Stregthen trusses

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House was built in '74 using trusses made of 2x4s. It needs a neew roof and the design of the house lends itself to replacing the asphalt shingles with Spanish terra cotta tiles. In this case probably the light weight concrete ones. Roofer said the existing trusses will never support the extra load and I agree.His suggestion was to build new trusses sistered next to the old one out of enginner lumber(wood I beams) not that difficult to do in this attic but probably very labor intensive. I need something me and some of my poker buddies can do without hiring a crew. I have at least couple of years to do this project before thes roof has to be replaced, I figure since wood I-beams are so strong. box beams would be even stronger. My plan is to use use 9" strips of OSB one each side of the joist and rafters and completing the box beam with another 2x4. This is something I could do myself over the course of a couple of years or less. Attic is tall and has a 6 ft wide walkway down the center. Insulation is sheets of rigid foam.
Also could build I beams in place using exsiting trusses as part of the I.
Any thoughts on my plan would be appreciated.
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On Mon, 10 Apr 2006 23:56:10 -0400, " snipped-for-privacy@bellsouth.net"

They use 2x4 trusses under concrete tiles too. You don't need the advice of a roofer, you need an engineer. It might be fine just the way it is.
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My reaction too. Have an engineer analyse the existing trusses. or Ask a truss builder to price new trusses for the new load. In addition, the clips and ties holding the trusses to the building probably don't meet code. An engineer can tell you that too. TB
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If the stuff you build ends up stiffer than the existing trusses, they will take most fo the load. If it is less stiff, it will not take load until the existing trusses have deflected somewhat. If your upgrade isn't matched to the existing trusses, you may end up not improving the situation at all.
When the roofer told you to sister the existing trusses, he should have said to make them the same as the existing ones or sister with components that will move with the existing ones _exactly_.
Before you go on with this, get the advice of an expert.
Mike
PS a box beam is only as strong as the shear strength of the join between the webs and the flanges. Do you know what it takes to make that right?
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yeah, call a truss company. they have engineers who are used to roof trusses.
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Of course, this is an opportune time to consider pulling the whole roof, trusses included, and putting in a cathedral ceiling, or another room, etc.
Dave
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I doubt your plan to make your own box beams would give you anything near the strength of a manufactured "wood I-beam". The companies that make the I-beams hire engineers and specialists in materials to design these things, and for that you end up something that is known to work. Your poker buddies are likely to just slap together scraps of whatever OSB and nails and glue they have lying around, hoping to get something that sort of looks like the real thing, at least in appearance.
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Actually we have made I beams with OSB and 2X4s for a boat house. Local supplier didnt have them and we didnt meet the minimum order. My poker buddies are a pretty savy group with a couple of EEs in the group. There is a stuctural engineer in the group but he say he hasnt done anything with wood in 30 years and any real engineering at all in 15 years. I havent been able to get them around to the house yet but in a couple of weeks we will be down at the house and I can get them together for poker and steaks. Then I can see what they know.
Some of the comments did get me thinking. Plan on building a test section of the box beam mades of OSB and 2x4 and see what the failure strength is and how much a given lenght deflects under load. I figure this will give whoever takes a look at it a good starting point. Any other way of doing except by my plan would probably be too expensive to meet my budget. Would be nice if I found out I didn't have to do anything at all to it.
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snipped-for-privacy@bellsouth.net wrote:

Stop guessing & get an engineer to take a look at the roof system,
There is no need to build a test specimen, that's what calcs are for
Shop around for an engineer who would be willling to design a fix (if any fix is needed) that could be built onsite by your group.
cheers Bob
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And how do you know it is anywhere near the spec for a real engineered beam?

Asking an electrical engineer to solve a structural problem is like asking my mother to put aside her knitting needles and design a new space shuttle. If you want a structural problem solved, you get a structural engineer.

A good starting point for what? You don't know what the existing roof can do so what will knowing about box beams tell you? It is entirely possible that the existing trusses have sufficient reserve capacity to support the planned roof upgrade. If they cannot, the box beam doesn't sound like the right solution anyway.

Then sell the house. Either do it right or walk away.
Better: Start with an expert and find out what you _need_, then find out what it will cost. Then decide on whether you should do this at all. You and your buddies don't sound like a good solution.
Mike
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Obviously you dont know the capabilites of my friends or me. If we dont know we learn. We really hate spending money for not knowing. That is the one thing we have in common. Im sure my EE friends will let me know if they cant handle it. They are at the point in their lives where they can learn something new for themselves without taking a class. My dad said this point in your life should come when you are about 12.LOL
I really thought I might get some practical info here, Something on the order of "I've done that before" or the info I got from the local college ths morning. Just wishful thinking I guess.
Went to test my box beam(built it this AM) at local junior college and found my box beam plan was way overkill. While I didnt get to test it(forgot it was Spring break), I did find out that just adding 3-1/2" OSB strips will more than triple the strenght of the existing 2x4 trusses. Now to find out the load of the shingles compared to the concrete tiles.
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snipped-for-privacy@bellsouth.net wrote:

"> > to put aside her knitting needles and design a new space shuttle. If

<SNIP>
"found my box beam plan was way overkill"
<SNIP>
"I did find out that just adding 3-1/2" OSB strips will more than triple the strenght of the existing 2x4 trusses. "
Really?
cheers Bob
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of the truss. I was thinking plywood would be even better but found the OSB is stronger. Plywood would be more than sufficent for my purposes and has the advantage of being easily glued to the existing truss, the OSB I am familar with has a waxy coating on it. Is it all like this? We discussed the possibility of actually building trusses like I suggested. In the real world it would be easier and cheaper to build them of enginnered lumber but thought my idea would be plausibe for stengthening an existing structure. He also thought there was a very good chance that the existing trusses would work "as is" considering the truss was designed to span the full width of the house and there are a lot of internal walls that could be load bearing. For hte most part ther is a load bearing wall that runs almost all of the way through the center of the house . The exceptions are the den area where the load walls would be about 1/3 the way in on each side and for about 4 ft where the dining room on the back side of the house overlaps the livingroom on the front ther is a 4 foot wide area where the trusses are only supported by the outside walls. Several years ago there was a problem with the ceiling cracking in this area. The problem was fixed by placing 2 beams across 6 ceiling joist to support the 2 ceiling joist that otherwise were unsupported except on the ends.
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You and your friends are obviously in your manic phase. I hope you come to your senses before you collapse your house on someone. You _think_ you know what you're doing or can figure it out yourselvest - I've got more than enough years as a structural engineer to know you don't.

You got advice but you haven't taken it. The advice that was repeated by several posters was "get a professional to check it out." Just because that's not what you want to hear, doesn't mean you didn't get good advice.
Mike
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snipped-for-privacy@bellsouth.net says...

EE = Electrical Engineering, right?
If you had appendicitis, would you hand a medical book and a scapel to one of your EE buddies?
Banty (designs computer chips, gets experts to deal with my house)
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Actually, you've got to be careful even asking people to perform within their nominal field, if you're in a sub-genre they're not used to.
I took a drawing of my attic to a game session with some of my friends that included PEs in three different specialties, because I wanted them to check my math, and it still took me about 90 minutes to convince them that the bottom chord of a truncated queen-post truss is a tension member.
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Sad but true. On the other hand the in house engineer at a truss plant can glance at a design and tell you what it will hold. The OP could work the problem backward and ask what a truss looks like that will handle the load and compare it to what he has. I know they use pretty standard looking 2x4 trusses here under tile roofs. The number and placement of the bracing will be the likely difference, if there is any.
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OK - I'll take your word for it ;)
Banty (metallurgist by training)
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Are you talking about a full-length continuous piece of OSB? Where do you find 3-1/2" x 20' strips of OSB? I know the engineered wood companies use single pieces for building their beams. But all I have is 8' lengths.
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snipped-for-privacy@bellsouth.net wrote:

I guess we didn't get the memo telling us of you and your friends' capabilities.
There's learning, and then there's learning. You may feel you've mastered the first, but it's obvious you need to work on the second.

Every so often someone pops up on one of the construction newsgroups and wants to do something similar to what you are contemplating. Invariably they get the same advice. The curious thing is that the replies come from many different people who are pros - engineers, architects, experienced contractors, etc. - it's not always the same people spouting the company line. Invariably, if the response isn't quite so rosy as the OP hoped, the OP's response is, "What do you know?!" I find that amusing.

Ummm, no, those strips won't triple the strength of the trusses. They may strengthen the chords, but chord strength alone does not make a truss. Wherever you got that information, don't go back there for more.
To the problem at hand: If you're in an area that requires building permits it's a sure thing that the building department will require sealed drawings. Since you have some engineer buddies, that shouldn't be a problem. I'm sure your buddies are up on the major code revisions that were implemented in the last few years, and if not, are capable of coming up to speed. I'm sure that they'll also realize that it's not the member strength but the connections that will be the Achilles heel of your modified truss design, and that adding intermediate supports to a truss that was not designed for them can create localized stresses that are beyond the capabilities of the original design. A quick way to make your attic ground floor accessable.
Everyone approaches such remodeling in different ways. I'd have to see the roof structure to make up my mind for sure, but from my understanding of your description, I'd be more likely to wait until the roof is ripped and add new trusses at that time. You'd have all of the roof work done in a day or two, as opposed to many days or weeks, you'd have a designed solution, and you'd have new sheathing. I'm sure your buddies pointed out that the sheathing will almost certainly have to be replaced as it will be undersized for the new load. Smart people keep up with current events, and a couple of EEs and a PE will have commented on the major changes in construction techniques with respect to wind loads, uplift and required metal connectors due to recent hurricanes. These things are a pain to deal with while you're lying flat on your stomach reaching into the eaves, but a snap when the sheathing is removed.
I think it's great that you want to do the improvement, and that you would like to learn how to do it yourself. These are admirable traits. Another admirable trait is to not dismiss the people you have asked to help you. It's kind of like insulting the waiter before your food arrives - not a good idea.
R
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