Table 1604.3 - Deflection Limits

[CladdingNYC]

Dear Engineers,

I'm trying to settle a disagreement I have with an engineer (I'm an architect) about Chapter 16, Table 1604.3. Footnote “f” states: “The wind load is permitted to be taken as 0.7 times the “component and cladding” loads for the purpose of determining deflection limits herein”

The engineer argued that this applies to the design of non-structural C&C themselves (in this case Alucobond panels), and argued that a 0.70 reduction factor can be applied to the Design Wind Load used for the Metal Composite Panel in calculating the strength and deflection.

My understanding is that table 1604.3 and its footnotes apply only to structural members and that the 0.70 reduction factor is meant to be applied to wind load acting on structural members. However, what I can’t explain is why anyone would use the C&C loads instead of simply using the MWFRS wind load in the first place? Isn't the MWFRS generally much less conservative than the C&C load anyway?

Also, while I don't think the IBC limits deflection for non-structural metal panels, the manufacuturer has a recommended limit of L/60.

I appreciate any thoughts on this.

 

[JAE]

The 0.7, in my view, shouldn't be used for strength calculations - only for checking serviceability checks (deflection checks).

Whether it is structural or non-structural to me isn't important (isn't a pane of glass in essence a structural thing?). We use the 0.7 factor on structural elements.

C&C loads are generally higher than MWFRS loads. We've debated the use of these in Eng-Tips in the past - but essentially the C&C wind pressures are based upon smaller tributary areas. Wind varies over a given surface and its peaks and valleys are more acute when your component (whatever it is) receives wind from a smaller area. Thus a single screw in a wall panel might see very high peak pressures. A shearwall which laterally resists perhaps half of a building will only really see the average value of all those peaks and valleys of pressure. Thus, MWFRS pressure is smaller.

As the tributary area gets larger (going from screws, to panels, to girts, to wind columns, to diaphragms) the overall wind effect approaches the MWFRS wind. 500 s.f. or 700 s.f. (depending on the code you use) is about the cutoff where C&C wind pressures end and MWFRS wind pressures begin.

 

[CladdingNYC]

Dear JEA:
Thanks for taking the time to reply. Good point about glass, but glass deflection is covered in other parts of the code. I think this section deals with the MWFRS and does not apply to cladding. I think this engineer is taking a footnote that has nothing to do with cladding and applying it in a totally inappropriate way.

 

[woodman88]

Non-structural items will have their own deflection criteria. Floor tiles can have a l/360, l/480, l/600, etc. Which means the floor joists and slab deflection should be designed to meet this criteria. You will need to check what the criteria is for the Alucodond Panels and design for that deflection.

Garth Dreger PE
AZ Phoenix area

 

[CladdingNYC]

Thanks Woodman88, I found Alucobond's recommendation and will hold them to that (L/60). I still would like to hear an example of how footnote "F" would be applied. I can't figure that out.

 

[woodman88]

AS I read footnote f, you design structural members per the higher of 1 times the MWFRS wind case and .7 times the Component and Cladding wind case. The .7 times the Component and Cladding wind case should control for most structural members.

Garth Dreger PE
AZ Phoenix area

 

[syarn]

try this example for seeing how to apply note "F" [ 0.7 * component cladding windload ] for check of steel stud.

http://www.steelnetwork.com/SSMA_S.aspx?link_id=12

my guess is for alucobond panels you would want similar data:

dead load, live load, wind pressures for MWFRS and C&C, panel width (depth), panel height, panel spacing and bridging (lateral bracing)....you have the deflection at L/60 [which seems very giving compared to what the secondary structure supporting the panels probably wants to be; for the whole cladding assembly the studs or girts the panels attach too may actually govern for deflection limits]

being an architect as well it seems that the deflection calculation is very involved (possible calculus integration equation starting with moment expressed as a function; remember that from high school & college-haha) and tables are typically used such as for the stud example above.

steve yarnall

http://svyarchitect.blogspot.com/

 

[JAE]

The whole section (1604.3 and table 1604.3) both refer to "structural members" (see 1604.3.1).

So the table is NOT referring to non-structural members.

The use of C&C wind is used on structural members if they are not part of the MWFRS and take wind from small tributary areas.