FM Global Loading

[CMalenofski]

After reading the 2015/2016 DS 1-28 I am confused about the application of the FM Global loads. They run through calculations comparing results to ASCE 7-10 with both ASD and LRFD loads. What I can't tell is if I am to be using these loads with ASD or LRFD capacity equations for my structural members. They are calling it a design pressure but a few times in section 3.10 they refer to the FM Global loads being the ultimate pressures.

The standard doesn't seem to indicate how the material capacity is to be calculated. Should it go into the LRFD equations with 1.0W or ASD with 0.6W? They seem to say not to use the 0.6 factor but you get a much greater capacity from phi*Mn than you will from Mn/Omega if you just use 1.0W. Is there a definitive answer somewhere?

 

[Once20036]

As you know, everything wind related got jumbled from ASCE 7-05 to ASCE 7010. In my experience, structural engineers generally understand what happened, but folks outside the professional are unaware or baffled. I've had countless conversations with MEP manufacturers about the required wind load rating for their rooftop equipment and its sometimes time consuming to convince them that old 90mph is new 115 mph, and when we say 115mph we're including the safety margin that they used to apply on their end.
FM global seems the same, they're not structural engineers but they've written a technical document used by structural engineers.

My experience goes back to FM 1-28 2012. This document says, "The guildelines in this data sheet are derived from the basis provided in ASCE Standard 7. The following exceptions have been adopted..." "Use an importance factor of 1.15 for components, cladding, and secondary structure framing..." "Use FM Approved exterior wall panels... with a minimum safety factor of 2.0" and "use a minimum safety factor (or net load factor, ultimate resistance / design pressure) or 2.0 for the securment of above-deck roof components. Use a minimum safety factor or net load factor of 2.0 for failure modes related to securement of metal wall panels. Use a safety factor of not less than 1.5 for other failure modes."

I interpret this as being all service level loads. They're an insurance company and the fewer claims they pay, the more profit they make. I have never seen them reduce loads below what ASCE requires.
In the third quote, they're saying ultimate resistance, but I believe that this is a case of a non structural engineer writing for structural engineers.
The Factor of safety of 1.5 tips my opinion in this direction as the code requires 1.6 under ASCE 7-05.

I believe FM 1-28 2015 follows the same logic. The 1.15 factor is replaced by them requiring the use of a higher wind map. They use an 0.6 factor to convert to service level loads, and then require the same 2.0 factor of safety that they had in `12. If you`re running an LRFD design, I believe that you need to increase the FM load by dividing by 0.6.

 

[CMalenofski]

I tend to agree and see the logic in that. Where it starts to fall down for me is that they require a factor of safety of 2.0. This seems to disregard that the material design also has a safety factor on it, increasing the total safety factor to greater than 2 if you just multiply ASD loads by 2 and then do an ASD design. For steel or aluminum we'd be at a safety factor of about 3.33. What you said above about mechanical contractors being able to take out their 2.0 because it's now built in seems to indicate that we don't need to use the 1.67 for steel or 1.65 for aluminum.

I'm just not liking the lack of specificity on application in this document. It's not seeming to have guidelines for the application of the load, just the calculation of it.

 

[Once20036]

I don't think they're disregarding the material design safety factor, I think they're aware of it and want more, so that they pay less.

I`m not sure that the mechanical contractors are able to take out their 2.0, or if they even have a 2.0 to begin with. Typically my time in 1-28 is for exterior walls.
I mentioned mechanical contractors just as an example for people confused by the change in wind code, as I believe that FM is confused.

Most of the time I`m looking to figure out the spacing of wind girts to support exterior metal panel walls. FM says that the panels themselves and the connection of the panels to the building needs include this 2.0 factor, but the design of the wind girts, connections, columns, MWFRS, etc are just designed per ASCE loads.

What are you designing that brings you to these questions?

 

[CMalenofski]

I'm designing a curtain wall system and their attachments. So it should be similar to your metal panels in some ways. I'm not totally following the panel and attachment being 2.0 on top of everything else but the framing behind it being treated differently since if that framing fails the system fails.

Below is a link to an article I just found Googling with one of FM Global's staff engineers seeming to agree we just need a net safety factor of 2.0.

https://steeljoist.org/wp-content/uploads/C-ConstrIssues-Fisher-Mar17-1.pdf

 

[Once20036]

I`m not familiar with the standards they reference, 4451 or 4470, but I`ll certainly dig into it.

The commentary describes for the roof deck what I described for the wall panels. The roof/skin are designed for 2.0, the deck/girt is not.

With metal panel, I think that the idea is that the steel structure is ductile. If this steel fails it will bend and bow, but likely not rip off the building, hence standard ASCE design loads.
If a single metal panel fails, or the connection of a metal panel, you now have a hole in your building and all that rain getting into your building.
I suspect this is based on their numerous claim investigations and seeing lots of panels ripped off but not seeing any/few failed wind girts.
Metal panel guys have told me that in order to have an FM approved product it needs to be load tested. I have received 1 exemption to this testing requirement in the past, based on similarities to a tested assembly.

Have you reached out to FM?
I think that will be a frustrating conversation, but its worth trying.
The last FM guy I talked to told me that I couldn't follow FM 1-28/16 section 2.7 "Use of ASCE 7-10" because he didn't have a copy of ASCE 7-10.

 

[CMalenofski]

I have reached out to them and had to leave a message. I'm hoping to hear next week. I think you are right that I'm in for frustration.

Thank you for all the insights!

 

[JLNJ]

Here's what I had written a while back to one of our architects explaining FM's loads and safety factors as best I could understand it:

Under the old ASCE 7-05 and earlier service-load system, FM required a Wind Importance Factor of 1.15 be applied, even if the building was not a Risk Category III or IV building (i.e. hospital, fire station, etc.). That defined a pressure which was doubled to specify the 1-XX wind uplift rated roof. The XX in psf represents the pressure the tested assembly can resist for 60 seconds without failing. So, under the old system, your ASCE component and cladding load might calculate out to, say, 32 psf. The FM roof designation would need to be p x I x 2.0 ---> 32psf x 1.15 x 2.0 = 73.6 psf. So an 1-75 roof system would be specified. The structural group would still use the 32 psf for the structural components (actually, less than 32 psf, based on member tributary area).

For the design of the structural system, this “lesser” number is accompanied by a Load Factor when combined with dead, live, seismic, and other types of loads. The Load Factors are different for different loads, and vary based on the way the loads are combined. The traditional load factor which accompanies wind is 1.6, so a combination which includes dead load, live load, and wind might be 1.2 Dead + 1.6 Wind + 1.0 Live. (These load factors are for “strength design”. Allowable stress design has different load factors.)

Under the new ASCE 7-10 wind specification, two things have happened. The importance factor has been built into the wind speed AND the resulting pressures from the new wind speeds are ultimate-level forces. Since the wind is now ultimate (or strength) level, the basic Load Factor for wind decreases from 1.6 to 1.0. So the same load combination I gave above is now 1.2 Dead + 1.0 Wind + 1.0 Live. In order maintain equal load levels, the basic wind speed needs to increased proportionally to yield 1.6 times the pressure, and, since wind pressure varies by the square of the velocity, the pressure is increased by the square root of the load factor ratios, i.e. (1.6/1.0)^.5 = 1.264911… So the traditional 90 mph wind speed is increased to 90 x 1.264911 = 113.8. ASCE rounds this up to 115 mph on their Risk Category II maps.

Now, instead of having ONE wind speed map and applying one of the three different wind importance factors, ASCE 7-10 has NO importance factor and three different wind speed maps.

Again, the pressure varies by the square of the wind speed, and because Factory Mutual wants the equivalent of the old Wind Importance Factor of 1.15, the wind speed needs to be bumped up by the square root of 1.15, which is 1.07238…. This Risk III/IV velocity (90mph x 1.264911 load factor x 1.07238 importance factor = 122 mph) is rounded down to 120 mph on the ASCE 7-10 wind speed maps.

To my way of thinking, we can use the pressures from the Risk Category III/IV maps and double those pressures for FM roof system determination, or use the Risk Category II maps and bump the pressure up by 1.15 before doubling them for FM designation.

Does all that make sense?

I guess the important thing an all this is that for cladding attachment, FM wants us to use Risk Category III/IV pressure numbers for ALL buildings, not just those specifically defined as Risk Category III or IV ( i.e. essential facilities, facilities which pose a substantial hazard to the community, and/or facilities which could pose a substantial risk to human life).