Sagging Garage Beam

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Old 07-21-15, 03:02 AM
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Sagging Garage Beam

Problem:

I have a 24' x 24' garage that was built with half trusses (like double wide mobile home trusses. Under the center of these trusses is a beam made of 2 12' long 4x4s. It is supported by a 4x4 post on each end and 3 4x4 posts along it's length. The posts average 6' apart, but there is some slight variation.

It looks like the two end posts and the very center post, numbered 1,3, and 5 in my pictures, are original to the construction and posts 2 and 4 were added sometime later.

There is a good deal of sagging going on between all of the posts, 1-2" of deflection easily. The 2 beam sections are also separating in the center. Additionally, one of the end posts, number 1, is no longer plumb and a few of the trusses on that end are barely sitting on it anymore. It looks like it's been like that for awhile. Some of the posts look to have some water damage and slight rot on the bottom so they may differ in length as well.



Solution:

My plan is use an I beam alongside or under the existing beam(s), jack it all up level (or at least in line) using the two end posts as a reference.

Fix any variation in center post lengths.

Place 2x8s, or 2x10s along each side with the joints staggared (8' and 16') nailing these to the existing 4x4s and the posts.

Straighten the shifted end post so it's centered.



Questions:

Ideally I'd like to get rid of as many center posts as possible while doing this, but if I have to keep them all I will, so I have a few scenarios.

Keep all posts - What size lumber would be best to sandwich the existing beam with?

Delete posts 2,4 - would it be possible to take these out and use larger lumber on each side of the 4x4s? Would boxing in the bottom help at all?

Delete Center post (number 3)- Same question

Delete all posts - Probably a pipe dream - but can I build a header strong enough to allow this to be a clear span. If not with wood, what could I do with a 4" wide flange I beam or 12" steel joists? (i.e. http://www.kalexsteel.com/steel-prod...ages/joist.jpg) I have those laying around.




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  #2  
Old 07-21-15, 03:24 AM
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Can you post a few pictures of the bottom of the posts? I'm thinking that is where the real problem lies, the posts are not being properly supported and if that is the case then any repairs that don't take that into consideration are doomed to fail.

(Hint, always give a double enter after each picture insertion to keep the pictures from "bleeding" into each other.)

You might be able to use a continuous steel beam to eliminate all but the center post, and maybe even that one as well but it would be a might hefty piece of steel. It would definitely require the services of a structural engineer to determine the size.
 
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Old 07-21-15, 04:02 AM
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I can get some pictures of the post bottoms, and they may be slightly rotted and settled. However the 4x4s being used as a beam are definitely sagged. The ends sitting on the posts are higher than the centers.

I plan to cut off any bad places on the bottom of the posts and either scab on a section of treated 4x4 or set them on some squares of treated 2x6 or 1x6 depending on how much they need to be raised.
 
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Old 07-21-15, 04:53 AM
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Here are some pictures of the bottoms of the posts and one showing the deflection/sag on the beam. There is staining and water damage on the bottom of all of the posts, but they're not really soft when poked with a screwdriver and they're not currently wet, probably haven't been for several years as far as I know.

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Post 3 (center Post)
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The left end of the level is tight against the beam for about a foot, there's about 3/4" of gap on the right end.
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Old 07-21-15, 05:05 AM
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The staining is probably from the posts being embedded in the concrete and wicking up moisture from the concrete. Since the beam is bowed in the middle [between supports] I'd assume that it is under sized. You'd either need to replace it with a bigger beam [or steel] or install a support in the middle.
 
  #6  
Old 07-21-15, 07:08 AM
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A few years ago I had to re-do my 24 x 24 center beam and it was a bit of a battle. Since yours is not loaded with storage it may be easier, but those aged trusses may not want to move. Just my opinion, but I would want a full length (substantial) steel beam or a couple of full length engineered beams, LVL is the term I remember.

If you located 1 2 x 12 LVL on each side of what is there and jacked them up it would level the truss bottoms and lift them off of the existing beams. Those sagging beams would be hard to straighten.

Without being there I can't suggest whether one LVL first and then the second, but basically you should be able to maintain structural integrity while slowly jacking the new beams into place. Once in place you should be able to tie both beams together, sandwich the existing, and (carefully) remove all supports.

Of course you would want an engineer to advise on the total load, bolting pattern and end supports, including the capability of the concrete below.

Slow and steady.
Disclosure, I am not a structural engineer.

Bud
 
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Old 07-21-15, 08:16 AM
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Bud,

Thanks for the response, I hadn't thought about the old beams not straightening out, but I think that will be ok. Like you said, as long as the truss bottoms all line up, I can nail the wood I place along the sides to the old posts and existing beam and the trusses will just sit on the new wood. Basically it will be like the side walls of a pole barn.

What kind of trusses were in your building? and what did you end up with in the end as far as supports. If you've got just 2 2x12 LVL, or that and your old existing beam, then that gives me hope for getting rid of my posts all together and I'll get an engineer to look at it. As it is, I know I can run a 2x10 or 2x12, possibly even smaller along each side and it will definitely be stronger than what's there, but taking out some posts while I'm at it would be awesome!
 
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Old 07-21-15, 08:31 AM
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I didn't use the LVLs on this project and it is traditional framing with overlapping 2x6's. The idiot that built it used triple 2x12's with no center support which would have been ok if he hadn't matched the butt joint of two sides of the 2x12's right in the middle. We had decked the attic for storage and it quickly became obvious. A new partition wall and now 5-2x12' (added one to each side) are holding fine. But the LVLs I have used are awesome and most suppliers can probably give you load information from your pictures. Lumber yards that sell trusses and beams like that will have the software and training to specify load requirements, and they are free, at least around here, if you are buying the beams.

There is no problem getting a full length LVL (except for getting it into place) for that 24' span and you should absolutely be able to eliminate all the supports (except the ends), you would just need really good end supports and need to verify there is a sufficient footing under the concrete.

Bud
 
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Old 07-21-15, 09:32 AM
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Bud has all the answers you need. I may duplicate some things he said but I just want to agree with all that was posted and give you a heads up on a few things.

- Hire an architect or engineer. These calculations are rather easy and it shouldn't cost a fortune. The loads in your location are required. Live Load, Dead Load, Snow Load and Duration, Roof Pitch, etc.

- The 24' span can most likely be supported using 1 or 2 LVL's with no center post.

- The Bearing End support area is critical. As Bud said, you may have to do some digging to improve foundation. Vertical support area at sides of beams is required to prevent rolling.

- The designer could also give you some advice on how to actually get this beam in place

For example:
A 1.75 x 11.875 LVL Beam can carry 74 Live Load, 93 Snow Load. These are general numbers and would be adjusted by an on-site inspection. The LVL would also be doubled up which would enable you to reduce the height of the beam.
 
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Old 07-21-15, 01:59 PM
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Ok, thanks guys. I think I know what my next step is: finding a local engineer that can calculate it out for me and see if the end supports are up to the job. You've all given me some great ideas, I appreciate it!
 
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Old 07-26-15, 08:57 PM
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Whoever used 4 x 4s as a horizontal, load-carrying beam wasn't thinking logically. Efficient beams are always deeper than they are wide, because bending moment capacity is dependent on the depth of the member, squared. All interior supports can be eliminated if a properly-sized beam is used.

If you find that LVLs are a bit pricier than you're comfortable with, there's always the option of using a salvage steel beam. And while you're in there working, you might give some thought to correcting the electrical wiring rats' nests--what you have is an accident waiting to happen.
 
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