The following is a way to load agricultural trusses in order to be competitive in the marketplace and at the same time produce sealable designs.  It is important to note that the applicability of these suggestions be verified by the authority having jurisdiction and or by the building designer for the truss designs in question.  MiTek is not specifying how to load trusses.  All of these design parameters must be verified by others.
Beginning in the Design Info screen, select as the building code IBC2006/TPI2002 as shown here:
(Click on image to enlarge)

Note also that the top chord restraint/bracing is set to 2’ on center and the bottom chord restraint/bracing is set to 10’ on center as is typical in agricultural framing. 

The next step is to set the loading.  Begin in the general tab and turn on wind and snow as shown here:

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In the example shown here, snow load has been selected.  If you are in an area where snow is not considered a factor such as the state of Florida, the snow load may be unchecked and instead roof live load may be selected.  If this is done, the minimum roof live load generally used in truss design ranges between 16 to 20 PSF and is based on the tributary area supported by one truss, as well as the pitch. 

The snow load value shown above is for example only and will be dictated by the location of the building.

Also note that no bottom chord live loads are generally required on agricultural type trusses.

Under the Snow load tab you will note that ASCE 7-05 is selected for you and is grayed out since IBC2006 was selected in design info.  ASCE 7-05 yields more competitive unbalanced snow loads than previous versions in general and particularly on agricultural truss designs.  The terrain and roof exposure should be set to those dictated by the job site.  The Ct factor should be set to 1.2 (cold roof) if the building is not going to be heated at some point.  Turn on “Apply slope reduction factors”.  The surface condition may only be set to slippery if the roof is a metal roof with no anti-snow-slide devises.  Occupancy category is of course set to “I” assuming the building to be “Low hazard to human life in failure”.  The building Lu should be the length of the building measured perpendicular to the truss span.
The ground snow should be selected from the snow map, or as specified by the authority having jurisdiction or the building designer. The “Ground” radio button should then be selected.  Finally, verify that the unbalanced snow load is set to “Do for any geometry” as shown below:

(Click on image to enlarge)
Note: If trusses are caped, the transverse roof pitch should be set to the roof pitch so that trusses with flat sections are not overloaded with rain on snow or controlled by the minimum flat roof live load.
Under the Wind load tab the exposure and occupancy categories were already set in the Snow tab.  The wind design method should be set to Main Wind Force Resisting System ASCE 7-05.  The roof zone for agricultural trusses may be set to interior with the exception of the gable end truss that needs to be run in the gable end zone.  The opening condition is typically set to “enclosed”. The wind Duration of Loads (DOL’s) may be set to 1.6.  In version 6.5 you may use the same dead loads as specified in the general truss loading tab on agricultural truss designs as shown here if the dead loads do not exceed 5 for the top chord and 4 on the bottom chord, the occupancy category is set to I.

(Click on image to enlarge)
Once again, all of these parameters must be verified by the authority having jurisdiction and or by the building designer.  Please feel free to contact your MiTek engineer for any further clarification on this topic or any other truss engineering issue.


 This page last modified on 4/11/2008