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walkereng
walkereng, Consultant
Category: Structural Engineering
Satisfied Customers: 2598
Experience:  Over 30 years of Structural Engineering experience.
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I've worked with a few engineers to design a roof system

Customer Question

Hello, I've worked with a few engineers to design a roof system for my home extension in Fort Worth, TX area. It's 50 foot wide and 60 foot long with 3:12 roof pitch. Engineers called for a structural ridge (gable to gable)beam 18WX50 with support in center
and then 9X7X1/4 HSS every X' based on purlin span capabilities. So I can go 10' and use 4'' C purlin or I can go 20' and use 8'' C purlin.This extension has lots of porches where I plan to use 2X2X1/8 tubing for purlin and not C purlin leaving it exposed.
Given price is about the same for tubing and it's easier to weld and I don't need clips. I'm thinking about just using tubing for purlin on main roof system as well as porches. Can you give me purlin equivalence for C vs tubing for both 4'' or 8'' C purlin
(14 gauge). I plan to space them every 4' with steel structural panel and sprayfoam. Thanks..
Submitted: 1 year ago.
Category: Structural Engineering
Expert:  walkereng replied 1 year ago.

Do you have any kind of sketch or plans you can share with me?

Customer: replied 1 year ago.
I tried to upload the plans, but there is a 5MB limit.
Customer: replied 1 year ago.
The question is more general in terms of engineering. How does C purlin compare to tubing in terms of strength for 4'' and 8''.
Customer: replied 1 year ago.
It's a pdf so compressing it doesn't shrink the file size.It's about 8MB.
Customer: replied 1 year ago.
Nothing is really time sensitive. Let me know how you want to proceed.
Customer: replied 1 year ago.
I uploaded a few screen shots. Hope that helps.
Expert:  walkereng replied 1 year ago.

try sending to***@******.***

Let me know on this Chat Page after you send it, then I will check for the file

Customer: replied 1 year ago.
Each of the three sections are new and we are building roof system over them. The walls are 8'' core filled block walls with #4 horizontal and vertical rebar. The gable ends are also built from block. W beam from gable to gable. HSS for rafters 10' OC. Then either C purlin or HSS for purlins.
Customer: replied 1 year ago.
Just emailed the full file.
Expert:  walkereng replied 1 year ago.

Let me take a look

Expert:  walkereng replied 1 year ago.

I’d like to point out that a Professional Engineer’s standard of care typically includes a site visit to assess field conditions and get an overall understanding of the structure. This can obviously not be accomplished through the internet. The information provided here is meant for planning purposes only (general sizing and budgeting) and is based on the information provided by you. All loading cases considered are for vertical loads only, no lateral analysis has been completed. The information should be verified by a professional engineer who can visit the site to ensure that potentially important information has not been overlooked or omitted.

Assuming a Live Load of 20 psf and a Dead Load of 10 psf, member span of 10' and a tributary load width of 4', you can use the following:

4" deep members

C 4x5.4 (5.4 plf)

4"x2" x3/16" structural steel tubing (6.87 plf)

8" deep members

C 8x11.5 (11.5 plf)

8"x2" x3/16" structural steel tubing (11.97 plf)

If you feel you have received a satisfactory answer to your question, click the Rating button that is appropriate. Experts are credited for each adequately Rated answer they provide. If you have additional questions, please let me know. Thanks

Expert:  walkereng replied 1 year ago.

I would use the structural steel tubing, due to the fact that it has much better lateral stability than "C" channels.

Customer: replied 1 year ago.
Are you talking C channel or C purlin which is typically light gauge metal?
Customer: replied 1 year ago.
We have metal buildings with same basic roof system. If bays are 20-25' we use an 8'' C purlin if they are 10' we use the 4'' C purlin or they also have a Z purlin. Both are light gauge 14 or 16. This is basically rule of thumb. None of them use tubing for purlin, but I see most porches, boat house roofs, etc. going that way to keep birds from building nests, etc. So I have to think that a 1/8 2X2 tube is as strong as a light weight 4'' C purlin. Here is an image. I think what your referencing is a C channel which is different and not typically used for purlin on roof systems. Let me know.https://www.google.com/search?q=c+purlin&espv=2&biw=1640&bih=925&tbm=isch&imgil=EBmrYKw8iL2e-M%253A%253BaC2TVnp8h4gHjM%253Bhttp%25253A%25252F%25252Fwww.textor.com.au%25252Fproducts%25252Fview%25252Fc-purlin&source=iu&pf=m&fir=EBmrYKw8iL2e-M%253A%252CaC2TVnp8h4gHjM%252C_&usg=__V6cHJSukKIV1GTtkVGz1DBuHb1Y%3D&ved=0ahUKEwicjI_3m8PJAhXGKiYKHfdLCEsQyjcILQ&ei=1g5iVpzJC8bVmAH3l6HYBA#imgrc=eV2BMianV6qIYM%3A&usg=__V6cHJSukKIV1GTtkVGz1DBuHb1Y%3D
Customer: replied 1 year ago.
Here you go. I'm talking about the roof purlin in the below image. The R panel is screwed to this.https://www.google.com/search?q=steel+building+anatomy&biw=1640&bih=925&tbm=isch&imgil=CgIN74NeuV087M%253A%253BoqKd-kdMwxG6BM%253Bhttp%25253A%25252F%25252Fwww.arcosteel.com%25252Farco-products%25252Fmetal-buildings-101%25252Fmetal-building-anatomy%25252F&source=iu&pf=m&fir=CgIN74NeuV087M%253A%252CoqKd-kdMwxG6BM%252C_&usg=__W_5suay6n--AUYDJi6fx0er4TGo%3D&ved=0ahUKEwjixr2PnsPJAhWJ8CYKHbU2ALgQyjcIKw&ei=IhFiVuKtCYnhmwG17YDACw#imgrc=CgIN74NeuV087M%3A&usg=__W_5suay6n--AUYDJi6fx0er4TGo%3D
Expert:  walkereng replied 1 year ago.

You are correct, sorry I was looking at C Channel sections.

You can use 8" depth C-Purlins that are 16 gauge for your span.

I still like the use of the 4x2 x 3/16" tubes.

Customer: replied 1 year ago.
I think to wrap this question up I need Tubing equivalent span strength for the following.
8'' C-Purlin 16 gauge = ?
4'' C-Purlin 16 gauge = ?I think what I'm hearing from you is 4X2X3/16 is equivalent (better for lateral forces) to 8'' C-Purlin for span capability.I need the same for the 4'' C-Purlin. Thanks..
Customer: replied 1 year ago.
Still waiting on this. Maybe you marked it as complete?
Expert:  walkereng replied 1 year ago.

sorry for the delay

you are correct, your best option for your 10' span with a tributary width of 4', is a 4"x2" x3/16" structural steel tubing (6.87 plf)

If you feel you have received a satisfactory answer to your question, click the Rating button that is appropriate. Experts are credited for each adequately Rated answer they provide. If you have additional questions, please let me know. Thanks

Customer: replied 1 year ago.
The 4X2X3/16 is for 8'' C Purlin equivalent. I still need 4''.
Expert:  walkereng replied 1 year ago.

that is the purlin you can use in either case, since the loading is the same

Customer: replied 1 year ago.
The other / previous engineers are telling me that rafter spacing is dependent on purlin span capability. I can go 10' with 4'' C-Purlin or I can go 20' with 8''. So I need tubing equivalent for each. I wouldn't think they are the same. BTW, the 14 gauge 8'' purlin is 3X the cost of 2X4X3/16.
Expert:  walkereng replied 1 year ago.

sorry for the confusion, let me run numbers for a 20' span purlin

Expert:  walkereng replied 1 year ago.

for a 20' span, you can use a 5"x4" x1/4" structural steel tubing (13.91 plf)

Expert:  walkereng replied 12 months ago.

f you feel you have received a satisfactory answer to your question, click the Rating button that is appropriate. Experts are credited for each adequately Rated answer they provide. If you have additional questions, please let me know. Thanks

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