Moment connection when sistering two wood joists

[millphil]

I'm working in an old brick building where there are 26' long 14" deep wooden floor joists (2x12+2x4 - old joists that are actually 2" wide) @ 16 OC bearing on two exterior brick walls. 9 of these joists are connected to a header 4' from the exterior wall. The header is connected a full length joist at each end.

The header is sagging quite a bit and the full length joists to which the header is connected are not designed to take the load from the header.

The plan was to take the header down; sister 14" deep LVLs to these 9 joists and connect them to the wall using joist hangers and a ledger beam connected to the brick with adhesive anchors.

But that leaves me stuck with problem of the connection between the LVL and existing wood joists.

1. I have moment connection with 5.1 kip-ft that is hard to make work in wood.
2. Any guidance on the splice length?

I would appreciate any suggestions!

 

[madmantrapper]

Are you saying you have a 26 foot long clear span (there abouts) spanned with a 2 x 14 joist? And the ends of the header are connected to a 2 x 14 on each end? I hope I have this incorrect.

Paul

 

[msquared48]

How does a moment connection come into play here?

Usually the concern is with the relative loads shared based on the different EI values of the joists and the size and number of stitch bolts needed to do this.

Mike McCann
MMC Engineering

 

[millphil]

Let me clarify:

The clear span between walls is 26' and most 14" deep joists span the full length and bear on the brick wall.

When the building was constructed, 9 joists were cut 4' short and connect into header shown on my attached a sketch.

To correct the aforementioned problems above; the "short" joists are to be extended to the wall by sistering an LVL to them. I was hoping to avoid using a full 26' long LVL and using something like a 10-12' long one meaning there would be about 6-8' of overlap on each existing short joist.

Maybe I'm over thinking the connection between the LVL and the short joist.

I was initially thinking of the connection being capable of handling the moment and shear that would be created at 4' in a 26' long simple span beam.

I am over thinking this?

 

[hokie66]

Probably. Yes, you need to design the lapped splice to develop the capacity of the joist. Just curious...how is the 2 x 4 connected to the 2 x 12? That might not add up to a 14" joist.

 

[RFreund]

As far as the connection you could see here:

http://www.eng-tips.com/viewthread.cfm?qid=333686

However I'm also curious about the 2x12 and 2x4...

EIT

http://www.HowToEngineer.com

 

[BAretired]

You could beef up the header and the joist at each end of the opening and simply leave the 22' joists alone, then add 4' long joists in the existing opening.

BA

 

[hokie66]

Agree with BA. That would be the most logical approach.

 

[msquared48]

Chances are that the two joists at the end of the header are going to be shear critical, not moment critical. You need to check both, and the header itself as mentioned above.

Mike McCann
MMC Engineering

 

[SteelPE]

Depending on where you are, you don't need to limit yourself to wood either. A steel channel combined with the suggestion of BAretried might do the trick as well. It may also be possible to eliminate the need for shoring with this method.

 

[woodman88]

The way I have seen your situation done, was to use the 2/3 rule. So with a 4' extension you have a 8' lap. The moment you design for is at the midpoint of the lap (e.i. 8' from the wall). As the connection will be within the lap you use a 6' moment arm and nail/bolt/screw for the force at each end of the lap with glue to control spliting of the lumber. My concern is the lumber (even with the glue) starting to split at the connections overtime.
Personally, I would replace the header and beef up/replace the full span side joists, as others has mention

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.

 

[millphil]

Fist off - thanks for all the comments and great suggestions!

In regards to the 2x4 on top of the 2x12; its quite difficult to see how well they are connected together. There is another 2x4 attached like a diagonal truss member (see sketch) which loosely connects the 2x4 to the 2x12. Despite this; I have only assumed the strength of the 2x12 for bending and shear but the full depth for deflection as there is additional stiffness from this brace.

Given my strength assumptions; I originally avoided the suggestions by BA as the existing 26' span joists are slightly overstressed in bending (10%) and I did not want to add additional bending caused by the header reaction (although small due to its proximity to the wall) to overstressed joists. Hence my original solution to extend the short joists and due away with the header.

However; the suggestion by BA and others is interesting if there was a better connection between the 2x4 and 2x12 such that the full depth of the joist could be counted on for strength; something like a series of truss plates or 2x6s nailed to the 2x4 and 2x12 that can transfer the shear flow between the two members.

In that event; beefing up the joists at the end near the header for the increased shear works well. In this case would the connection between the joist and the additional member be designed for "M1" and "V1" shown in my sketch?

 

[BAretired]

millphil wrote:
In that event; beefing up the joists at the end near the header for the increased shear works well. In this case would the connection between the joist and the additional member be designed for "M1" and "V1" shown in my sketch?

The joists at each end of the header need beefing up for moment as well as shear. The maximum moment may be more than M1 depending on how many joists are supported by the header (your text indicates 9 but your sketch shows 7). If 7, the maximum moment occurs at about 20' from the end or 2' from the header.

It would be nice if you could use a full length LVL or steel channel as reinforcement, but it would be impossible to place it in position, so you can't do that. I would suggest using as long a member as possible, say 25' with full bearing at the larger reaction. The existing member should be checked for the lesser reaction which it must carry by itself.

The existing floor seems a mite shallow for the span. If deflections are a problem, you might want to recommend stiffening the joist beyond the opening.

BA

 

[BAretired]

correction: I meant "joists beyond the opening".

BA

 

[dhengr]

Millphil:
Given the floor plan you showed earlier, and the moment you said you were trying to develop at 4', I did some calcs. and thought your typical joists might be over stressed and/or that the load on them was kinda high. With the 24' dimension on your plan and the question about 7 or 9-22' shorter joists.... why not remove the header and the short joist, and use these short joists to sister alongside the longer over stressed typical joists, to bring them up to code. Then, put in 7 or 9 new 26' long LVL’s to replace the old shorter joists. I’m not sure I understand how the 2x4's are connected to a 2x12 to turn it into a 2x14 and they could give you some trouble in the sistering effort.