Cold Formed Z Purlins
Cold Formed Z Purlins
(OP)
I am looking at an existing PEB with 8 1/2" Z purlins spanning 25 ft. @ 5ft o.c. Snow load = 30 psf, DL =2.5 psf. The purlins are lapped 2.5 ft. Does the lapping detail constitute continuous beam action? I am having a hard time making these work for the original design loads much less coming up with a reasonable fix for upgrading to 40 psf snow load.






RE: Cold Formed Z Purlins
Also, in light gauge design the I.P. is sometimes considered a brace point.
RE: Cold Formed Z Purlins
I assume you mean they are lapped 2.5' over the supports. The purlins are then free to deflect independently (i.e. no moment transfer from one to the other).
PEMB are usually built with almost no "fat" whatsoever. It is highly unlikely that you will be able to squeeze another 10 psf out of it without some upgrading.
RE: Cold Formed Z Purlins
how would the transfer of moment work? I assume the flanges are not connected to one another in order to transfer rotation (and moment).
Disclaimer: I have only specified 2 PEMB's and I designed them both as SS. I'm not really challenging your assumption, just curious as to how it would work.
Also, why would the IP be allowed to be considered a Brace Point in Cold Formed, when it is specifically prohibited by AISC, and there is research suggesting that it is unsafe to do so?
RE: Cold Formed Z Purlins
ANY that I have analyzed (reverse engineered) at best just make it with NOTHING or less to spare. Trying to add 10 psf worth of snow might be almost impossible. Not only will the purlins probably be overloaded - everything down the line will be overloaded also....
Good Luck
RE: Cold Formed Z Purlins
Thanks for all the responses.
RE: Cold Formed Z Purlins
RE: Cold Formed Z Purlins
As an example, the 16 gage 8x2.5 Zee section for a 25' span and a long lap condition (which is closest to what you describe)
simple span 65 plf, 2 span 76 plf, 4 span 89 plf, 6 span 91 plf
Reading the foreward, the design charts were computer generated (no indication of test supported data) and have numerous conditions for the values to be valid one of which is both flanges to be laterally supported based upon axial load charts (Cee section charts are provided, Zee section charts are not).
This really doesn't help your situation other than you might contact the Light Gage Structural Institute (assuming they are still around) and see if they have developed more complete information or design charts over the years.
It appears that they do provide for some amount of moment transfer based upon the lap condition however they give no supporting information as to how they determined the degree of moment transfer or how they modeled the lap conditions to obtain the various degrees of moment transfer.
RE: Cold Formed Z Purlins
http://www.loseke.com/lgsi.html
RE: Cold Formed Z Purlins
-Check the light gauge manual either 2001 or 2008 and assumptions that are accepted as good practice are listed in some design examples.
The lap must be designed to do what is required.
AISI does not prohibit the use of the I.P. as a brace point.
RE: Cold Formed Z Purlins
I'll take you at your word on the light gauge..
As far as AISC, read the commentary to section 6.3 in the AISC 13th Ed. design Manual, p.16.1-425.. "..for beams with double curvature, the inflection point cannot be considered a brace point because twist occurs at that point.."
RE: Cold Formed Z Purlins
DaveAtkins
RE: Cold Formed Z Purlins
RE: Cold Formed Z Purlins
RE: Cold Formed Z Purlins
Composite action won't apply here. The two purlins are nested together side by side, so the section properties are just doubled at the lap.
DaveAtkins
RE: Cold Formed Z Purlins
Do you detail Z-purlin laps something like this?
The full installation guide can be found here.
http://ww
The cold-formed steel market is quite a competitive industry in my country with lots of research going into producing the most efficient section for design purposes.
RE: Cold Formed Z Purlins
But I have 2 concerns here, the length of the lap is too short and secondly from your post there appears to be no positive connection at the end of the lap.
In reality I would say that this would only give partial continuity to the spans
RE: Cold Formed Z Purlins
Yes, those are the types of shapes being used in the U.S.
DaveAtkins
RE: Cold Formed Z Purlins
RE: Cold Formed Z Purlins
Assuming you have standing seam roof that doesn't brace the purlins, be sure to look at the detail that 'braces' the purlins. In my experience the PEMB designers live in fantasy land if they think their details really brace the purlin flanges.
RE: Cold Formed Z Purlins
RE: Cold Formed Z Purlins
RE: Cold Formed Z Purlins
1) Inflection point as a brace point. Research by Dr. Tom Murray at Virginia Tech shows that on a Z-shaped purlin that the inflection point functions in a similar manner to a brace point. The distinction between AISC and the Z under AISI rules is that a typical AISC section does not directly rotate under load and therefore the deflection pattern is the same on both sides of the inflection point. With a Z-shape the purlin rotates about it's center, and the rotation is opposite depending upon whether top flange or bottom flange is in compression, hence providing some stiffness at the inflection point.
2) Continuous laps generally require a minimum of 2 bolts in the web at each end of the lap plus some form of attachment bolts to the supporting frame, either through the lower flange or through the web into a supporting clip. As noted by others, just the lapping does not do it, you need the bolts. I'm not aware of any manufacturer who would suggest they get continuity without lap bolts. Minimum lap length per certain AISI rules is 1.5d on each side of the support, where d is the depth of the purlin.
3) Since 1996, there are only two ways to treat the bracing of purlins. a) Ignore any benefit from the paneling for lateral support and provide intermediate discrete bracing. Design the purlin based strictly on the unbraced lengths between the discrete braces; or b) Perform a series of base tests to determine the benefit of the paneling. The base test is a two-purlin simple span test in a vacuum box modeling the system as it is to be built in the field. Test is run to failure. From the test result and a calculation to determine full strength of the purlin assuming 100% lateral support, a calculated R factor may be determined. For all subsequent design, the capacity of a particular purlin then becomes R times full allowable capacity. Typical R factors run in the 0.50 to 0.70 range. R-factors must be determined for both gravity and uplift loadings. Purlin systems designed before 1996 may have used other approaches for determining the beneficial aspects of panels (or lack thereof).