Expert: Arshad Khan - 5/30/2011

Question
QUESTION: Dear Mr. Arshad Khan,

We have designed a virendeel truss spanning 40 m and is being erected. While erection we are facing this particular problem- incase the temporary supports which were given to the truss at one third points while the truss was being seated on the two supporting pedestals are removed the truss lifts up at the two support points so much so that the nuts tend to come out from the threads- could you please advice on the reason for this and what could be the solution. The truss design has been checked and is correct.

Thanks and Regards,

Narendra Hattangadi

ANSWER: Hi Narendra,

Maybe I am understanding the exact issue, but surely if you remove the 1/3 point erection supports, there is no where else for the truss to support itself apart from the 2 supports at each end? In that case it cannot lift up as you say.

However, if you are talking about maybe the two bolts on the outer side of a base plate, this may be caused by the rotation of the plate due to truss deflection, so surely the solution is to tighten the nuts on the bolts before removing the temporary supports?

---------- FOLLOW-UP ----------

QUESTION: Dear Mr. Arshad,

Thank You so much for your reply.

Actually , we have provided three nuts on each of the bolts and fully tightened but still the nuts tend to come out of the thread. So what could be the problem- or is it that we should firts erect two adjacent trusses in the braced bay connect all the roof purlins,the tie members at bottom chord & the top and bottom chord bracings and then remove the temporary  truss supports - do you think this would help ? Please kindly advice.


Thanks and Regards,

Narendra Hattangadi

Answer
Hi Narendra,

You have not confirmed whether the base plates are rotating, but this sounds like your bolts are under-designed. I do not think the additional measures you mention will have much of an effect on the bolts problem. So try and use larger bolt diameters.