Broken timber visible in basement ceiling


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Old 01-29-03, 12:59 PM
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Broken timber visible in basement ceiling

I have an OLD Stone home. The basic footprint of the home is rectangular, with all four walls of stone and the gable ends being the short sides. (looks like a typical colonial home)

The interior wood framing that supports the floors and central staircase consists of a large beam 12 x 12 that extends from the center of one gable end to center of the other gable end. perpendicular to this beam are the floor joists which are 3x8 timbers, set into slots in the stone wall and mortised into the main beam, spaced about 24" on center with the exception of the area that underlays the stairwell/center hall. On either side of the Hall the joists are 4 X 8 timbers with a 7 foot span between them. (this allows a large enough area up through the center of the house for the stairs from basement to attic) on top of these Jumbo Joists, rest the walls that separate the rooms from the hallway. The walls are constructed with 12-24 inch wide planks one inch thick set side by side (like exterior sheeting) resting on the floorboards (and jumbo joist) and face nailed to the joist above, then covered with wood lath and then plastered. The wall for the floor above is similarily built.

It is on of these Jumbo Joists in the basement that have broken. It snapped (probably 50 years ago) near the center of it's length where a crosswise member is joined to it with a through tennon joint. (the cross member is one of two present along its 7 foot length and they provide support for the hallway floor above).

How can I repair this broken structural member? I can't replace it because I won't be able to lift the wall above out of the way to get at it, and I won't be able to slip a new one in place anyway because of the 2 through tennon joints that will need to be put back in place, not to mention that I need to slip one end of it into the support built into the stone wall and the other into the tennon cut into the main beam of the house. I was thinking of sistering it with a beam of the same size, but I am not sure of how to join the two, or how to join the new beam to the main beam of the house for support. I figured that the end that would normally go into the stone wall could be supported by a lolly post so long as the support beneath the post was set up to distribute its load across a reasonably large area of the basement floor(concrete).

I also thought about jacking it back up to nearly level (depending on how much the plaster cracks - I know that I can only lift it about 1/8 inch per day at the most) and then supporting it from below with a small steel I-beam, with posts set at each end of the I-beam. (this will keep them out of the way of the useable basement space that I have) BUT I am worried about the I-beam just sitting there on top of the posts and squeezed between them and the joist. Will I need to secure it in some way from twisting or sliding one way or the other? The Jumbo Joist that needs to be repaired looks like it was hand cut by a sawyer 200 years ago when the house was built, and I doubt that it is a perfectly level surface that can provide even bearing for the I-beam.

I hope that some of this makes sense.

Thanks for taking the time to read this and for any information that you can give.

HB
 
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Old 01-29-03, 05:31 PM
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I have a pretty good picture of your problem. Have thought of and thrown out several possible solutions. Question: Is this broken joist cousing some sort of structural problem in you home?
Or do you just want to fix it ?
 
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Old 01-30-03, 05:42 AM
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Jack, there is a problem with the broken beam, the crack is directly below the edge of a door frame. The door frame drops about 1-2 inches across it's top (running from the stone wall toward the central beam of the house) and without support below the wall, its entire weight is hanging from the beam above that makes up part of the floor on the second story so I'm worried about it causing problems up there later on.

Also, the floor is springy in this area. I noticed this the other day while I was standing above this spot and my wife walked past me. Her whopping 100 lbs passing over this spot made me feel like I was standing on a trampoline.

The thing is that we always thought the slant in the hallway floor from the stone walls to the center of the house was due to the fact that the main beam was green when it was set and it sagged within the first few years from the weight above it. I guess we were wrong about that one!

I've since thought about how to sister a timber to this broken one and I think that I will sandwich a piece of angle iron between the broken timber and a new timber that is the same size as the original (4x8). I will support the end near the stone wall on a jack post, and attach it to the main beam with a joist hanger fabricated to accomodate the full width of the existing beam, new beam and angle iron. For a little extra support a post will be placed under the joist hanger as well. Here's another question - would the use of 1/2" bolts through predrilled holes through the entire assembly be strong enough, or would the holes compromise the strength of the timbers? Also, I think I remember reading somewhere that holes should never be drilled in the top or bottom 1/3 of a timber. Is that true?

Thanks for any advice.
 
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Old 01-30-03, 07:39 AM
brickeyee
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I do not have my books handy for computing the strength of that beam. (The way I usually attack these problems is to create a patch that is equal in strength to the broken member, and long enough to spread the bolt loads out and not crush the wood.) I would patch it with steel flitch plates and 2x lumber on both sides. The flitch plate should be the same size as the 2x and at least #10 (about 1/8 inch thick). Through bolt using all thread and large washers to spread the load onto the wood. Sistering on only one side can cause twisting depending on how the loads are applied and how large they are. If the patch is as strong as the beam you can remove the jacking after patching. The rules about not drilling in the top and bottom 1/3 are general rules for wiring and plumbing to avoid weakening the beam. The rules for structural reenforcement are different since you are trying to transfer loads.
 
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Old 01-30-03, 08:44 AM
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Brickeye, I can't sister it on both sides since the cracked beam receives tennons from some cross members. The best I can do is trim the through tennonns so that they're flat with one side of the timber and then sister on that side only.

I'm hoping that the steel joist hanger that i'll have to have made will help prevent twist since it'll be 1/4" steel and house both the old and new timber and I was going to have its side flanges extend at least 3 inches out to either side of the pocket for the joists so that it will have plenty of area to attatch to theface of the main timber. Also, the jack posts that I install at either end of this thing might help - or is that incorrect?

Thanks for the reminder about spreading the load of the nuts and bolts across the face of the beams. I plan to stack increasing diameter washers at both ends of the bolts to try to achieve at least a 2" diameter of seating area against the beam.

As for the bolting pattern, I was going to go with a bolt every 12 inches along this 7 foot length, alternating high to low. Is thqat enough or do I need something more like double this number, placing one high and one low every 12 inches?

Thanks again.
 
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Old 01-30-03, 09:40 AM
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*This idea might be a workable solution if it can be confirmed by someone with load structure knowledge to be suitable for properly supporting the loads.*

Temporarily support the tennioned cross members (build a deck support from 4*4 and 2*10's set on a 2*10...) to span both cross members) ...Cut the cross members vertically 3/16" before the connection to the joist...Align and Sister the fractured joist on both sides...Connect the cross members to the joist with hangers.

With the span of the joist hanger on the main beam being 5.5 to 6 inches. I am wondering if the repair will be more firm by adding a bottom flange to the fabricated joist hangerwhich can be secured to the main beam.
 
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Old 01-31-03, 04:44 AM
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Stephen, thanks for the idea. Didn't occur to me to just use joist hangers for those cross members....

As for adding a bottom flange to the joist hanger at the main beam, I'm kind of leary about poking holes in the bottom of a beam thats under so much tensile stress. I was hoping that wide flanges across the face of the beam would provide enough strength to hold everything in place.

Thanks for the suggestions.

HB
 
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Old 02-01-03, 02:55 AM
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Your getting all good answers. Just remember that your sistering has to be stronger then the original beam. 1/4 plate steel will be fine. 12" bolt spread should be fine. I have only one concern and it may not even be valid since I cannot see it. It seems by what you say, that this crack or break has been there for some time. If that is the case, then your beam has a set in it. I just jacked up the center beam on a home three stories over 100 years old and am still in the process of it. Here is what I did. On one end of the beam (by the rock foundation) I put in blocking (
I took railroad ties and cut them in 2 foot lengths) 2 blocks side by side then 2 more blocks on top of those at 90 degrees and so on and built up to about 16" below the beam. I did this also about each 7 feet along the beam. Took a bunch of ties. Then I put 20 ton jacks on each blocking and slowly began jacking. The first day I jacked 10 pumps, 3rd day 10 pumps, etc. After 9 days, I have lifed the beam, the mail floor and 3nd floor about 1 1/2 inches. Only have 1/2 to go. Now will pour a 12 deep footing with losts of steel in it for the final jacks to set on. Oh yes, when I get the post to its final elevation, I w3ill leave the jacks there for 3 weeks to make sure no setteling occurs
 
 

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