Wood Ledger Bolted to CMU Wall - 45 year old building

[sturr]

I have been asked to evaluate the gravity load capacity of a wood-framed floor system within a building that was built in 1968. The floor consists partly of 3 x 12's in joist hangers mounted to a 3 x 12 wood ledger. The ledger is bolted to the CMU with 5/8" bolts at 4' o.c. Everything in this system that I have analyzed yields an allowable total floor load of more than 90 psf with the exception of this ledger connection. The wood is Douglas fir from 1968, tight-grained, clear (beautiful stuff by today's standards).

Using today's code, the allowable load per bolt per the 2005 NDS Table 11E is 610 lbs. Working backwards (the tributary floor width to this ledger is 8.5 feet) the allowable total floor load based on the ledger connection is only 18 psf (Ouch!). I know this allowable bolt load is based on 1 of 6 yield modes, all of which contain varying safety/reduction factors.

My question is: Does anyone know if back in 1968 there was significantly different allowable loads for the design of these connections? Perhaps the factors of safety were much less or there was a lack of understanding of these connections? I hate the thought of telling the owner this is a problem, even though the floor has held various occupancies throughout the last 45 years with no apparent damage whatsoever.

Thank you!

 

[hokie66]

There was no such misunderstanding in 1968, but there was at least as much ignorance then as there is now. Sorry, but you have to advise the owner that the connection of the ledger to wall is a very weak link. But first, I would want to make sure there are not shear plates on those bolts, which might make the wood side of the connection work...you would have to check that. Do the bolts into the masonry work?

 

[msquared48]

YES, the values were very different. Oldest code I have at the office is from '76 I believe

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

Mike,
Doubtless there are differences, but a 5/8" bolt in single shear perpendicular to grain has never been equivalent to the end reaction of two or three 3 x 12 floor joists, which I assume it is carrying.

 

[RFreund]

Also you may try to use the actual yield mode equations with different properties (i.e. different modulus of Elasticity, density etc.) and see if it yields a different result. You may be able to find these properties in an old NDS or if you create a spreadsheet you maybe able to see what you would "need" and compare this to what is "reasonable".... just a thought.

EIT

http://www.HowToEngineer.com

 

[RFreund]

Ooops forgot to add that I'm still not too sure you'll get to 100psf... but a fun exercise none the less!

EIT

http://www.HowToEngineer.com

 

[woodman88]

I remember back in the 1980's that my chief engineer was laughing about someone specifying the use of a dense coastal DF-L grade for its higher design values. He said that even in the 60's it was hard to get and that now (back in the 80's) in was impossible to get. But I do not think that the values would of been high enough for your situation. My NDS's prior to the 1991 are all missing, so I can not go back and check any values for you.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[IsaacStructural]

On the positive side, it sounds like something that could be corrected with post installed anchors drilled and bolted through the ledger and into the brick, one could change that spacing to 12" or 16" O.C. pretty easily, no? Epoxy or expansion anchors are both possibly, depending on local regulations.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[msquared48]

Rather than do that, I would consider adding a periodic or continuous, whatever you need, 3X6 steel angle (LLV) bearing seat below the ledger, after set bolt it diretly to the CMU wall, aand lag bolt it to the bottom of the ledger.

I would also make sure that the existing ledger satisfied the minimum 200#/foot lateral load for the diaphragm without goint into cross grain bending. I would be willing to bet that there are no direcct ties to the CMU other than the existing bolts thru the ledger.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[sturr]

Thank you all for your responses. I see that I am not alone in thinking there wasn't a drastic difference in 1968. I will look into adding anchors, but I'm not sure if there is a bond beam behind the ledger. I know the wall is grouted at 4' o.c., which could be why there is only a bolt every 4'. I think I am going to propose a load bearing furring wall be installed under the ledger down to the slab. It is understood (in writing) that I haven't been asked to evaluate the building as a whole, but in a seismic event these bolts may be subject to out-of-plane wall forces, so adding the wall would also add some safety.

 

[msquared48]

For comparison, in the 1973 UBC, a 5/8" bolt through a 3X ledger in single shear across the grain was good for 810# according to Table 25-F.

The current code tells you 610#. That's about a 25% reduction, but ewill still yield only about 24 psf for the floor load, way below the 90 psf you apparently need.

You just need to add the steel angle bearing ledger.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[sturr]

Thanks Mike, your post came in while I was writing my last one.

 

[nac521]

@M2 - would an angle under the ledger detail add further cross grain loading to the ledger for out-of plane forces?

Make sure that you consider the load to the slab if you decide to add a load bearing stud wall. If it is a floating slab, I would be hesitant. I would anchor the ledger to the wall with additional anchors using Hilti Hit Hy 70 system (which is good for hollow or solid block). I may also consider adding a tie or angle from the top of ledger to the block wall to get the out-of plane forces directly into the floor diaphragm preventing any cross grain bending of the ledger.

 

[msquared48]

nac521:

No, not if you added something like a PA51 oe aimilar over the diaphragm sheathing and after-set bolted to the CMU wall. You could also do this to the side of the purlins, through the 3X ledger, and into the CMU wall, but it would be more difficult. Another op[eion , depenc=ding on the depoth of the joists and le3dger, would be to attach the tie to the bottom of the joists and after-set bolt to the wall. Just depends what works best for the situation.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com