ARMeerkat
Structural
- Oct 24, 2010
- 2
I've spent 25 years as a structural engineer designing just about every type of commercial and industrial buildings. These buildings include PEMB's. While I've never designed a full PEMB, we have specified hundreds of them and provided the foundation design to support them. One question is always asked by the PEMB manufacturer, "Is the roof live load reducible on the main frames?" It's always bothered me that they feel the need to ask this question, and now I've figured out why.
Whenever checking the roof load capacity of an existing PEMB we can never get the roof purlins (z-purlins) to pass. The design load book that we utilize is the Light Gage Structural Steel Framing Design Handbook (LGSI). I can never get the example problems to match up with the actual design provided by the PEMB manufacturer. I've always assumed, like others, that this is due to some hidden code provisions, or maybe due to actual testing of the purlins. While recently going over the design of an existing PEMB, and once again knowing that the purlins would not pass code, my curiosity got the best of me and I started researching to try and find the answer as to why. The following is a narrative of my thought process and what I found.
The load tables for the z-purlins are broken down into simple-span, two-span, four-span, and six-span conditions. Each of these span conditions is further broken down based on the length of the end laps on the purlins. The premise being that the more end lap you have the more continuity you have, therefore more load capacity and less deflection. So, even though the typical purlin span is only 25 feet, the capacity is increased as though it is continuous over multiple spans. And this is what got me to thinking... if the PEMB manufacturer is assuming the purlin to be continuous over multiple spans, could he also be assuming that the purlin is a single continuous member over the length of the building? If they are assuming a single continuous member over the length of the building, then the tributary are of that purlin would be great enough to justify a roof live load reduction.
So, we puled out three different PEMB projects from three different PEMB manufacturers. All three buildings had the same roof dead load and standard 20-psf live load requirements. The projects were also in three separate states, which became very important. After running the numbers based on the PEMB shop drawings, we could only get one of the buildings purlin design to pass. The other two buildings were then recalculated using a purlin tributary area based on the building length and purlin spacing. We reduced the roof live loads accordingly and checked the purlin design against the LGSI Handbook. Both of the buildings purlins passed within a couple of percent (definitely an AH-HA moment). The difference in the building in which the purlins did pass code is that it is located much further north and had a snow load of 20-psf, which is not reducible, and therefore could not be taken advantage of.
When the PEMB manufacturer asks "Is the roof live load reducible on the main frames?" what they are really doing is omitting the roof purlins from the question. Why ask specifically about the main frames only? Why not just ask if the roof live load is reducible per the code? It is a very dishonest question, designed to create a loop hole by elimination. Don't ask, don't tell.
When a PEMB bulding is 200-ft long and the roof purlins are spaced 4-ft on center (very common) the purlins tributary area (per the PEMB) is 800-sf. The PEMB manufacturer reduces the purlin design roof live load from 20-psf to 12-psf, and never mentions a word about it. We've always thought that PEMB's were deisgned to a frogs hair, but it turns out it's even less than that. Prove me wrong.
Whenever checking the roof load capacity of an existing PEMB we can never get the roof purlins (z-purlins) to pass. The design load book that we utilize is the Light Gage Structural Steel Framing Design Handbook (LGSI). I can never get the example problems to match up with the actual design provided by the PEMB manufacturer. I've always assumed, like others, that this is due to some hidden code provisions, or maybe due to actual testing of the purlins. While recently going over the design of an existing PEMB, and once again knowing that the purlins would not pass code, my curiosity got the best of me and I started researching to try and find the answer as to why. The following is a narrative of my thought process and what I found.
The load tables for the z-purlins are broken down into simple-span, two-span, four-span, and six-span conditions. Each of these span conditions is further broken down based on the length of the end laps on the purlins. The premise being that the more end lap you have the more continuity you have, therefore more load capacity and less deflection. So, even though the typical purlin span is only 25 feet, the capacity is increased as though it is continuous over multiple spans. And this is what got me to thinking... if the PEMB manufacturer is assuming the purlin to be continuous over multiple spans, could he also be assuming that the purlin is a single continuous member over the length of the building? If they are assuming a single continuous member over the length of the building, then the tributary are of that purlin would be great enough to justify a roof live load reduction.
So, we puled out three different PEMB projects from three different PEMB manufacturers. All three buildings had the same roof dead load and standard 20-psf live load requirements. The projects were also in three separate states, which became very important. After running the numbers based on the PEMB shop drawings, we could only get one of the buildings purlin design to pass. The other two buildings were then recalculated using a purlin tributary area based on the building length and purlin spacing. We reduced the roof live loads accordingly and checked the purlin design against the LGSI Handbook. Both of the buildings purlins passed within a couple of percent (definitely an AH-HA moment). The difference in the building in which the purlins did pass code is that it is located much further north and had a snow load of 20-psf, which is not reducible, and therefore could not be taken advantage of.
When the PEMB manufacturer asks "Is the roof live load reducible on the main frames?" what they are really doing is omitting the roof purlins from the question. Why ask specifically about the main frames only? Why not just ask if the roof live load is reducible per the code? It is a very dishonest question, designed to create a loop hole by elimination. Don't ask, don't tell.
When a PEMB bulding is 200-ft long and the roof purlins are spaced 4-ft on center (very common) the purlins tributary area (per the PEMB) is 800-sf. The PEMB manufacturer reduces the purlin design roof live load from 20-psf to 12-psf, and never mentions a word about it. We've always thought that PEMB's were deisgned to a frogs hair, but it turns out it's even less than that. Prove me wrong.
