L/90 Total Load Deflection in Roof Framing Members
L/90 Total Load Deflection in Roof Framing Members
(OP)
I'm assuming responsibility for a project that involves the use of 19 m roof framing members. These members have been designed for code prescribed live load deflection limits and ponding. No problems there. The total load deflections amount to L/90 however. While I can find no code limitation prohibiting the use of L/90 for total load deflection, the system is twice as soft as anything that I've designed myself in the past.
How do others feel about deflections of this magnitude?
Thanks for your help.
KootK
How do others feel about deflections of this magnitude?
Thanks for your help.
KootK






RE: L/90 Total Load Deflection in Roof Framing Members
Is there enough slope to ensure water doesn't flow the wrong way? I guess with enough slope then not a problem.
If this is a roof with no brittle finishes on it then...well... I'd still be nervous.
RE: L/90 Total Load Deflection in Roof Framing Members
It hasn't yet been decided what the ceiling finish will be. It may well be gypsum mounted to light gauge framing suspended from cables. That brings another question to mind. If gypsum sheathing is mounted to light gauge framing suspended from the roof framing, rather than being directly attached to the roof framing, can the deflection requirements be relaxed any? I would think not. It seems to me that the curvature induced in the suspended gypsum would still match the curvatures of the roof beams more or less.
Code limitations or no, my response parallel's yours JAE... ick.
RE: L/90 Total Load Deflection in Roof Framing Members
I would be more concerned about live/snow load deflection rather than total deflection - especially if you camber out some/most of the dead load.
If the drains clog up - and the deflection increases, I assume you have a secondary drain system and that the roof can adequately support the additional water up to the level of the secondary drains.
RE: L/90 Total Load Deflection in Roof Framing Members
http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."
RE: L/90 Total Load Deflection in Roof Framing Members
Its the L/90 combination likely to ever occur? Or it is strength state where you're just happy if its still standing after such an event?
I too have never strayed beyond about L/150, but Australian codes are very different to a lot of others.
RE: L/90 Total Load Deflection in Roof Framing Members
when you say total load I take this as referring to a combo of DL + LL load not ultimate.
http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."
RE: L/90 Total Load Deflection in Roof Framing Members
We have Ultimate load cases, that we ensure our structure can withstand. These loads are highly highly highly unlikely to be reached or exceeded, hence deflections at this limit are too much of a concern.
But if its a Serviceable combination, such as we have here of DL + 0.7 LL or similar, something that is likely to occur during the buildings lifetime, then I'd personally say L/90 is a bit far. Similar to the slenderness limits we have really. Just there to ensure a general level of robustness for a structure.
RE: L/90 Total Load Deflection in Roof Framing Members
@Ben & Rowing: the load case under consideration is indeed a serviceability load case. Is the L/150 non-mandatory limit in the Australian code meant for comparison against service level total load (LL+DL)?
With very large deflections, I wonder at which point the following become concerns:
1) The applicability of small deflection theory.
2) The ability of the "pinned" connections at the supports to accommodative the rotation of the beam ends.
RE: L/90 Total Load Deflection in Roof Framing Members
In the US we design to the 2009 IBC. Table 1604.3, Deflection Limits notes the following: Roof members supporting plaster ceiling, Live Load, L/360, Snow, L/360, D+L l/240.
If my math is right, 19 meters is about 62' and your calculated deflection of L/90 amounts to a little more than 8". I think that this is "excessive"
RE: L/90 Total Load Deflection in Roof Framing Members
Also think about how any sheeting/clading/fixings will be installed after the majority of the DL is applied, the deflection from the DL itself isn't much of an issue, its the additional LL that will be the problem for claddings etc.
RE: L/90 Total Load Deflection in Roof Framing Members
RE: L/90 Total Load Deflection in Roof Framing Members
RE: L/90 Total Load Deflection in Roof Framing Members
Even if the L/90 only occurs in a 1/50 year event, I would think at that span even a 1/10 year event would cause problems.
RE: L/90 Total Load Deflection in Roof Framing Members
Some older AITC codes did recommend certain deflection limits.
For the record, I would never do either roof ponding, or L/90. L/120 minimum for an unoccupied structure, and L/180 if occupied. I think you re just asking for maintenance problems and roof leaks. As JAE mentioned, leaves and ice buildup (sometimes working in collaboration together, mind you) do clog drains.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: L/90 Total Load Deflection in Roof Framing Members
RE: L/90 Total Load Deflection in Roof Framing Members
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: L/90 Total Load Deflection in Roof Framing Members
Suggest you insure the erection subs are well aware of this prior to bidding. There may be some unusual erection requirments (depending on the geometry of the work)
RE: L/90 Total Load Deflection in Roof Framing Members
BA
RE: L/90 Total Load Deflection in Roof Framing Members
@CANEIT: fortunately, it is all open space below. For about three stories actually.