Truss Analysis and Design
Civil Engineering students and Young professionals are encountering the need to truss analysis and design for roofs and bridge structures. Trusses are typically made from metallic materials such as Steel and Aluminum.
Structures are designed by official specifications; e.g. AASHTO (LRFD, LFD & ASD), AISC (LRFD, ASD), and resist loading such as; Live, Dead, Thermal effects (Expansion & Contraction), Impact, Wind, Snow, Rain, Earth Quake, Member imperfection (lengthening or shorting), and camber.
Trusses are modeled as two force members e.g. they only resist axial (tension and compression). Member ends are assumed to be pinned only transferring point loads between members and the support of the structure. This idealization maximizes the loading effect on the member, which is proper for that member’s design and evaluation.
Further, a closer look at member ends they are not pinned. A gusset plate attaches the members to every node point. This attachment appears to be more rigid than a pinned connection. Gusset plates were the primary cause of the failure of the I-35 Bridge in Minnesota. The connection design requires more complex analysis and idealizations of the plate as a compression, tension, shear, and combinations of these effects for the plates and fasteners (bolts, rivets, etc.).
Other information about Trusses can be found at Structurae.net.
Truss Gusset Plate over Elk River
Remember the Mechanics (Strength) of Material Courses
Engineers idealize structures into the basics that are learned our education; axial, bending (flexure), shear, torsional members. In reality, all four conditions exist in all structural elements, but the main load caring capacity is specifically dominated in one of these four of these states.
Trusses come in simple and continuous span configurations or some combination of these. Simple trusses members can be identified as tension and compression elements by observation. Old bridge plans it is easy to identify tension and compression members by the member size and configuration.
Trusses loadings are transferred through node points (panel points), as point loads. All loads need to be transformed to point loads in order to perform the analysis. Therefore all dead and live distributed loading must be converted to point loads by a tributary method. Moving loads must be calculated by influence lines. Influence lines were developed by application of Betti’s Theorem and Müller-Breslau’s Principle.
Hand calculation procedures such as the method of joints and sections have been used in the past to analyze these structures. Trusses are evaluated by software programs e.g. (SAP 2000, Strudl, Staad, AASHTOware BRR & BRDR, etc.). Therefore, educational institutions are not spending as much time on these hand methods anymore which could be used to check the results of these programs to assure the correctness of these programs. It is good for students and junior engineers to check there answers before completing their work with the use of these methods.
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