

u.

J

•iH 6 c

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CO

CO

1—( 3

WJ

•H

29
<U bC C C O -H • H U

4-> rt •H [1-

0)>• H

C 1/1 6 J-P<o cou

Siooco

•H

•tJ

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30
3. 5 Test Results - Twenty-Four Hour Sustained Load Test . A uniform load of 45.7 psf (0.2D + 5L) was applied to the panel with

sand bags (D = 3.5 psf; L = 30 psf). The panel deflected 0.77 in at midspan

upon load application and increased to 0.81 in after 24 hours. The residual

deflection immediately after removal of load was 0.050 in and recovered to

0.037 in after 24 hours. Figure 20 shows this time-deflection history.

4.0 Discussion of Results

4 . 1 Test Performance of Roof Panels . It was computed that the shear stress in the roof panel with a live load

of 30 psf would be 6.5 psi neglecting the contribution from the edge members.

The method of attaching the edge members to the panel (stapling) did not

appear sufficient to justify computations assuming complete composite behavior.

The measured shear strength for the unaged specimens of Samples C and

D averaged about 23.9 psi. Aging reduced the strength to about 19.7 psi

(Sample C) . These strengths appear to be adequate when compared with the

computed 6.5 psi shear stress for the 30 psf roof load. However, these

'Strengths were obtained in dry specimens when in fact it must be assumed

that the core will be damp at some time in service.

Jenkinson [4] and others have shown that honeycomb similar to that used

in this panel material will lose about 50 percent of its dry (50 percent rh)

shear strength when conditioned at 100 percent rh. Thus, the shear strength

of the core when reduced 50 percent for dampness would be 12 psi for the

unaged and 9.9 psi for the aged core. The above discussion shovv's how the shear capacity of the core in the

roof panel would affect its load carrying capacity. However, the flexural

tests on the full-size roof panels indicate that the shear capacity of the core is not a controlling factor and that the edge members do contribute

to the flexural strength of the panel. The three roof panels tested to

failure with short-term uniform loads show that the failure mode is facing

buckling rather than shear. This means that the tensile (or compressive)

strength of the panel materials in the flatwise plane is the controlling

factor in the roof panel. The computed test shear stress for the soaked

roof panel, assuming no edge members, was 18.8 psi at the failure load of

104 psf. This computed stress when compared with the 12 psi shear strength

for wet unaged core indicates that for this test the edge members contributed

about 36 percent to the shear resistance of the damp roof panel. The same

type of comparison indicates that the edge members contributed only about

10 percent to the shear resistance of the dry panel. However, these roof

panel tests did not reveal the effects of aging or variability in tlie properties of the panel material.

31
00

oo^-. toin^tocvj — o

Ni 'N0ii03nj3a NVdsam

32
The minimum failure load of 104 psf, for these panels, when adjusted

down for the aging effect of 24 percent (see section 2.3.3) is 79 psf.

A variability factor (v) of 0,41, computed from the tensile test data

for Samples C and D, could be used because of the failure mode in all the

full-scale, roof -panel tests. Adjusting the load capacity of the wet and

aged roof panel (79 psf) for this variability reduces the rated capacity

to 49 psf [79 (^j—j-^j-1^) ]

. This variability factor of 0.41 may be

conservative, but present knowledge concerning localized buckling failure

in sandwich panels is insufficient to justify a lower value.

The sponsor recommends that the design ultimate load be 1.4D + 1 . 7L

after allowance for aging, environmental effects and variability. From the

discussion above and this recommendation the design live load should not

exceed 26 psf.

Normally it is expected that a structural component will exhibit some ductility; that is, support a significant load while undergoing inelastic

deformation. This was not exhibited by these sandwich roof panels.

The sudden buckling failure which occurred near, and parallel to, the core splice was typical for all short-term flexural panels and appeared to

be in part caused by inadequate adhesive bonding between the honeycomb

and steel facings . This inadequate bond was apparently caused by a difference

in the thickness of the two core pieces and is an indication of a quality

control consideration. As a result, it was recommended either: CI) core splices be eliminated by using full length core sheets; or (2) core material

be chosen for consistent thickness and splices made such that the shear

strength of the splice and that of the bond near the splice be equal to

that of the honeycomb core without a splice.

5.0 Summary and Conclusions

This paper describes a series of structural evaluation tests performed

on components and materials intended for use in one of the Operation

BREAKTHROUGH housing systems. Four samples of steel faced, paper honeycomb,

sandwich panel material and four prototype roof panels were evaluated.

The samples of sandwich panel material were used to evaluate the

variability of panel material properties and the effect of aging on tensile

and shear strengths. The roof panels were used to determine the probable

behavior in service considering the effects of adverse environmental conditions

— Assuming a normal distribution, the requirement that structures be designed

for an overcapacity of (1 + l.Sv) times the required capacity would mean

that approximately 95 percent of that population of structures would have

at least the factored load capacity.

33
on ultimate strength and mode of failure. In addition, the performance

of one panel under sustained loadinj^ was evaluated.

The following conclusions can be made from the test results:-

1. The roof panels, when loaded uniformly, will fail

in flexure rather than in shear even when the core

is damp.

2. The uniformly loaded roof panels failed suddenly

by local buckling of the compressive skin and

exhibited very little ductility.

3. The flatwise tensile strength of the sandwich panel

material is a controlling factor in the strength of

the roof panels.

4. Aging of the sandwich material reduced the flatwise

tensile strength about 40 percent in the prototype

panel specimens (Sample C) , but only 24 percent in

the production panel specimens (Sample D) . This

difference in the aging effect is attributed to the

disruption of the adhesive film, wliich offers some protection to the metal facing, in the prototype panel.

5. The coefficient of variability for the tensile

strength of the small specimens was 41 percent.

This includes variability in the adhesive bond as effected by fabrication techniques,

6. The maximum uniform load which a wet and aged roof

panel should be expected to support is 49 psf.

7. A wet and aged roof panel will meet the sponsor's

recommendations for 26 psf live load using a variability of 0.41.

8. Defects in the wood edge members, such as large

knots, may affect the load capacity of the roof

panels

.

9. The long-term performance of the panel material

would be affected by the following quality control

items

a. Thickness of adhesive relative to the

condition of the edges of the paper honeycomb core.

If the edges of the core has been roughened by

sanding or some other method prior to lamination

the adhesive may have to be thicker in order to

bond to the solid portion of the paper.

34
b. Relative movoinent between facing and core during, or after, lamination. Tbis movement will

"squeegee" the adhesive away from one side of the core cell edge.

c. The bond between the primer and the basic

steel sheet.

d. Difference in thickness between two pieces

of core used in the same panel. The thicker piece

prevents good contact of the thinner piece with the

adhesive

.

References

1. Cullen, W. C, Solar Heating Radiative Cooling and

Thermal Movement - Their Effect on Built-Up Roofing,

Technical Note 231, National Bureau of Standards,

Washington, D. C, December 1963.

2. Sharpe, L. H. , Some Aspects of the Permanence of Adhesive

Joints, Applied Polymer Symposia, No. 3, Pg. 353-359, 1966.

3. 1971 Annual Book of Standards, Part 16, American Society

for Testing and Materials, Philadelphia, Pennsylvania.

(Test methods C 297, C 393, C 481 are included in this

book .

)

4. Jenkinson, P. M. , Effect of Core Thickness and Moisture

Content on Mechanical Properties of Two Resin-Treated

Paper Honeycomb Cores, U. S. Forest Service Research

Paper FPL 35, Forest Products Laboratory, Madison,

Wisconsin, September 1965.

35

FORM NBS-1 14A (1-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. 2. Gov't Accession

No. EBS BSS-51

3. Recipient's Accession No.

111 Lt A1\U OU Dill Ltl

Structural Evaluation of Steel Faced Sandwich Panels

5. Publication Date April 1974

6. Performing Organization Code

7. AUTHOR(S)

J. H. Pielert, T. W. Reichard, L. W. Masters

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/ Work Unit No. 4213450

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Department of Housing and Urban Development

Washington, D.C. 20410

13. Type of Report & Period Covered Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card No.: jK-^OS^

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) A series of structural evaluation tests performed on components and materials intended

for use in one of the Operation BREAKTHROUGH housing systems is described. Four samples

of steel faced, paper honeycomb, sandwich panel material and four full size prototype

roof panels were evaluated.

The samples of sandwich panel material were used to evaluate the variability of panel

material properties and the effect of aging on tensile and shear strength. The roof

panels were used to determine the behavior in service considering the effects of

adverse environmental conditions on ultimate strength and mode of failure. In addition,

the performance of one panel under sustained loading was evaluated.

17, KEY WORDS (Alphabetical order, separated by semicolons) Accelerated aging; adhesive bond; ductility; flexural shear; housing systems; local buckling; material variability; moisture condi- tioning; Operation BREAKTHROUGH; paper honeycomb; structural sandwich; sustained load

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