Z Purlin on PEMB

[kmart30]

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?

 

[KootK]

Where does that 3PSF code rule show up? Assuming that it exists, I vote for letting this go. Structural engineering and small business entrepreneurship are both activities that require some mettle / risk tolerance. You have bigger fish to fry.

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.

 

[hokie66]

I have never tried to calculate the capacity of a purlin system. The capacity of purlin systems as tabulated by manufacturers is based largely on full scale testing, not calculation. If this is a reputable manufacturer with a history of quality, I think you should be fine. 3 psf is nothing anyway, and should assist in wind uplift capacity.

 

[SteelPE]

Hokie,

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.

 

[ajh1]

Load tables are probably somewhat of a dinosaur in this day and age. They are only really going to be valid for simple span conditions (rare) or perfectly matched bay sizes with full uniform load only. As soon as you get mixed bay sizes of any amount and/or deal with patterned live and snow loads, load tables become so complex to publish that they aren't generally used. The challenge with back analyses like this is the lack of available information. What is the yield stress of the purlin? Do you know the cross-section precisely? If the existing roof is also a standing seam, when was the roof built? (Prior to 1996 the code was silent and many manufacturers felt that a standing seam panel would brace the top flange of the purlin. That was outlawed with the 1996 AISI spec.) Is there discrete bracing on the purlin? If not, are the appropriate load factors for a base test available? If prior to 2007, distortional buckling was not a considered limit state, so would have an impact on current evaluations.
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.

 

[kmart30]

Im in Florida so Im dealing with the code here but I know its all based off the IBC. Its in the Existing Building Code section 707 I believe. There are a few exceptions but when adding a second roof layer you are allowed up to 3psf; or if its a piece of equipment or something else you are allowed up to a 5 percent increase.

 

[kmart30]

Here is what they are trying to do if anyone was curious....

 

[SteelPE]

OK, I see where you are getting the additional 3psf from. I have never really looked at this section before. I do everything I can to stay out of the "alterations level" section of the code as I find it doesn't take much to trigger large upgrades in the code.

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.

 

[DaveAtkins]

Remember, at laps over interior supports, the purlin has twice its normal capacity, because you have one purlin nested inside another. Perhaps this is how the manufacturer is getting it to check out.

DaveAtkins

 

[kmart30]

Here is the reference I typically use from a local manufacture. Section properties and load tables with "lap lengths" are shown towards the bottom of the pdf. It says the allowable loads are based off the 2001 AISI Specification so I believe its a good reference and not just test values from a manufacture. Page 47 goes into where there information is based from.

 

[ajh1]

Interesting info: You do have things a bit simpler for gravity conditions given that you are in Florida. Current codes permit ignoring patterned live loads as long as you maintain a basic uniform live load of 12 psf. Tributary reduction never really comes into play for Z-purlins anyway because you simply don't have enough square footage per purlin to hit the 200 sqft target.
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.

 

[P1ENG]

If you were to consider a load share between the 8" Zee and the 3" Zee, will that help you get the 3 psf increase? As long as you calculate the screw to have a tensile capacity equal to the shared portion of load through the 8" Zee, it should be good. Provided they are fastened together with a tight frequency they have to have the same deflection curve.
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

 

[hokie66]

SteelPE,
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.

 

[hokie66]

Maybe not your call, kmart30, but most metal reroofing work involves removing the existing roofing rather than overlaying it.. Your sketch shows having to saw out strips of the old roofing over the purlins for installation of the new battens, which is a lot of work, very time consuming. If I were forced into the overlay scheme, I would want to use wood battens, laid on top of the existing roofing and screwed through into the purlins.

 

[JedClampett]

Depending on where you are in Florida, the likelihood is that wind uplift controls, especially at the purlin (C & C) level. So the load you're adding is actually reducing the net wind uplift. But the bad news is that if the purlins don't work for downward loads plus 3 psf, I don't see how they're going to work for wind uplift.
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.

 

[kmart30]

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.