Maximum Metal Roof Deck Diaphragm Aspect Ratio? 1

[structuresguy]

I am wondering if there is any published recommendation or requirements for maximum unfilled metal roof deck diaphragms. I know for plywood diaphragms, the codes limit it to 4:1 aspect ratio. But I can't find any similar limit for metal deck. There is a discussion going between the engineers in my office about the acceptibility of a 4.3:1 roof diaphragm. We can size the deck and fasteners for the required strength and stiffness, but some are concerned it is just too long and narrow.

Also, is there any recommendation for maximum diaphragm deflection limit? I limit my diaphragm deflection to ensure that the maximum story drift for the columns at mid-span of the diaphragm is not exceeded.

Thanks in advance.

 

[structuresguy]

I should mention that I know the code has a limitation for diaphragm aspect ratio if using empirical masonry design. But I always use strength design, so this does not apply here.

Thanks.

 

[DaveAtkins]

If you can justify the diaphragm deflection, I think you are OK.

Have you checked the chord forces? They may be quite large for a long, shallow diaphragm.

DaveAtkins

 

[jike]

Just make sure that you can meet strength and stiffness requirements. There is a maximum deflection limit if you have masonry walls.

 

[frv]

Once you start getting to an aspect ratio of 3:1 or more, you start getting really high diaphragm shear and your deflection is really large.

As DaveAtkins mentioned, your chord forces will be very high as well (although all you need to do is bump up your deck angle.

But I don't think there is any code-imposed aspect ratio limit.

 

[MiketheEngineer]

I agree with all above - as a rule of thumb I use 2:1 but that definitely depends on the eave height (wind loads controlling). At 10' or 12' a 3:1 might work but at 16' or 18' a 2:1 is more likely.

I work with steel clad post frame type construction

 

[JedClampett]

If I understand your question, the deck manufacturer's have their limitations on span/width. These are ususally stated in their ICBO Report. For example, Verco (

http://www.icc-es.org/reports/pdf_files/ICBO-ES/2078plan.pdf),

has their requirement in Table 7 in Report ER 2078P. Vulcraft has a very similar table.
As a warning, these tables are very difficult to comprehend. Whenever I try to explain them, I end up confusing myself and who I'm talking to.

 

[structuresguy]

Thanks for the posts. In general, I try to stay at 2:1 as well, as I find it results in fairly easy to manage roof loads and connections.

But in this case, this was not possible. I can get everything to work fine, chord forces, deflection, diaphragm capacity, combined uplift/shear etc... The deck connections are pretty robust, 36/7 w/ 11#10 TEKS sidelaps using 18 gage 1.5" B deck, but everything works.

I just wanted to verify that there were no code limits that I was not aware of.

Thanks again.

 

[sandman21]

Section 2109.2.1.2 2006 IBC, metal deck with no fill 2:1

 

[structuresguy]

Thanks sandman. However, the section you reference is only for empirical masonry design. If I am doing masonry, I always use engineered design, so that table does not apply, strictly speaking. Though in general it is probably good practice to stick close to those guidelines.

 

[sandman21]

Sorry missed your second post, I never use empirical design but I still follow those ratios.