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May 2023 #15286 Page #92
Correctly Defining Truss Members
By MiTek Staff
hen designing trusses for special loads or special conditions, it is important to define the individual truss members correctly. Without proper member definitions, trusses could be inadvertently under-designed and/or have incorrect analog. The most common member types are webs and chords. Top chord members form the top of the truss and are subjected to compressive and bending stresses. Bottom chord members establish the bottom of the truss and usually are carrying combined tension and bending stresses. Webs are members connecting the top and bottom chords of the truss, and typically carry axial (compression and tension) forces only.
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When you design the truss, you assign uniform loads such as top chord live load (TCLL), top chord dead load (TCDL), bottom chord live load (BCLL), and bottom chord dead load (BCDL). These loads will be automatically applied to chords only, webs will never be automatically loaded with any of the dead or live loads, except when the truss is designed as an attic truss. Let’s review some examples. When adding a raised horizontal member over the bottom chord that will be used for storage, it should be defined as a bottom chord. Defining this member as a web will not load this opening automatically to support floor / storage load. Simply by changing the definition of this member to a bottom chord may cause the member failure with the new system generated load as shown below. If the member is defined as a web, additional load must be applied manually.
Raised member called “web”
Raised member called “bottom chord” Continued next page
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