BREAKAWAY WALLS. The breakaway wall investigated here is a common stud framed wall consisting of dimension lumber with plywood sheathing. A typical ...
Dynamically loaded light-frame wood stud wallsExperimental verification of the analytical model Michael Collins1 and Bo Kasal2 ABSTRACT Light-frame wood structures may deform well beyond the elastic limit when loaded by dynamic forces such as earthquakes and sea wave impacts. A finite-element model of a light frame wood wall was developed to simulate such response. The finite-element model includes nonlinear connection properties, elastic constitutive laws of wood material, large deformations and inertia forces. The finite element wall model was subjected to an impact load imparted by a rapidly moving sea wave. To verify this model, a wave-channel experiment of a full-scale wall was conducted. The wall was instrumented with reaction load cells, displacement transducers and strain gauges on plywood sheathing and framing. A closed-loop hydraulic system generated the time varying loading function. The resulting reactions, deformations, and strains were recorded as functions of time. High-speed cameras monitored failure modes and the behavior of the wall. Comparison of experimental and analytical results yielded reasonable agreement. The analytical model captured observed failure modes, including rigid-body motions after connection failures. The model may be used to analyze similar nonlinear systems loaded well beyond the elastic limit. INTRODUCTION Buildings located in regions defined as high hazard coastal areas that are prone to flooding are required by the Federal Emergency Management Agency (FEMA, Coastal Construction Manual 1986) to be constructed on open foundations such as piles or columns. The loads imparted by flooding and wave attack to these types of foundations are generally much smaller than the loads transmitted to solid foundations. Under the National Flood Insurance Program (NFIP) the area beneath the raised structure can be enclosed and used for storage, building access, and vehicle parking. However, the enclosure must be designed to “breakaway” at some design load level to prevent transfer of excessive loads to the foundation. The walls must therefore be designed to resist minimal load levels and yet fail at a larger predetermined load level. To accomplish this goal accurate prediction of the behavior of these walls is necessary. BREAKAWAY WALLS The breakaway wall investigated here is a common stud framed wall consisting of dimension lumber with plywood sheathing. A typical design wall attaches to the foundation or piles through the use of nailer plates that are securely attached to the foundation or floor beams. The connection between the nailer plate and the foundation is significantly stronger than the connection between the nailer plate and the wall. This design ensures that the wall separates from the nailer plates and maintains overall structural integrity. Breakaway walls are primarily intended to resist lateral loads such as wind and wave forces. Breakaway walls subjected to wave action encounter three different wave types, which create three different loading conditions. The three classifications of waves are: unbroken, broken, and breaking waves (Shore Protection Manual, 1984).Unbroken wave loading was used in this investigation due to the difficulties in measuring and specifying time and spatial pressure distribution associated with the other two wave types.
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Graduate Student, Dept. of Civil Engineering, North Carolina State University, Raleigh, NC. Assoc. Professor, Dept. of Wood and Paper Science, Dept of Civil Engineering, North Carolina State University, Raleigh, NC.
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WAVE FORCES In order to verify the behavior of breakaway walls loading functions must be known. The unbroken oscillatory wave is the easiest to describe analytically and can be approximated by linear wave theory, except in shallow water. The Shore Protection Manual (1984) suggests that the pressure profile generated by an unbroken wave crest at the wall face varies linearly with depth. The manual does not fully describe the entire time history of the loading. However, it contains enough information to assume that the change in pressure from the hydrostatic pressure is a linear function of the wave height. The resulting reactions from the finite element model were compared with the wave channel test results and the loading functions were accordingly scaled. EXPERIMENT The investigation and analytical verification of the behavior of breakaway walls under the action of oscillatory wave force is a part of a larger project, which included several configurations of breakaway walls and wave types. The walls were tested at the Oregon State University wave channel facility where a closed loop hydraulic system generated the waves. Wave gauges were installed along the length of the channel and on the face of the wall. The wave channel setup used for the test is shown in Fig. 1.
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Figure 1 Wave Channel Setup (Yeh, 1997) The test wall is a standard US “2x4” construction (39 x 89 mm). The studs were placed 61 cm (24 in) on center and sheathed with plywood 12.7 mm (0.5 in) thick. The stud framing was fastened with two 16d (d=4.1 mm, l=89 mm, d= diameter, l= length) nails, end nailed. The plywood was fastened with 10d nails (d=3.8 mm, l=76 mm) located 15.2 cm (6 in) on center around the perimeter of each sheet and 30.4 cm (12 in.) within the field of the sheet. Prior to testing, the modulus of elasticity for each stud and plywood sheet was determined. In order to verify the behavior of the wall, the wall was instrumented with six strain gauges on the studs and two rosettes on the plywood as shown on Fig 2. A displacement transducer was also attached at the geometric centroid of the wall on the stud side to measure deflections. The wall was fastened to a test frame and instrumented as shown in Fig. 3. The wall’s sole plate was securely screwed to the test frame
to ensure that the failure would occur within the breakaway wall. The instrumented wall was subjected to eight different unbroken wave configurations. Variables included the wave height, wavelength, wave period, and still water level.
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Figure 3 Test Frame Configuration (wall profile) ANALYTICAL MODEL The model of the breakaway wall was developed using Ansys (Swanson Analysis Systems,2000), a commercial finite element package. The model used elastic shell elements for the plywood and framing and nonlinear, non-conservative
spring elements for nail. The model used three nonlinear springs for every nail; one for each principal direction. The spring properties were obtained from nail tests by Phillips (1990). Fig. 4 gives the finite element mesh of the wall model.
Figure 4 Finite Element Model of Breakaway Wall In the wave channel experiment for the instrumented wall tests, unbroken wave loads were used. Two of the unbroken wave loading cases are used here for verification. Both waves have a period of 7.62 s and a wavelength of 27.8 m (91.2 ft). The height of the first and the second wave trains were 45.7 cm (18 in) and 61 cm (24 in) respectively. During the test, the same wall was subjected to both loading conditions. However, this analysis did not take into account the previous load history from the different wave configurations. Neglecting the prior load history assumes that the wall remained elastic during loading. However, this investigation did perform a time history analysis for the selected wave loadings. In addition, the standard nonlinear spring element in Ansys does not account for strength degradation correctly when the spring undergoes unloading. RESULTS The first loading condition was chosen because it produced a sizable wall response that remained within the elastic range. The second loading condition was chosen to verify the model’s ability to capture the wall’s behavior in the inelastic range and at failure. The comparison of the strains for the first case is given in Fig. 5. For clarity, the graph only shows the results from two of the lower strain gauges (gauges 2 and 8). However, the other strain gauges yielded similar results. The first thirty seconds of the test were analyzed in order to demonstrate the veracity of the model. The reactions from the bottom of the wall are compared in Fig. 6. The second loading condition generated a nonlinear response of the wall and resulted in a failure in the stud to bottom plate connection. The model captured the separation at this location at the same time. Again, the analysis covered the first thirty seconds of the test to minimize computational time. The strains and reactions are plotted in Fig. 7 and Fig. 8, respectively. Analytical results were obtained using two different loading functions, first a less accurate one for the period from 15 sec to 22 sec followed by a more accurate one for the period from 22 sec to 30 sec. The differences in the two responses around the two peaks in these figures illustrate that the model reliably reproduces the actual behavior given the correct loading function. The analytical results also indicate that in the first seven sets of tests, the wall responded in the elastic range. Had the system responded in a non-conservative manner, it would then have a reduced capacity during later tests. As mentioned previously, the current model does not have degradation included and would not match the test data very well if degradation had occurred. Thus it appears the wall remained in the elastic region during the previous tests.
CONCLUSIONS The nonlinear model with elastic members and nonlinear, non-conservative connections simulates well the behavior of a light-frame stud wall loaded by a cyclic pressure. The model accurately captures the failure mode, deformations, strains, and reaction forces. Further improvements will include plasticity and load- history dependent constitutive formulations to model the connections. Analysis of wall response to other types of waves (such as broken and breaking waves) using the model discussed above can be conducted providing that a correct loading function is known. In order to analyze breakaway walls for other wave types more research is needed to quantify time and spatial distribution of a pressure exerted by breaking and broken waves on a non-rigid wall. ACKNOWLEDGEMENTS Federal Emergency Management Agency and the National Science Foundation sponsored this research through grants to North Carolina State University and Oregon State University. The results reported in this paper do not necessarily reflect the views of the sponsoring agencies. REFERENCES Phillips,T.L. (1990).”Load Sharing Characteristics of Three-Dimensional Wood Diaphragms,” MS thesis, Washington State University, Pullman,WA. Shore Protection Manual, 4th Ed.,Vol.II.(1984).Corps of Engrs., Department of the Army,Waterways Experiment Station, Vicksburg, MS. Swanson Analysis Systems, Inc., www.ansys.com. Houston, PA 2000. Yeh,S.H. (1997).”Behavior of Breakaway Wall,” PhD thesis, North Carolina State University, Raleigh N.C.
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Figure 5 Comparison of Analytical and Experimental Strains-Loading Condition 1 Breakaway Wall Loaded by an Unbroken Wave (T=period, L=wavelength, h=wave height) 20.0 Analytical 15.0
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Time (s) Figure 6 Comparison of Reaction Forces of a breakaway wall—Loading Condition1 Breakaway Wall Loaded by an Unbroken Wave (T=period, L=wavelength, h=wave height
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Time (s) Figure 7 Comparison of Analytical and Experimental Strains -Loading Condition1 Breakaway Wall Loaded by an Unbroken Wave (T=period, L= wave length, h = wave height) 25.0
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Time (s) Figure 8 Comparison of Analytical and Experimental Reactions -Loading Condition1 Breakaway Wall Loaded by an Unbroken Wave (T=period, L= wave length, h = wave height)
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