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Uplift @ each bearing

Uplift @ each bearing

Uplift @ each bearing

(OP)
When truss has 3 bearings, is there some way to calculate uplift at each bearings?

RE: Uplift @ each bearing

YES.  

However, you need to engage the services of a local licensed structural engineer in your area.  He can do this very easily.

Mike McCann
McCann Engineering

RE: Uplift @ each bearing

No problem at all if you have the proper truss design software.  If not consult a local structural engineer or a truss manufacturer to run some preliminary designs for you.  The value of that interior bearing can vary greatly depending on where it is located and how the truss web configuration lays out.

RE: Uplift @ each bearing

It is not a good idea to support a truss in threee places. The three points are going to be at different heights and the truss is too rigid to sag across them.

RE: Uplift @ each bearing

Multiple bearings for trusses can reduce reaction-load requirements and reduce the cost of the truss component. It is important to have level supports for the truss to rest on, otherwise internal stresses in the truss members will occur which were not designed for. A truss engineer can determine the maximum downward force to the foundation and the maximum uplift force to design connection hardware. These reacton forces vary depending on vertical and lateral loads applied for the specific site conditions.

When construction introduces uneven support conditions, a new truss design is required that considers support settlements. The new engineering analysis may require truss repair-modification design(s), or the original truss may be adequate as originally built, but the answer is not known until the engineer does the analysis.

RE: Uplift @ each bearing

Sometimes it is better to use two independent trusses or under the guidance of the truss engineer - simply cut the bottom chord over the bearing to make it act like two trusses.  Make sure you have enough bearing area.

RE: Uplift @ each bearing

A bigger problem with trusses supported at three points is the condition where two supports are close together and the last support far away.  This is often modeled in cases where there may be a 6-8' porch and say a 30-40' backspan over living space.  In the standard/balanced load case one may find a rather large uplift.  If the truss is not adequately strapped to resist this force as well as the structure below, the truss may overstress or cause a tension member to go into compression which may or may not require lateral bracing.  For this case I generally model the truss as cantilevered or model the porch beam as a vertical roller with the remaining supports modeled as horizontal rollers.  

woodengineer

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