L/90 Total Load Deflection in Roof Framing Members

[KootK]

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

 

[JAE]

That's a big deflection - and I wonder if having L/90 does in fact have no ponding issues.
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.

 

[KootK]

Thanks for your response JAE. The roof in question is basically a big bowl with drainage in the middle. The deflection should actually contribute to positive drainage. It will pond like nobody's business when the drains clog but the water should definitely go the right way. The project is in Canada. Our ponding criteria seem to be surprisingly easy to satisfy. The plan right now is to camber the roof beams for a dead load value that we're confident in. Say, 75% of an optimistic (light in this case) dead load.

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.

 

[JAE]

If the gypsum sheathing is provided with enough control joints in it then that might help.
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.

 

[rowingengineer]

L/90 sounds a bit of a stretch (I generally like to stop at L/150 as a review limit), but if in the right context it may perform sufficiently.

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."

 

[BenAustralia]

Under what load combinations is the L/90?

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.

 

[rowingengineer]

I don't know of any code which mandates deflection limits, the part that Ben is referring to in the Australian code is a not mandatory but only provided for information.

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."

 

[BenAustralia]

Yeah well that's my point.

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.

 

[KootK]

@JAE: we do have a secondary drainage system. In my jurisdiction, however, we have to deal with the maximum probable one day rainfall regardless of the drainage condition. Grey haired engineers around here are fond of reminding me that most of our severe storms begin as hail storms. Apparently the hail gums up both primary and secondary drainage systems almost instantly.

@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.

 

[OldPaperMaker]

KootK,
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"

 

[BenAustralia]

The table in the Australian code details guidlines for deflection limits for a lot of different elements. The limits are usually for seperate cases, such as just DL or just Wind, and not combinations. It also states combinations unlikely to occur don't apply.

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.

 

[KootK]

@Ben: any chance you'd be willing to post a scan of those tables? I'm curious now.

 

[BenAustralia]

I can try later, they are electronic files on my PC that are secured, so I can't snapshot or print them to PDF.

 

[CANPRO]

Is it all open space below? If there are interior partitions its going to be an interesting detail between the walls and ceiling.

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.

 

[msquared48]

rowingengineer:

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

 

[KootK]

@MS^2: Why the difference between occupied and unoccupied structures? Simply because deflection in an occupied structure is likely to be perceived by someone?

 

[msquared48]

A pole barn, as opposed to a residence, retail or commercial space, etc.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[FLCraneBuilder]

L/90 is way scary to me...
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)

 

[BAretired]

L/180 is what I have used for combined Live + Dead Load.

BA

 

[KootK]

Yeah, L/180 has been my fall back position as well.

@CANEIT: fortunately, it is all open space below. For about three stories actually.