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Re: MORE ABOUT TRUSSES...

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Juan,

Worst points are those that produce the maximum effect on the truss. For bending, a single pick point at midspan for shear, two pick points - 1 @ last panel point each side (panel point is where verticals and diagonals intersect top or bottom chord). The value of that load is the weight of the truss plus some impact value.

The ceiling weight an collateral load are issues that occur because there is often bending induced in the bottom chord because ceilings and other appendages are not attached to the truss chord at or near the panel point. A two force member (bottom chord) is now being asked to behave similar to a simply supported beam with axial load. I also specify a "ROVING" point load for the top chord of 400 lbs between panel points for construction loads.

The moment to depth calc is as follows .... Simplify your truss as if it were a simple beam. Calculate the moment in the beam using usual methods. If you divide the moment at any point along the span by the depth of the truss at the same point, you will get the compression force in the top chord and the tension force in the bottom chord.

For seismic, you should also specify/design for  an additional axial load in the top chord for any collector trusses and those at the edges of subdiaphragms.
-gerard

>>> jjtreff(--nospam--at)hotmail.com 05/29/01 03:46PM >>>
Hi everyone,
Recently I posted a question on roof trusses. Many people responded me with 
very good advise but I have some new doubts and would like to know your 
opinion.

Mark advised me to include 5 extra kips at the worst joints or nodes due to 
lifting. Did I understand right?  How would you pick the "worst ponits"?  Do 
you do it only in the lower chord nodes?

Then John advised me to add 10 psf for ceilling weight. And Greg told me to 
add from 3 to 10 psf for collateral loads.

Do I add this loads to the Dead Load condition, to the Live Load condition 
or as a new Collateral load condition?

For the initial truss depth, Greg adviced me to do a quick moment/depth calc 
and check against ASD tension check. Coul you be a little bit more explicit?

Thank you all for your kind and valuable help.

Regards,

Juan José


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