When 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. [For all images, See PDF or View in Full Issue.]
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.
Another important scenario to consider is top chord bearing roof trusses / raised bearing trusses. If you have a top chord bearing truss or a truss with a raised bearing sitting on a block, you will have two end verticals. Be sure to define the member sitting on the bearing as a block and define the member stacked next to it as a top chord. This will ensure correct analog, proper loading and plating, and the program will automatically check for top chord reaction limits. This is handled automatically if you use the top chord bearing feature in the program.
Some other scenarios to consider are the following:
- Parapet verticals should always be defined as top chord so that wind loading gets applied correctly.
- Exterior horizontal and vertical members of cantilevers should be defined as top chord and bottom chord, respectively, for proper wind load application.
- Stacked chords should be defined as the same chord type when a single chord isn’t sufficient.
- All horizontal members of ceiling trays should be defined as bottom chord to ensure that dead load is added to account for the weight of the ceiling material.
For additional information, or if you have questions, please contact the MiTek Engineering department.