Member and Joint Design Examples-LFD and SLD

Design of a truss member by the AASHTO LFD and SLD Specifications is illustrated in the following examples, The design includes a connection in a Warren truss in which splicing of a truss chord occurs within a joint. Some designers prefer to have the chord run continuously through the joint and be spliced adjacent to the joint. Satisfactory designs can be produced using either approach. Chords of trusses that do not have a diagonal framing into each joint, such as a Warren truss, are usually continuous through joints with a post or hanger. Thus, many of the chord members are usually two panels long. Because of limitations on plate size and length for shipping, handling, or fabrication, it is sometimes necessary, however, to splice the plates within the length of a member. Where this is necessary, common practice is to offset the splices in the plates so that only one plate is spliced at any cross section.

Load-Factor Design of Truss Chord

A chord of a truss is to be designed to withstand a factored compression load of 7,878 kips and a factored tensile load of 1,748 kips. Corresponding service loads are 4,422 kips compression and 391 kips tension. The structural steel is to have a specified minimum yield stress of 36 ksi. The member is 46 ft long and the slenderness factor K is to be taken as  unity. A preliminary design yields the cross section shown in Fig. 13.8. The section has the following properties:

Ten 11⁄4-in-dia. bolt holes are provided in each web at the section for the connections at joints. The welds joining the cover plates and webs are minimum size, 3⁄8 in, and are classified as AASHTO fatigue category B.

Although the AASHTO LFD Specification specifies a load factor for dead load of 1.30, the following computation uses 1.50 to allow for about 15% additional weight due to paint, diaphragms, weld metal and fasteners.
Compression in Chord from Factored Loads. The uniform stress on the section is

= 35.9 ksi

For the dead load of 0.98 kips / ft, the dead-load factor of 1.50, the 46-ft span, and a factor of 1/10 for continuity in bending, the dead-load bending moment is

Tension in Chord from Factored Loads. The following treatment is based on a composite of AASHTO SLD Specifications for the capacity of tension members, and other aspects from the AASHTO LFD Specifications. This is done because the AASHTO LFD Specifications have not been updated. Clearly, this is not in complete compliance with the AASHTO LFD Specifications. Based on the above, the tensile capacity will be the lesser of the yield strength times the design gross area, or 90% of the tensile strength times the net area. Both areas are defined below. For determinations of the design strength of the section, the effect of the bolt holes must be taken into account by deducting the area of the holes from the gross section area to obtain the net section area. Furthermore, the full gross area should not be used if the holes occupy more than 15% of the gross area. When they do, the excess above 15% of the holes not greater than 1-1⁄4 in in diameter, and all of area of larger holes, should be deducted

Service-Load Design of Truss Chord

The truss chord designed in Art. 13.10.1 by load-factor design and with the cross section shown in Fig. 13.8 is designed for service loads in the following, for illustrative purposes.
Properties of the section are given in Art 13.10.1.

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