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walkereng
walkereng, Consultant
Category: Structural Engineering
Satisfied Customers: 1719
Experience:  Over 28 years of Structural Engineering experience.
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WE WANT TO SPAN 20 WITH AN I-BEAM THAT SUPPORTS A ROOF WITH

Resolved Question:

WE WANT TO SPAN 20' WITH AN I-BEAM THAT SUPPORTS A ROOF WITH A TRUSS SPAN OF 26' IN NORTHERN VA. WHAT SIZE DO I NEED? WE WANT A BEAM WIDTH OF ABOUT 4".
Submitted: 1 year ago.
Category: Structural Engineering
Expert:  walkereng replied 1 year ago.

walkereng :

I can help

walkereng :

let me read your question a bit closer

walkereng :

Can you take a look at the following link and answer the questions at the bottom of the sheet? Your answers will allow me to accurately model the loading and size your beam. Thanks


 


https://docs.google.com/open?id=0B3LjUB_rxIbcNVlreGljdFZUZ3kxTHhvVDFnRV9uQQ

walkereng :

Would it be a New Beam #5?

Customer:

The beam is closest to the center, #3 and supports only the asphalt shingled 4/12 roof. We have run lolly columns to the foundation of this one story with a basement house.

walkereng :

Will it also support the ceiling and drywall?

walkereng :

Are you still there?

Customer:

Yes. What we have done is to remove a semi-load bearing wall between the kitchen and living room and, had originally put in two 12" x 1.75" microlams bolted together, but it does'nt seem like quite enough as we are getting about a 1/2" sag. we also cut away the trusses above the kitchen and put in vertical supports from the plate that sits on the beam to the top of the trusses.

Customer:

We are putting a cathedral ceiling in the kitchen.

Customer:

We were looking at a beam roughly 4" x 10" at 15 lbs/ft. Do you think this will be enough? I can send you a picture if you like.

walkereng :

You have to be very careful about cutting truss members. The internal truss members are designed specifically for the configuration they were shipped to the job site for. You can not just cut out sections and replace that with new elements.


 


Truss members all have different forces in them. Some members are in compression and some members are in tension. When you cut out an internal member, the entire force distribution in the truss will change.


 


On another note, most pre-manufactured trusses are not designed to have bearing points at their midspans

walkereng :

I would not recommend that you do this until you have talked to the truss manufacturer that fabricated your trusses and get them to analyse the new configuration and agree with your modifications.

walkereng :

What city are you close to in VA so I can look at your design snow load requirement?

Customer:

This is an older house ca 1950 with the trusses built on site and nailed together with plywood plates. The previous owner had already removed the wall and made half the room a cathedral ceiling. It sagged a good 1 1/2". we are trying to fix the problem. let me send you a picture. We are in Manassas.

walkereng :

You can attach it to the chat window with the icon in the toolbar

Customer:

Can you give me an address we can sen a picture from an I phone?

walkereng :

tryXXX@XXXXXX.XXX

Customer: Thanks

 

Customer:

The trusses are 24" oc and we realize it may take a little while to figure this.

walkereng :

I just got the picture, let me print them out

walkereng :

Before we get started 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.

walkereng :

One problem I see is if you go to a steel beam, connections will be harder to accomplish between your timber members and your steel beam

Customer:

We have thought of that, and I have worked with I-beams before. We are sending more pictures to try to give you a better grasp. We will probably have some small holes drilled to attach wood tot he bottom to aid in drywalling. I appreciate the lateral support issues you have mentioned.

walkereng :

I can actually see the sag in the beam in one of your pictures. A big issue is that you have a sag in your beam with only Dead Load on it. The snow load for your area is 25 psf, when that is added you will really have sag and possible cracking and failure

walkereng :

Another issue is your roof rafters in the cathedral ceiling side are way over-spanned.

walkereng :

Are you going to sister in some deeper roof rafters that can span your approx. 13' with a 25 psf snow load on top of your Dead Load?

walkereng :

I just printed off 8 pictures. I am leaving my office right now. I am going to have to put some thought into this. I will be able to give you some advice for planning and budgeting purposes, but when all is said and done, you will really need to have an engineer come out for a site visit and give you the final guidance.


 


I am going to switch over to the Q&A format and we can continue with the e-mail process. When I come back online, I will send you an e-mail to tell you I am back, so check your e-mail periodically (use the e-mail account that you gave to JustAnswers). You do not need to rate any answer yet until we are complete and you are satisfied. Thanks.

Expert:  walkereng replied 1 year ago.
Our chat has ended, but you can still continue to ask me questions here until you are satisfied with your answer. Come back to this page to view our conversation and any other new information.

What happens now?

If you haven’t already done so, please rate your answer above. Or, you can reply to me using the box below.
Expert:  walkereng replied 1 year ago.
I looked over your information. I can give you some information for possible planning and budgeting purposes. You really need to have an engineer come out for a site visit to see your entire roof/ceiling configuration and then give you his recommendations.

If I assume your beam will take one half of the roof and ceiling load (13' wide of your 26' wide roof, 25 psf snow load) I get the following:

If you want to stick with a 4" wide steel beam, the largest beam with a 4" width is a W12x22 (50 ksi) steel beam. It is 4" wide, 12.25" deep and weighs 22 plf. This beam will deflect 3/4" at its midspan under full dead load and live load.

If you can go to a 5" wide steel beam, you could use a W14x26 (50 ksi) steel beam. It is 5" wide, 13.875" deep and weighs 26 plf. This beam will deflect 1/2" at its midspan under full dead load and live load.

I just made real rough assumptions and did not get into how the existing roof trusses have been compromised by being cut and reconfigured.

Like I said earlier, you should sister larger roof rafters in your cathedral ceiling area to allow them to span between your exterior wall and their new support point over your new beam. If your new rafter span is approx. 12', you could use 2"x12" Douglas Fir #1 grade members at 24" o.c. (existing truss spacing).
Expert:  walkereng replied 1 year ago.
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.
Let me know if you have any more questions
Expert:  walkereng replied 1 year ago.
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
walkereng, Consultant
Satisfied Customers: 1719
Experience: Over 28 years of Structural Engineering experience.
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