Cold Formed Z Purlins 2

[landongws]

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.

 

[ash060]

Typically the purlins are designed as continuous, and the lapped section provides double the capacity because there are two purlins.

Also, in light gauge design the I.P. is sometimes considered a brace point.

 

[frv]

I wouldn't think so.

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.

 

[frv]

ash060-

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?

 

[MiketheEngineer]

There have been a number of threads about these - just take a look (search).

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

 

[landongws]

I will be adding reinforcing to get the additional 10psf snow load capacity. The tough part for me was getting the steel to work for the original 30psf snow load. This included the purlins and the tapered steel rigid frames. I was just looking for any tricks that the PEB industry might be using. I have seen the splices of purlins in the field and they are not exactly a tight fit and there is no flange to flange attachment so the continuous moment transfer is a mystery to me.

Thanks for all the responses.

 

[ToadJones]

You're probably going to find that the steel frames will not work. Some of these frames deflect a scary amount under design snow loads....and you're looking to add 33% more snow load.

 

[JD2]

I have a Design Handbook (Light Gage Structural Steel Framing System Design Handbook) published by the Light Gage Structural Institute dated about 1991. In it, they give design values for various light gage sections considering three different beam span conditions (2 span, 4 span and 6 span) considering 4 different "lap" conditions (simple span, long lap (2'-3 1/2"), max lap (4'-3 1/2") and super lap (6'-3 1/2")) They also have shown what they are calling a super lap and an extended lap for 10" and 12' sections.

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.

 

[jropert]

Here's a good website that has some design charts and information on typical laps, member capacities, etc. for Cees and Zees that I use for this type of design (gives you at least a quick "feel" for what size members will be in the ball park).

http://www.loseke.com/lgsi.html

 

[ash060]

frv

-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.

 

[frv]

ash060..

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.."

 

[DaveAtkins]

JD2 is correct--these are designed as continuous. One purlin is nested within the other--that is how continuity is achieved.

DaveAtkins

 

[ash060]

Yes, AISC does address the issue, but AISI does not as far as I know

 

[asixth]

And the purlin bolts at the lap locations must also be designed for the longitudinal shear component if you want composite action to occur.

 

[DaveAtkins]

asixth,

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

 

[asixth]

DaveAtkins,

Do you detail Z-purlin laps something like this?

The full installation guide can be found here.

http://www.stramit.com.au/downloads/Purlins, Girts & Bridiging Detailing & Installation Guide.pdf

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.

 

[csd72]

normally lapped purlins are designed as continuous and the critical section is at the end of the lap where it reduces from 2 purlins to one.

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

 

[DaveAtkins]

Looks like a great resource, asixth.

Yes, those are the types of shapes being used in the U.S.

DaveAtkins

 

[structuresguy]

THanks asixth

 

[dcarr82775]

The purlins are designed as continuous. The purlin bolts are necessary for continuity. Simply nesting the purlins does not make it a continuous member.

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.