Expert: Bruce E. Johnson - 2/18/2011

Question
I have a large commercial building in Maine that is 32 years old.
It has 67' long clear span trusses.
I recently put office spaces in and hung my suspended ceiling from the trusses. When we got a snow load the ceiling sagged about 1 1/2" Was it wrong to hook the the ceiling to the trusses and is it normal to get some deflection in a 67' long clear span truss? Thanks
Sam

Answer
Hi Sam, I imagine your truss system was designed for a particular live load, 20-40 psf or possibly higher.  The dead load is the constant (this is the load required by the roof system to just support its own weight).  With the heavy snow load coupled with the new ceiling load you may have exceeded the design limits of the structure. That being said, an inch and a half of deflection over 67 feet really doesn't seem like that much for such a span considering your snow load plus the added ceiling weight.  It wouldn't hurt to have an engineer take a look at your roof system to make sure that the structure is still in tact and to do the math on how much deflection your roof can withstand.  I have seen spans of this size built with 8-10 inches of crown in them or more so that they will deflect as much under ultimate stress. Whether or not it was "wrong" to suspend your ceiling from the trusses isn't necessarily the case in point here.  Free span roofs are made to deflect.  It's the amount of deflection the roof was designed for.  If you have your old construction plans and specifications for the trusses you can see what the design professional allowed for live and dead loads along with allowable deflection.  Worst case scenario you may need to reinforce some or all of the trusses if the deflection has gone beyond the design parameters of the original building design but I have a feeling that the live load from additional ceiling isn't that much, maybe 5psf and the deflection you are noticing is normal and only became evident when you framed up your walls and established a point of reference.  One more note:  Most building designers allow for the addition of ceiling loads but specify that the load be supported by the top chord of the truss instead of the bottom chord.  The deflection at the top chord is magnifyed to the bottom chord to some extent so that if the ceiling were suspended from the top chord it might have shown slightly less deflection.  I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com