BCSI Guidelines for Continuous Lateral Restraint & Diagonal Bracing of Metal Plate Connected Wood Trusses
BCSI Guidelines for Continuous Lateral Restraint & Diagonal Bracing of Metal Plate Connected Wood Trusses
optimization using SBCRI truss assembly test data.
Wood trusses have slender members loaded with high compression forces; therefore, temporary and permanent truss member continuous lateral restraint (CLR) and diagonal bracing is critical to preventing premature failure by buckling of the top/bottom chords and/or web members. Truss member CLR and diagonal bracing is typically installed according to the recommendations provided in Building Component Safety Information (BCSI): Guide to Good Practice for Handling, Installing, Restraining & Bracing of Metal Plate Connected Wood Trusses, produced jointly by SBCA and TPI.
Question
I’ve heard that industry testing of BCSI has been conducted at the SBC Research Institute (SBCRI). What kinds of tests have been performed and how will they affect BCSI CLR and diagonal bracing recommendations?
Answer
BCSI was created to provide industry recommended methods and procedures for installing CLR and diagonal bracing that are comprehensive and result in safe construction practices. It uses a prescriptive approach to CLR and diagonal bracing for trusses up to 60' in length and up to 80’ in length when CLR and diagonal bracing is designed and inspected by a Building Designer [registered design professional (RDP)] per ANSI/TPI 1 Chapter 2 and the International Building Code (IBC). The International Residential Code (IRC) adopts these provisions by reference to ANSI/TPI 1 and BCSI.
Both BCSI and ANSI/TPI 1 were developed by leading representatives of the truss design and manufacturing industry, based on engineering mechanics theory and their collective experience. BCSI is intended as a guide and does not and should not supersede a Building Designer’s CLR and diagonal bracing specifications. Due to its prescriptive approach and range of possible truss configurations, some BCSI provisions may be more stringent than a CLR and diagonal bracing plan generated by a qualified Building Designer based on job-specific structural analysis. To improve the prescriptive CLR and diagonal bracing methods of BCSI, the unique testing capabilities of SBCRI have been used to better understand the true distribution of load through CLR and diagonal bracing members in an assembly.
BCSI Lateral Restraint & Diagonal Bracing Tests
SBCRI conducted tests on an assembly of five 39' common trusses (see Figure 1) under 33 different CLR and diagonal bracing conditions. The assembly was first tested with CLR and diagonal bracing applied according to BCSI guidelines, to establish baseline performance. Different elements of the CLR and diagonal bracing system were then removed, and the assembly was re-tested to determine how the forces in the CLR and diagonal bracing changed, and if the assembly experienced buckling failure.
In all tests, load was applied to the bottom chord of the end truss, representing the loads applied by three construction workers standing at the quarter points of the truss. The load was increased linearly up to a maximum of 420 pounds (2.1 x 200 lb man-load). Load reactions were measured at the bearings of the five trusses and at the horizontal ground CLR and diagonal bracing locations. This reaction information determined the path the load followed from its point of application to the assembly supports.
In addition, the lateral force in one of the top chord CLRs was measured by replacing the lumber with a threaded rod that contained a load cell (see Figure 2). By measuring the force in the CLR at this location, comparisons could be made to the lateral restraint requirements calculated using the current design procedures (such as the “2 percent rule”). Current design procedures, including the “2 percent rule,” are generally a conservative assessment of lateral forces that result in good performance but may present unnecessary challenges for framers (see the article, “System Stability in Wood Truss Assemblies during Construction” in the Sept/Oct 2007 issue of SBC Magazine).
The lateral deflections of all CLR points and the quarter-point vertical deflections of the loaded truss bottom chord were recorded as well. The load and displacement measurements for the different CLR and diagonal bracing configurations can then be used to calibrate and verify the results of computer models of the truss assembly.
Finite Element Modeling
The results of the CLR and diagonal bracing tests are being used to validate a Finite Element Model (FEM) of the truss assembly (see Figure 1). Once the model is fully calibrated and verified, FEM will be used to predict the actual forces in the truss members and CLR and diagonal bracing components. Knowing the actual forces of the truss and CLR and diagonal bracing members under load will determine if the members are susceptible to buckling and the amount of lateral restraint needed to prevent buckling. The reactions produced by FEM will be compared to the measured reactions to ensure that the model is accurately distributing forces throughout the assembly. In addition, the non-linear modeling capabilities of FEM will be used to predict when and where buckling failures occur. The predicted buckling load and buckled shape will be compared to the buckling load and shape observed during tests to verify that the model produces accurate results (see Figure 3).
All of the information gathered from the 33 tests conducted by SBCRI and FEM can be used to evaluate current design truss CLR and diagonal bracing methods. Once validation of the model is completed, FEM can analyze other truss configurations and assemblies. These models will determine the critical
CLR and diagonal bracing components, which will allow for optimization of their design. Using a system-based design method to determine the stability of the truss assembly will likely result in a reduction, perhaps significantly, of the CLR and diagonal bracing forces currently expected when designing members individually. The test data and FEM results will ultimately help revise BCSI guidelines to result in more economical restraint and CLR and diagonal bracing methods.
Concluding Thoughts
This work is being undertaken because, more often than not, framers do not follow BCSI guidelines. Some framers find BCSI too time-consuming and difficult, and they generally believe it is overkill. It’s important for BCSI to provide an answer, through engineering analysis, testing and generally accepted engineering principles, that solves framers’ concerns and provides solutions that make framers say, “Now that bracing process makes sense to me!” If done well, temporary CLR and diagonal bracing will become permanent CLR and diagonal bracing, which is installed just once, versus being installed, taken off, and permanent CLR and diagonal bracing installed separately. If this process is simple for framers to deploy, safe truss installation will improve. Everyone can embrace concepts that make installation easier and safer, and in the process, prevent a serious injury or save a life. SBC
For more information on temporary and permanent lateral restraint and diagonal bracing of trusses, see the BCSI book (sbcindustry.com/bcsi.php) and B-Series Summary Sheets (sbcindustry.com/bcsi.php#bseries).
Non-Linear Finite Element Analysis
To determine if a structure will buckle, a finite element program can use a non-linear analysis that accounts for large displacements. Most structures are analyzed for member and connection resistance assuming that the displacement/rotation of the structural members and/or connections is small. This member resistance analysis can use the initial un-deformed geometry of the structure, which makes solving for the forces in the system simpler and is usually quite accurate.
However, using the undeformed (undeflected) shape of the structure does not check for buckling of compression members, since this involves large displacements (like the 1" displacement assumed for the generation of the 2 percent rule). To determine if a structure will buckle, an analysis must use the deformed shape of the structure. An example of a non-linear analysis involving large deformations is shown in Figure 4. Under large deformations, the truss has a different geometry, which implies that it will have a different stiffness response to the vertical load.
For a non-linear analysis, a finite element program accounts for the large displacements by conducting an iterative solution. The geometry of the structure changes over time, with each step of the analysis using the deformed shape calculated in the previous step. Using this method, buckling failures can be determined. The type of non-linearity resulting from large displacements is called geometric non-linearity and is only one form of non-linearity that can be considered by finite element programs.
SBCRI is undertaking this work to provide better truss system buckling behavior analysis. A non-linear finite element resistance program and model are being calibrated to the test data. This model will define and refine the required temporary and permanent lateral restraint and diagonal bracing required to efficiently resist the applied gravity-load-inducing and wind-load-inducing lateral loads. The goal is to provide a generally accepted engineering CLR and diagonal bracing solution that is more accurate and framer friendly.
