Progress 12/01/00 to 11/30/04
Outputs
The objective of the investigation was to investigate the affect of superimposed dead load on the lateral load capacity of wood frame shear walls. The research included static and cyclic tests of wood shear walls, static and cyclic tests of sheathing-to-stud nailed connections, and nonlinear finite element modeling of the full wall. A series of ten static tests were conducted to determine the effect of a vertical load on the lateral load capacity of wood frame shear walls. Tests were performed on 2.4 m by 2.4 m walls, for varying vertical load, with and without hold-down anchors. The presence of vertical load on a wood frame shear wall was found to have a significant effect on the lateral load capacity of the wall. The ultimate load capacities of walls with hold-down anchors increased by 20% and 28% for walls with a vertical load of 12 kN/m and 25 kN/m, respectively, as compared to the wall without vertical load. Results also show that hold-down anchors do not add
significantly to the strength of the wall when a vertical load is present. Finally, the results presented suggest that current code specified allowable shear forces are conservative when a vertical load is present; the wall has additional reserve capacity than is reflected by current code specified allowable shears. A series of twenty-one cyclic tests were conducted to determine the effect of vertical load and hold-down anchors on the cyclic lateral load capacity/response of wood frame shear walls. Tests were again performed on 2.4 m by 2.4 m walls: seven different configurations were investigated including walls without hold-down anchors and a vertical load of 0, 6, 12 or 25 kN/m, and walls with hold-down anchors and a vertical load of 0, 12 or 25 kN/m. Results show that the effects of vertical on the cyclic response of wood frame shear walls are to (1) increase the lateral stiffness, and (2) increase the energy dissipation capacity. Results also show that hold-down anchors have no
effect on the stiffness and energy dissipation of the wall when the wall is subjected to the allowable vertical load. A series of tests were conducted on nailed sheathing-to-stud connections to study the effect of superimposed compressive stress on the load-slip behavior of the connection. Identical connections were tested under static monotonic load and cyclically, under varying levels of compressive stress. Results show that the initial stiffness and energy dissipation capacity of the connection increases slightly with the superimposed compressive stress, while the stiffness, energy dissipation and ultimate capacity all decrease slightly with the compressive stress at larger displacements. A detailed finite element model of a wood frame shear wall was developed using ANSYS. Static non-linear analyses of the wall were conducted for different vertical loads and different hold-down conditions. Results show that accurate modeling of the support conditions and the framing connection
details is important for capturing correctly the overall behavior of the wall. Results show good correlation between the finite element model and the experimental data.
Impacts
The wood frame industry is facing stiff competition from alternative building materials for low-rise construction. This includes steel, masonry, recycled products, and most recently, polymer and bio-based composites. If wood is to remain competitive, every effort must be made to gain the maximum benefit from the material at the least possible cost, while still achieving the design objectives. Current building codes in the United States do not consider the effect of vertical load in calculating the allowable lateral load of a wood frame shear wall. The study provides data which indicates that vertical has a beneficial effect on the lateral load capacity of the wall: the lateral strength increases with increasing vertical load. Code specified allowable shear forces could be increased to reflect the beneficial effect of vertical load, thereby, resulting in a more cost-efficient design of wood frame shear walls.
Publications
- Hite, M., and Shenton III, H.W., (2002) Modeling the Nonlinear Behavior of Wood Frame Shear Walls, Proceedings of the 15th ASCE Engineering Mechanics Conference, June 2-5, 2002, Columbia University, New York, NY.
- Johnson, A.R., Dean, P.K., and Shenton, H.W., Effects of Vertical Load and Hold Down Anchors on the Cyclic Response of Wood Framed Shear Walls, in review, ASCE Journal of Structural Engineering, Nov. 2004
- Dean, P.K. and Shenton, H.W., 2004, Experimental Investigation of the Effect of Vertical Load on the Capacity of Wood Shear Walls, in press, ASCE Journal of Structural Engineering.
- Shenton, H.W., Effect of Vertical Load on the Capacity of Wood-Frame Shear Walls, 58th Annual Forest Products Society Meeting, June 27-30, 2004, Grand Rapids, Michigan
|
Progress 12/01/00 to 11/30/01
Outputs
(I.) Progress has been made in the investigation of the effect of vertical load on the lateral load response of wood frame shear walls. A series of tests have been conducted on nailed sheathing-to-stud connections to study the effect of superimposed compressive stress on the load-slip behavior of the connection. A special fixture was designed and fabricated to simulate the dead load stress on the connection, while the lateral load was applied by the computer controlled universal testing machine. Identical connections have been tested under static monotonic load and cyclically, under varying levels of compressive stress, up to the design maximum allowable for a stud-frame bearing wall. Results show that the initial stiffness and energy dissipation capacity of the connection increases slightly with the superimposed compressive stress, while the stiffness, energy dissipation and ultimate capacity all decrease slightly with the compressive stress at larger displacements.
Connections are known to play a dominant role in the overall behavior of the shear wall; therefore, these same effects may translate to the overall wall behavior. The effect of the compressive stress on the connection behavior may be important for understanding the effect of the compressive stress on the full wall behavior. Full-scale wall tests will be conducted to determine if these effects, and/or others have an influence on the behavior of the wall under varying vertical loads. (II.) A finite element model of an 8' by 8' shear wall is being developed using the program ANSYS. The 2D model uses beam elements for the frame members and plane-stress elements for the sheathing. Individual fasteners are modeled using a pair of discrete springs that connect the frame to the sheathing (one in each principal direction). The fasteners are modeled using linear springs; future work will involve enhancing the model to include the nonlinear effect of the springs. Linear analyses have been
conducted and the results show good correlation with experimental data in the small displacement regime. The model will be used to study the effect of vertical load on the shear wall, for varying boundary conditions and modeling assumptions (for example, fixed-based, pinned-base, with and without hold-down anchors, effect of the rigidity of the frame connections, etc.). The model will be calibrated against the full-scale shear wall tests. (III.) Considerable progress has been made toward full-scale testing of walls with superimposed vertical load. Plans have been completed for modifying the existing test frame to include the effect of vertical load. Materials have been purchased and fixtures are being fabricated. The first tests are expected to take place in the early part of 2002 and will continue for more than a year. A full range of tests will be conducted, including both static and cyclic lateral loads. The primary test variable will be the magnitude of vertical load. The effect
of vertical load on the ultimate capacity, ductility, stiffness and energy dissipation of the wall will be investigated.
Impacts
The wood frame industry is facing stiff competition from alternative building materials for low-rise construction. This includes steel, masonry, recycled products, and most recently, polymer and bio-based composites. If wood is to remain competitive, every effort must be made to gain the maximum benefit from the material at the least possible cost, while still achieving the design objectives. Superimposed dead load may in fact enhance the performance of wood frame shear walls under the action of lateral loads; however, the affect is currently not considered in the design of shear walls in the U.S. Therefore, low-rise wood frame structures may have greater factors of safety than one might expect (this is especially true for two, three and four story structures). In other words, they are over designed. The research proposed will lead to a better understanding of the behavior of wood frame shear walls under combined cyclic lateral and vertical loads. This will lead to a
better utilization of materials and more cost effective designs.
Publications
- "Modeling the Non-linear Behavior of Wood Frame Shear Walls" by M. C. Hite and H. W. Shenton has been accepted for publication and presentation at the 13th ASCE Engineering Mechanics Conference (May, 2002)
|