TEE RIGIDITY AND STRENvInformation Reviewed and ReaffirmedLOAN COPYUNI - PDF document

TEE RIGIDITY AND STRENvInformation Reviewed and ReaffirmedLOAN COPYUNITED STATES DEPARTMENT OF AGRICULTUREIn Cooperation with the University of Wisconsin THE RIGIDITY AND STRENGTH OF FRAME WALLSThe purpose of the study reported here was to obtain a better understandingand appreciation of the principles involved in wall construction, which tendto make frame dwellings and other small frame buildings substantial struc-should also help prospective home owners to appreciate what they should in-A house has comparatively few main parts. Of these parts, the ones thatRept. No. 896-1- determine the amount of end thrust various types of wall construction arecapable of withstanding panels approximating in size the side of a room,Test Material and ProcedureTest PanelsPanels 9 by 14 feet were selected as representative of story heights and ofto 7 feet 4 inches high by 12 feet 1-5/8 inches long.All panel frames consisted of 2-by 4-inch upper and lower plates and 2-by4-inch studs spaced 16 inches, except at the ends of the 14-foot panelswhere the spacing was 12 inches from the outside edge of the end post to3/8 inch to which a third 2 by 4 was nailed with itsoutside edges flush, this left a nailing ledge for lath. The frame was puting boards were square-edged and 8 inches wide. Sheathing and framing wereat the Laboratory direct from southern yellow pine logs. One joint was used.4 feet long, spaced 1/4 inch and nailed with 3d nails.3/4 inch. The plaster, which was of good quality, was mixed175 pounds of sand.The lower, or sole plate, of each panel was bolted and in some cases boltedwas further anchored against thrust by a stirrup from the heavy timberbolted or bolted and nailed to a heavy timber, which furnished the resistanceRept. No. 896 to lateral buckling always supplied to the walls by the upper floor system.duce an overturning tendency, which normally would be prevented by the upperat one end to the base of the testing.machine and at the other to a bearingplate on top of the heavy timber to which the upper plate of the panels wereings to provide for free longitudinal movement of the panel. The hold-downwas observed for various increments of load. Figure 1 is a diagrammaticsketch of the panel setup. The apparatus for measuring deflection con-which was stretched a fine wire. A steel scale was fastened to the upperflections were read by observing with a telescope the movement of the scaleof dead weight distributed along the upper plate, produced deflections inthe panel of 50 to 75 percent of its deflection at elastic limit.Discussion The results of nearly 50 tests of large wall panels are given in tables 1description of each panel appears in the tables together with its stiffnessnails in each board at each stud crossing was obtained by averaging the the upper plate with respect to the lower plate is about the same for each5 falls along the base5 curve are divided by 4.3. It is to be noted that while thewas obtained by direct comparison of maximum loads, is over 8.Consequently, this factor is an increasing one. When the factor is a chang-sponds to the factor for a 1/2-inch deflection, has been taken as the stiff-3C. The effectiveness3.0. A good3.6 is finally reached. On the other hand, panel No. 34 has a decreasingfactor. A good average value out to a 1-inch deflection is 4.2, whereas themaximum load factor is 3.5. Thus there are three types of reduced curves,crease over the base curve, and one that shows a gradual drop from the basesudden and pronounced drop in load at a movement of the upper plate of from3/4 of an inch to 1-1/2 inches, depending upon the member, size, and arrange-ment of the, openings. The load at which this drop occurred was taken as thebut no pronounced drop in load occurred until the upper plate had a movementof from 2 to 4 inches. The load at which this drop occurred was taken asthe maximum load. In the case of panels covered with sheathing only, a load4 inches, which either held fairlyparticularly it is known that a higher maximum would be obtained when theno practical significance because of the enormous distortion that accompaniesfactor is given as over 8.Rept. No. 896-4- Horizontal Sheating (Panels Without Openings) tudinal thrust is due to the reaction couples of each pair of nails at thetively large distortions, since relatively large movements of the upper9 feet high by 14 feet long, sheathed horizontally with square-edged,crossing, sustained an average maximum load of 2,588 pounds applied to the3 inches.the nails become far more effective in offering resistance to distortion.The boards may be either in compression or in tension, according to thehorizontal sheathing for the panels unpierced by openings and averaged overthe giving of the plate was reflected in the horizontal movement of the upperless effectiveness than when the boards were in compression. In actualWhen openings were framed into the panels, the relations were somewhat-5- At the recorded maximum load, diagonal sheathing boards in compressionFigure 3, A, is a photograph of a diagonally sheathed test panel. The hold-Bracing (Panels Without Openings) In order to make horizontal sheathing more rigid, various forms of bracing3, B), cut-in between the studs aboutmidway between the lower and upper plates, was found to increase the stiff-4 inches in cross sectionand were nailed at an angle of about 22-1/2° to the horizontal.4 inches, cut-in between the studs on a line (fig. 3, C) in-6o percent and increasedthe strength 40 percent. The percentage increase is dependent upon the anglethat the bracing line makes with the plate. Had these braces been closer to45° with the plates, the stiffness would have been somewhat greater. As3, C, the left brace is in compression and its effectiveness is dependent3, D and E, show nominal 1-by 4-inch strips let into the3, D, is incompression and the other in tension when a longitudinal thrust is applied3, E, the stiffness factor was increased to about 4.The maximum load for the panels shown in figure 3, D and E, was about threeNumber and Size of Nails (Panels Without Openings) 896 approximately one-third. There is a slight increase in the friction between40 percent more than that for panels with either'two or three nailsin their lateral resistance to the loads applied to the panels. When either50 percent greater than40 percent.70 percent.End- and Side-Matched Sheathing (Panels Without Openings) end- and side-matched stock was the same as that of a panel sheathed withof boards and hence the number of couples is increased in about the sameWood Lath and Plaster (Panels Without Openings) It was found that lath and plaster produced a more rigid panel than did anying, there was very little increase in rigidity, but a considerable increaseabout the same for all cases. When the first plaster crack appeared, the4, A. Load was applied at the upper left-hand corner of this panelThe plaster sheet itself is extremely rigid as regards longitudinal thrust.-7- Table 1 also gives the results for a panel (No. 25) sheathed horizontallysheathed with well-seasoned lumber.throw that produced a deflection of 50 to 75 percent of the deflection atlumber, was vibrated 1,000,000 cycles after 1 month's seasoning. Panel No.Two panels horizontally sheathed and two diagonally sheathed were made ofVery often the sheathing on a house may be subjected to the weather for along period, during which time there is considerable working of the nails30 in table 1 ) was made of8, table 2),4 to approximately 3 and the8 to 5. Let-in braces, arranged around a window3, F, gave a stiffness factor of 3 when a load wasapplied at the upper right-hand corner.horizontal sheathing dropped to about three-quarters of the stiffness andFigure 3, G, shows a diagonally sheathed panel with a window and door in it.896 therefore, were in tension. From a factor of nearly 4, the stiffness drop-ped to 1.4 with this arrangement and the strength from over 8 to 4. It wasdouble window studs threw considerable bending in window studs. This, ofcourse, reduced the stiffness of the panel as a whole. With the positionH, thehand corner. The panel shown in figure 3, H, had a stiffness factor of1.3. Six-inch bevel siding applied to thistype of panel with 7d box nails raised the stiffness factor for the diagonal5.4, respectively. Horizontally sheathed3, I, gave a stiffness factor of 1.5 and a strength factor of 2.2.for horizontal sheathing, but its strength factor was only slightly morefactor to 2.4 and 2.8, respectively, and the strength factors to 2.2 and 4.4.Plaster with horizontal sheathing and let-in braces arranged around the open-ings as shown in figure 3, I, gave a stiffness factor of 4.1, which waswhich was somewhat smaller than that for the diagonal sheathing.Due to concentration of stress at the corners of the openings, plaster cracksdeveloped at relatively small loads as compared with the loads causing thethe first crack occurred between 800 to 1,500 pounds. The upper plate hadas with the unpierced panel this movement amounted to 3/8 inch or more.4, B, is a photograph of panel No. 23 taken after test. Load wasSignificance Plaster on wood lath may furnish all the rigidity necessary for most pur-896 itself through diminishing, if not entirely eliminating, needless annoyancesrigidity than horizontal sheathing without bracing. Either diagonal ormore rigidity than was needed perhaps, but the hundreds of old houses stillstanding bear witness to the fact that rigidity went hand in hand with per-of the heavy type of construction employed then. The modern adaptation ofing or the present-day balloon frame with the studs carrying through for twostories represent a great economy of material over the old style of bracedexpense of material than the balloon type. Good bracing and horizontalmembers may cause buckling of the sheathing if diagonal sheathing is used.Diagonal sheathing or good bracing with horizontal sheathing will still givetain fundamental principles should be kept in mind so that when constructionmethods are employed to reduce costs the methods will be such that willComparison of Fiberboard with Diagonal Wood Sheathing3 inches on centers at all vertical edges, 6 inches on centers on896 The results3.Diagonally sheathed panels tested without siding tend to bend the studs,3 for walls with door and window openings.out siding that the rigidity factor is nearly doubled and the maximum loadfactor more than doubled by addition'of the siding, when the sheathing isstressed in compression, which, as indicated by table 3 is the weaker di-84 percent the rigidity factor and 64 percent the maximum load factor ofFrom the partial summary for wall panels without openings given in table 4,it may be computed that those sheathed with fiberboard had 86 percent the47 percent the maximum1entitled "Rigidity and Strength of Frame Walls Sheathed1937.Rept. No. 896-11-.4-18 :s;. 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