FM Global Wind uplift

[SteelPE]

I do not have much experience with the FM Global building requirements and I have a project where it popped up at the last minute and I am now trying to ply catchup. I am trying to replicate the values in FM Global Data Sheet 1-28 table 3 (attached). I don't know if I am doing this wrong or not, but looking at my calculations which used ASCE7-05 and the numbers given in the table which are suppose to use ASCE7-05 and they don't match.

For example, given exposure C, V=100mph, H=30ft I calculate a wind pressure of 25.1 psf while FM calculates a wind pressure of 29psf. I know FM is suppose to yield higher uplifts, but they seem to specifically reference ASCE 7 and I am wondering why my numbers do not match.

Amy insight would be appreciated.

 

[hawkaz]

Didn't run the numbers- but you are different by a factor of 1.15. Importance factor for Category III and IV is 1.15. Coincidence?

 

[SteelPE]

I did not notice that relationship. It doesn't mention what I value they use to derive the tables. They make mention of I=1.15 when trying to calculate secondary members but don't say anything about what I was used for the tables. Maybe that is it. Someone on here must have experience with FM standards.

 

[jberg]

This is from memory, but I believe that 1.15 factor is included in the FM Tables. You are correct that FM will yield higher uplifts as compared to ASCE 7, but one thing to watch is the purlin spacing if you are using a metal panel roof (i.e. Fabral, MBCI, etc.). Under FM 1-28 and also FM 1-31, Metal Roof Systems, you need to use a roof panel that meets the FM loadings (say 1-105 in the field for example). Metal roof panel suppliers will give you a FM design table for this, say 1-105 requires a panel at 5'-0" max purlin spacing for the field. However, after that the FM data sheets use prescriptive requirements that state to use 1/2 of the field spacing for the edges and 1/3 of the field spacing for the corners. For this example, you would need purlins at 2'-6" for the edges and 1'-8" for the corners, even if the panel standard uplift load tables say it can span further. The actual design pressure you determine from the tables versus the allowable capacity of the metal panel for a given span does not come into play.

Sorry for the rant, but I just wanted to make you aware of the issues I have had recently in dealing with the FM design requirements.

JWB

 

[ajh1]

Yes, the FM charts use a 1.15 importance factor. In their view, any building worth insuring by FM is clearly an "essential facility".

 

[glass99]

Yes, I have worked with FM global criteria and can confirm that they are more conservative.

 

[SteelPE]

glass,

I don't really care if they are more conservative or not, I would just like to understand where their wind loads come from.... especially since they are referencing ASCE7 which I would like to think I know how to use.

nowing how to reproduce the chart goes a long way to understanding how to use it. I know a few engineers who can only design things that come out of charts. Drives me nuts when they are interpolating or extrapolating charts and could reproduce the chart if their life depended on it (and.... go figure, it sort of does).

 

[OHIOMatt]

To further expand this discussion, I am working on a project that involves FM as well. It is a new PEMB. The design criteria that they have provided is for is a partially enclosed building with an importance factor of 1.15 for wind. Obviously this blows the preliminary design away. Anyone know why FM would require a partially enclosed design. We have a few man doors and that is about it for openings save the odd louver. Does FM assume that the siding will fail?

 

[j19]

The 1-28 data sheet is only applicable to:

1. Building components and cladding and their immediate supports (wall siding, roof assemblies, etc.)
2. Opening protection (doors, windows, etc.)"

The following sentence is at the bottom of 1-28's page 6:

a) Use an importance factor of 1.15 for components, cladding and secondary structural framing.