Fowler, David W.

Houston, James T.

Johnson, F. B.

English

Missouri University of Science and Technology (Missouri S&T): Scholars' Mine

Missouri University of Science and Technology Scholars' Mine International Symposia on Low Cost Housing Problems Civil, Architectural and Environmental Engineering Conferences 26 Apr 1972 Structural Behavior of Wall Panels for Lower Cost Housing David W. Fowler James T. Houston F. B. Johnson Follow this and additional works at: https://scholarsmine.mst.edu/islchp  Part of the Civil Engineering Commons Recommended Citation Fowler, David W.; Houston, James T.; and Johnson, F. B., "Structural Behavior of Wall Panels for Lower Cost Housing" (1972). International Symposia on Low Cost Housing Problems. 63. https://scholarsmine.mst.edu/islchp/63 This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Symposia on Low Cost Housing Problems by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu. STRUCTURAL BEHAVIOR OF WALL PANELS FOR LOWER COST HOUSINGbyDavid W. Fowler,* James T. Houston,* and F. B. Johnson*The quest for lower cost housing has witnessed the use of countless construction techniques ranging from conventional on­site construction to completely prefabricated modules. While labor costs, labor shortages and quality control are largely re ­sponsible for the growing trend away from on-site construction, prefabricated modular construction has also been subject to limitations. Between these extremes, panelized construction utilizing prefabricated panels represents a logical and practical compromise for many building systems.Panels vary widely according to size, construction and ma­terials . While modular widths of two to four feet are the most common, some builders use full width panels. Some panels are constructed using conventional stud construction while other uti­lize sandwich or stressed-skin design made of a wide variety of core and facing materials.This paper summarizes the structural performance of stressed skin and sandwich panels used or proposed for use in lower cost housing. Several of the panels were used in the Austin Oaks Low Cost Housing Development Project. (1) All of the panels had a nominal length of eight feet and a width of four feet or less.TYPES OF PANELSThree basic types of panels were tested in the program: asbestos-cement post and sheet panels, aluminum-skin paper- honeycomb panels, and plywood skin panels with wood perimeter members and polystyrene cores. A summary of the panel de­scriptions is given in Table 1.Asbestos-Cement PanelsAsbestos-cement is a construction material that has been in use for many years. It has excellent durability, fire resistance, and compressive strength. Its compression and tensile strength, several times higher than concrete, are due to the asbestos fiber reinforcement. Asbestos-cement has been used extensively forTABLE 1; SUMMARY OF FLEXURAL TESTSPanel Thickness,in.CompressionFaceTensionT1 3-1/2 Asbestos-cement Asbestos-cementT2 3-1/2 Asbestos-cement Asbestos-cementT3 3 0.030 in. Aluminum 0.030 in. AluminumT4 3 0. 030 in. Aluminum 0. 030 in. AluminumT5 1-3/4 0. 030 in. Aluminum 0.030 in. AluminumT6 1-3/4 0.030 in. Aluminum 0. 030 in. AluminumT7 2-1/8 3/8 in. Douglas Fir 1/4 in. MahoganyPlywood PlywoodT8 2-1/8 3/8 in. Striated l /4  in. MahoganyCedar Plywood PlywoodT9 2-1/8 3/8 in. Striated 1/4 in. MahoganyDouglas Fir PlywoodT10 2-1/8 l / 4 in. Mahogany 3/8 in. AsbestosPlywood FiberboardT i l 2-1/8 3/8 in. Asbestos 1/4 in. MahoganyFiberboard PlywoodT12 2-1/8 3/8 in. Tempered 1/4 in. MahoganyHardboard PlywoodT13 2-1/8 l /4  in. Mahogany 3/8 in. TemperedPlywood HardboardT14 2-1/16 Aluminum-Clad 1/4 in. Mahogany5/16 in. Douglas PlywoodFir PlywoodT15 2-1/16 1/4 in. Mahogany Aluminum-CladPlywood 5/16 in. DouglasFir Plywood♦Department of Civil Engineering, University of Texas at Austin.housing in Europe, Puerto Rico, Latin Am erica, and South Am eri­ca but has witnessed only limited use in the U.S. housing industry.A section of panelized system utilizing asbestos-cement is shown in Figure 1. The system consists of hollow extruded as­bestos-cement columns on 16 inch center connected by two 3/8 inch asbestos-cement panels. The system is not a true prefab­ricated panel since the panels are bonded to columns with ad­hesive after the columns have been attached to the slab with steel angles. A horizontal steel rod is used in the top of the panel to tie the wall together. Fiberglass batt insulation is placed in the cavity.16"Fig. 1. Cross Section o f Asbestos-Cement PanelAluminum Sandwich PanelAluminum has been a primary aircraft construction material for years, but thus far has found limited use in housing. Its ex­cellent corrosion resistance and high strength-to-weight ratio have encouraged several manufacturers to produce panels with structural aluminum skins with lightweight cores.The panels used in this test program consisted of nominal0.030 inch pebble -texture prefinished aluminum skins with a resin-impregnated honeycomb paper core . A phenolic resin was applied separately to the core and the aluminum skins, and after the panel was assembled, heat was used to activate the adhesive. Exterior wall and roof panels were three inches thick while in­terior panels were 1-3/4 inches thick. All panels were 48 inches wide.An aluminum channel attached to the floor is used to hold the panel; one-eighth inch pop rivets attach the panel to the channel.A vinyl lock-strip provides the connection between adjacent panels.Plywood Stressed-Skin PanelsPlywood stressed-skin and plywood sandwich panels have been in use for many years in the aircraft and construction industries. The development of low density cellular plastic core materials and improved adhesives coupled with the trend toward prefabrica­tion have witnessed the development of many types of plywood­faced panels.Figure 2 illustrates the construction of the panels used in this test program. Using both wood stringers and polystyrene for the core, the panels are actually a combination of stressed skin and sandwich construction. Both plywood skins are bonded to the wood preimeter members and the polystyrene core to produce a panel four feet by eight feet by approximately 2 -1 /8  inches thick. The interior skin in all cases was prefinished Philippine mahogany plywood. The exterior skins used were: sanded Douglas fir  ply­wood; Douglas fir plywood; V-grooved striated cedar plywood; asbestos fiberboard; and tempered hardboard. The thicknesses were 3/8 inch in all cases except the aluminum-bonded plywood which was 5/16 inches thick.392 x 4  Grooved StringerExterior Facing Material (Varies) 2 x 4  Tongued f  y  Stringer|V2 Expanded Polystyrene/4  Philippine Mahogany Panelingzy.l'io 2 VCross-Sectional ViewFront View(Top Skin Omitted For Clarity) Fig. 2 . Section and Elevation o f Stressed-Skin PanelThe panels are connected together by the tongue-and-groove joint. An aluminum batten strip is used to weather-proof the joint.FLEXURAL BEHAVIORThe flexure tests were performed in accordance with ASTM E-72-61 except as otherwise noted. Panels T7 through T13 were tested with a span of seven feet; all other panels had a span of seven and one-half feet. At least two panels of each type or thick­ness were tested except in the case of three of the plywood panels (P7, P8, P9), in which case the exterior skins differed only slightly in strength. The behavior of each type panel will be discussed briefly. The results are tabulated in Table 2. The ultimate unit load has been reduced to correspond to an eight foot span for a typical wall. The factor of safety is given for a 20 psf superim­posed lateral load. The deflection, A, and the deflection-to-span ratio, A /L , are also for an eight foot panel height and a 20 psf load.TABLE 2: FLEXURAL TEST RESULTS1Panel UltimateloadpsfFactorofSafetyA,in.A /LT1 114 5.7 0.062 1/1550T2 111 5.6 0.086 1/1110T3 88 4.4 0. £09 1/459T4 112 5.6 0. 216 1/444T5 31 1.6 0.133 1/722T6 28 1. 4 0.184 1/522T7 169 8.4 0.373 1/257T8 144 7. 2 0.373 1/257T9 165 8. 2 0.373 1/257T10 96 4.8 0.328 1/293T il 196 9. 8 0.396 1/242T12 152 7.6 0.381 1/252T13 141 7.0 0.366 1/262T14 112 5.6 0.394 1/244T15 122 6.1 0.364 1/264All results have been adjusted to correspond to eight foot panels; factor of safety and deflections are based on 20 psf loading.Asbestos-Cement PanelsThe asbestos-cement panels were tested using one repetitive unit, 16 inches wide, consisting of a half-column section on each side of the two flat sheets instead of the 48 inch width specified by ASTM. The asbestos-cement panels exhibited a nearly linear load-deflection response to failure. In both panels, a sudden tension failure occurred on the bottom face between the quarter- point loads.The moduli of elasticity were found by test to be 3, 000,000 psi in compression and 3,400,000 psi in tension. The ultimate compressive stress was 13,600 psi; the ultimate tensile stress was 1,980 psi. It was determined that the calculated ultimate load could be very closely predicted using: (1) elastic theory;(2) a neutral axis location found by assuming the compression modulus for the entire session; and (3) an effective flange width from the face of the column equal to four times the panel thick­nesses. Good agreement was also obtained for deflection response using the same assumptions. The panel proved to be by far the stiffest of all the panels tested with an average A /L  ratio of less than 1/1200. The factor of safety for a 20 psf load was in excess of 5 for an eight foot panel.Aluminum Sandwich PanelTwo 3 inch panels and two 1-3/4 inch panels were tested to failure. After panel T3 had failed in buckling due to stress be­tween the reaction and the quarter-point load, the other three panels (T4, T5, and T6) were tested with four uniformly spaced live loads instead of the usual two as specified by ASTM. This was done to spread the applied load as much as possible to lower the stress concentrations in the thin aluminum skin at interior load points. The oilier 3 inch panel, T4, also failed by shear buckling. The 1-3/4 inch panels both failed by flexural buckling of the compression skin near the midspan.All four panels exhibited essentially an elastic load-deflection response to failure. The deflection of the panels, with the ex­ception of T4, was predicted within 20 percent using previously proposed theory considering both shear and flexural deflections.(2) From tests, the elastic modulus of the aluminum was found to be 10, 350,000 psi, and the shear modulus of the core was de­termined to be 1,710 psi for the 3 inch panel and 327 psi for the1-  3/4 inch panel. The deflections for a 20 psf superimposed load were quite low, and the factor of safety against failure was in excess of four for the 3 inch panel; for the partition panels, the factor of safety averaged 1.5.Plywood PanelsThe plywood panels exhibited considerably greater deflection than the other two types of panels, but still less than l/240th of the span for the 20 psf service load. With the exception of the aluminum-clad plywood, the panels exhibited a nearly linear load- deflection response, with a slight reduction in stiffness in the load range. The aluminum-clad plywood panels exhibited greater non­linearity near the maximum load. In fact, the load-deflection curve for T14 became practically horizontal before failure finally occurred. All panels except T10, with the relatively brittle as­bestos-cement fiberboard on the tension side, deflected at least2- 1/2 inches before failure occurred; panel T10 deflected 1.6 inches.The plywood properties used for calculating deflections were those recommended by the American Plywood Association (3) since these are the values used by the design engineer. For the non-plywood skins, properties were found from tests. For the asbestos fiberboard the properties were: compressive and tensile moduli, 450, 000 psi and 613, 000 psi respectively; ultimate com­pressive and tensile strength, 3000 psi and 1130 psi. The tem­pered hardboard possessed compressive and tensile moduli of 455,000 psi and 618, 000 psi; ultimate compressive and tensile strengths were 5000 psi and 2600 psi respectively.The mode of failure varied. Shear failure in the perimeter framing members occurred in T7, T9, T12, T13, and T15. The bond failed between the top skin and framing member in T8 and T14, although T14 continued to deflect at constant load. Panel40T8 also developed cracks in both faces. Tension failures occurred in the skins of T10 and T i l .The predicted deflections were calculated using the theory for plywood stressed-skin panels (4) with the exception that no reduc­tion in effective skin width was assumed since the core provided support as in sandwich panels. (5) For the panels T7, T9, T14, and T15, the calculated deflections for a 20psf superimposed load were approximately 20 percent less than the observed values; the deflections for panel T8 were in excellent agreement. However, the theoretical deflections were larger than the observed values for the panels with asbestos-cement fiberboard and hardboard facings. At the 20 psf service load, the deflection-to-span ratios were nearly the same for all panels ranging from 1/242 to 1/293. The factor of safety for service loads was in excess of four for all panels.COMPRESSION TESTSThe compression test specimens each had a height of eight feet with the same width as the flexural panels. The panels were tested in accordance with A£TTM E-72-61.The asbestos-cement panel carried a total axial load of 28,900 pounds. The maximum deflection was 0.332 inches.Failure was due to a rupture in the tension face.The aluminum sandwich panel developed a load of 3750 pounds before local buckling of the facing occurred at one end of the panel. The maximum deflection was observed to be negligible.The three aluminum-clad plywood stressed-skin panels tested developed an average ultimate load of 20, 200 pounds before elastic buckling occurred. The lowest ultimate load was 16,340 pounds. The maximum deflections ranged from 0.8 to 1.5 inches.INPACT TESTSImpact resistance was measured using a falling ball test very similar to ASTM D1037. The 2 inch steel ball was dropped from an increasing height on the specimen until a visible crack appeared on the upper surface or the maximum height of 80 inches had been reached. The maximum drop height, permanent set at last drop, and mode of failure are recorded in Table 3. The asbestos-cement panel was the most brittle and failed at a height of only 30 inches. The two plywood specimens did not rupture. The aluminum-clad plywood had a greater permanent set since the aluminum surface recovered less than the mahogany plywood.The 3 inch aluminum panel failed whereas the 1-3 /4 inch panel did not. Apparently the thinner panel had more compressive stiffness due to the shorter buckling length of the core material.TABLE 3: RESULTS OF IMPACT TESTSPanel MaximumDropHeight, in.Permanent Set, in.Mode of FailureAsbestos-cement 30 0.005 PunchThrough1/4 in. Mahogany Face of Stressed skin Panel 76 0.031 NoneAluminum-clad plywoodface of stressed skin Panel 72 0.095 None1-3/4 in. Aluminum Sandwich Panel 72 0.595 None3 in. Aluminum Sandwich Panel 50 0.425 Split in skinPANEL COSTSThe relative costs of the panels, including erection but no allowance for finishing of unfinished surfaces, are given in Table 4. The costs are given for sake of comparison only and may vary widely from one locale to another. It should be noted that maintenance costs and insurance rates will vary and can have a significant effect on costs.POTENTIAL OF PANELSAll of the panels tested in flexure and compression indicated a satisfactory factor of safety and stiffness. Panels T5 and T6 had a factor of safety less than two in flexure, but should be ade­quate as interior partitions. In compression the panels are capable of carrying typical roof spans with an adequate factor of safety.The asbestos-cement panels tested represent a structurally sound but inefficient system. Asbestos-cement is a material with enormous, but as yet untapped, potential for use in prefabricated building components. To be used economically, however, mono­lithic cellular panels are dictated to reduce labor and material (including adhesive) costs.The aluminum sandwich panels proved to be structurally ade­quate. However, thermal and acoustic deficiencies seriously impair their suitability for use in housing. Polyurethane low- density foam is being applied to each face of the honeycomb core in an attempt to aUeviate the thermal problem. The exterior skins are subject to damage by hail.The plywood stressed-skin panels have been used for several years in the southwest primarily with the aluminum-clad plywood exterior skin. The panels have more than adequate strength, can be furnished in a variety of exterior finishes, and possess a warmth not found in other materials. The primary disadvantages are maintenance and lack of fire resistance for the all-wood panels.TABLE 4: SUMMARY OF PANEL COSTSPanel Relative Cost,$/sq. ft.Asbestos-Cement 2.043 in. Aluminum Sandwich 1.363/8 in. Douglas Fir 1.303/8 in. Striated Douglas Fir 1.303/8 in. V-grooved striated cedar 1.383/8 in. Asbestos-Cement 1.593/8 in. Tempered Hardboard 1.385/16 in. Aluminum-Clad Douglas Fir 1.57REFERENCES1. Johnson, Franklin B ., J . Neils Thompson, Robert I.C arr,J r ., and David W. Fowler, “ Austin Oaks 68” , Proceedings of the International Symposium on Low Cost Housing Prob­lems Related to Urban Renewal and Development, Civil Engineering Dept., University of Missouri-Rolla, October 8, 9, 1970.2. Weidlinger, Paul, Aluminum in Modern Architecture, H,New York: Reinhold Publishing Corporation, 1956.3. American Plywood Association, Plywood Design Specifications, Tacoma, Washington, 1970.4. American Plywood Association, Plywood Design Specifica­tions. Supplement No. 3, Design of Flat Plywood Stressed- Skin Panels, Tacoma, Washington, 1970.5. American Plywood Association, Plywood Design Specifica­tions, Supplement No. 4, Design of Flat Plywood Sandwich Panels, Tacoma, Washington, 1970.41

Structural Behavior of Wall Panels for Lower Cost Housing

https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=1062&amp;context=islchp

Structural Behavior of Wall Panels for Lower Cost Housing

Abstract

Similar works

Full text

Available Versions

Missouri University of Science and Technology (Missouri S&T): Scholars' Mine