Z Purlin on PEMB
Z Purlin on PEMB
(OP)
Roofer is trying to retrofit a new standing steam metal roof over an existing metal roof over Z purlins. We are adding about 2.5 to 3 psf in DL but I cant get them to check out. However, when I reference load tables from a local manufacture for these purlins it checks out. Its not a lot of load we are adding but I know there isn't usually any "fluff" designed into these PEMB's. I know these purlins typically get designed as a system with multi-spans and laps at supports for added moment capacity but still cant get it to work even though its close. The new roof system will have smaller z shape like purlins fastened to the top of the existing ones with the new panels attached to these. I know this will also stiffen up the existing purlins but I am not sure how to incorporate this into my calcs.
Also, the code allows up to an additional 3psf of load to existing roofs without having to check the capacity of the existing supports. The roofer knows of this code as well so if I go back and say sorry no dice it could really stir the pot up. What is the play here?
Also, the code allows up to an additional 3psf of load to existing roofs without having to check the capacity of the existing supports. The roofer knows of this code as well so if I go back and say sorry no dice it could really stir the pot up. What is the play here?






RE: Z Purlin on PEMB
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Z Purlin on PEMB
RE: Z Purlin on PEMB
Manufacturer's never calculate the capacity of their roof purlins? Then what is the AISI code fore?
I recently was introduced to someone who writes code for the metal building programs. I routinely give him my roof purlins (any size) and he can calculate the capacity of the purlin system..... which almost always never work (maybe this is what you are referencing).
KootK,
I believe this is the 5% limit allowed by chapter 34 of the IBC.... but in order to get 3 psf to equal 5% of the load, the load on your roofing system would need to be 60psf.
RE: Z Purlin on PEMB
I'm not aware of any specific 3 psf rule, although some manufacturers will include a 3 psf collateral load at a minimum. This is generally intended to account for sprinklers, some lights, etc. not a reroof, but if the other items aren't present that may be available for this purpose. Going way back, the old Wisconsin code mandated a minimum 3 psf collateral load.
Now to the positive side, if the original roof is a standing seam roof the act of screwing down the reroof supports through the lower panel may create the effect of turning the floating standing seam roof into a through-fastened roof. Depending upon the spacing of these supports you would potentially be able to claim full lateral support to the purlin flange due to the fastening. Distortional buckling needs to be considered although a lot of people don't necessarily consider it for a through-fastened gravity condition. There are mixed test results on that score.
RE: Z Purlin on PEMB
RE: Z Purlin on PEMB
RE: Z Purlin on PEMB
It's really up to you to decide which direction to go in. You are currently giving code references that allow you to add the roofing without issue. As ajh references, a lot has been added to the design of these systems in the past 20 years or so. So it is going to be impossible justify a building that is older than this for the new code. It's all a matter or what you are comfortable with at this point.
RE: Z Purlin on PEMB
DaveAtkins
RE: Z Purlin on PEMB
RE: Z Purlin on PEMB
A few comments on what you attached.
* Charts based on 2001 AISI, therefore they do not take into account distortional buckling provisions introduced in 2007.
* Note indicates to boost allowables by 1.33 for wind. This is not valid with most current building code provisions.
* They make a sort of waffling statement regarding loads are only valid if compression flange is ADEQUATELY supported laterally, better read that as "has a through-fastened roof panel". AISI uplift provisions will knock your allowables down substantially further without bracing. (I recognize this is not an issue for what you are currently doing.)
* Base test results do not come into play as this is a component provider who is not going to have a tested combination of panel and purlins.
*** My biggest point is make sure you know exactly what thickness of metal corresponds to each of their "gage identification". We had an issue with Metal Sales a number of years ago when they somewhat arbitrarily reduced their 14 ga. section from 0.075" to around 0.070" without full disclosure to the folks buying their product.
RE: Z Purlin on PEMB
load through 3" Zee: EI_3z / (EI_8z + EI_3z)
load through 8" Zee: EI_8z / (EI_8z + EI_3z)
Alternatively, you could look at combined members as composite provided the screw connecting the two zees has a shear capacity that transfers the longitudinal shear to keep the zees from slipping past each other: V*Q/(I*b)
Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant
RE: Z Purlin on PEMB
I didn't say that manufacturers don't calculate capacity. Just that I have never done it, but rather relied on available load tables, which are largely based on full scale testing. But that is in Australia, not in the US. We don't have PEMB here, but we use purlin systems for almost all of our commercial and industrial buildings.
RE: Z Purlin on PEMB
RE: Z Purlin on PEMB
Is the original PEMB supplier around? I doubt they'll help you a whole lot, but they might give you a hint. Maybe the material properties of their purlins routinely exceeds your assumptions. Or possibly a non involved manufacturer (Butler, Varco Pruden, etc.) might give you some help.
RE: Z Purlin on PEMB
hokie66, the new section getting attached to the existing purlins is prefabricated with a notch at the same spacing as the existing metal panels "standing seams" so it fits flush in between these areas.
JedClampett, yes wind will more than likely control but I haven't been given the site specific info yet. They have product approvals on their new roofing system going on top of the existing which is rated I believe up to 170mph but obviously checking the existing purlins is a must. I doubt the original supplier is still around as well but I will see if I can contact one of the manufactures you mentioned and see if I can get some help.