Steel Roof Framing Deflection Absolute Limit?

[MJC6125]

When y'all design roof framing (commercial project specifically), do you put an absolute limit on the LL/SL deflection of the roof framing due to anticipated/potential interior partition walls going up to deck with a slip track head condition? My understanding is that typical slip tracks only allow 3/4" or 1" of deflection (one way).

If you have, for example, interior girders spanning 40' and joists spanning 50', to limit the absolute deflection of these members to 1" total (at mid span of both the joist and girder), you would have to design them for a deflection ratio of L/1080.
40'/1080 + 50'/1080 = 1"
I think this is going to result in some rather large roof framing members. Larger than typical I would assume, but I haven't actually run through examples.

Do you design for an absolute total deflection in most cases? Or do you only check it when you know you'll have a full height interior partition wall at a certain location and hope future remodels or tenant buildouts don't create full height partitions in larger deflection areas?

Lastly, if you are designing for a total absolute deflection, how do you typically specify or control what deflection the deferred submittal items should be designed to? I.E. if you have a WF girder beam, do you limit it's deflection to 1/2" and then tell the steel joist or truss supplier to also limit their deflection to 1/2" (ending up with worst case 1" total deflection)?

 

[Agent666]

I wouldn't limit it to the total deflection, only the serviceability live load or varying component, or serviceability wind/snow. Typically the dead load deflection is locked in when they build a partition up to the underside. So you only need to account for deflections that might occur after this point.

On your last point, I think you do need to be mindful of the summation of deflections like noted. But it does depend what your serviceability criteria are, if its just visual you might not even have line of sight along the soffit of beams or joists. Alternatively to address visual aspects of deflection you can introduce some camber to beams or joists.

 

[r13]

Camber the beam. Roof deflection will cause ponding situation too.

 

[DaveAtkins]

Regarding the slip track question, there are many different kinds of deflection connections, some able to absorb more than 1" of deflection. For example, there are slide clips with vertical slotted holes. But it is a good rule of thumb to limit live load deflection to 3/4" or 1".

DaveAtkins

 

[MJC6125]

I think my use of the word "total" deflection was confusing. I'm actually only talking about transient load deflection (no dead load), but I mean total in terms of the joist's actual deflection is affected by how much the girder deflects.

Because I'm only looking at transient load deflection, I don't think cambering applies here.

I also know there are slip connectors that you can place at each stud. I have a feeling the installed cost of these is much greater than a slip track, and I'm more curious about what is typically done. I don't think those slip connectors are typically used on interior non-load bearing walls.

DaveAtkins, are you saying limit the live load deflection of each member to 3/4" or 1" or limit the total live load deflection of the structure to 3/4" or 1"?

 

[BAretired]

I would not let potential future partitions dictate beam or joist deflection unless specifically requested by the client. Design them both in accordance with the structural code and let the partition installers make adequate allowance for deflection. Nothing wrong with noting deflection on the drawings (optional).

Camber does not resolve the problem as relative displacement is the concern here.

BA

 

[DaveAtkins]

When I design cold formed steel wall framing (even interior wall framing, which sometimes occurs in a pre-engineered metal building, where the deflections can be large), I design for the live load deflection only. My reasoning is the dead load deflection has already occurred prior to the wall framing being installed.

The slide clips with vertical slotted holes are used quite often, and I don't think there is a huge price premium for them. If you don't use this type of clip (i.e. when you use a slide track alone), you must install an extra row of bridging near the top of the wall. So there is extra labor cost there.

DaveAtkins

 

[r13]

Camber takes out the dead load deflection, lessen potential for ponding, and provide a level/flat base for track installation. I would also consider camber the dead load plus some live load deflection, or the amount in excess of the tolerance. This is a service criteria the designer needs to meet, rather than push it down the ladder.

 

[Tomfh]

I look at what the internal walls and ceilings are doing and how they are connected to the roof, as in theory it can cause issues if the roof moves up and down and the walls don’t. We often use telescoping slip joints with ~200mm overlap where wall structure meets roof.

I sometimes wonder how necessary it really is.