Flitch Beam Splice at Wall Bearing

[RareBugTX]

Hi. Am designing a Flitch beam with (3) 2x12's and two steel plates 8"x5'-0"x1/4" at the heAder bearing location for a garage opening that was increased from 16' to 17'. I determined the moment and distributed the stresses to wood and steel by their stifness ratios. Question I have is how determine appropriate lag bolt spacing and number other than the prescriptive 12" oc staggered top and bottom. Anybody has done anything like this based on. Rational approach. Thanks.

RareBug

 

[KootK]

I'm having a hard time understanding the situation. Can you post a sketch? Is your beam five feet long or 17' long? Is it a two span beam?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

General concepts:

1) Usually the centroid a of your wood and steel plies align so your bolt design need not consider VQ/It shear.

2) Usually your load is delivered to the wood plies and not the steel plies. Your distributed bolts need to deal with that.

3) At reaction points, usually only your wood plies are in contacts with the bearings. As such, you often need a gaggle of bolts at these locations to transfer the load out of the steel plies.

4) Usually folks use through bolts with washers rather than lag bolts. I think that it's good to have some interply frictions working in your favour.

5) The bolt spacing influences the local buckling potential in the steel plates. I've never known anyone to consider that explicitly in practice however. We discussed it in a thread here once and considered treating a chunk of plate between bolts as a column of sorts.

6) If your plates don't make it to the reaction points, or either the steel or wood plies need splicing mid-span, things get pretty complicated. Let's cross those bridges only if necessary.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[focuseng]

Kootk is right on. Additionally, I say no to: Lag Screws and "carriage" bolts. Woe to the carpenter who says otherwise.

Other than that you need to know the % of load you are distributing in and out of the wood members to steel. This is a function of the members strength and stiffness and your confidence in your methodology. Some engineers say the steel takes 100% as a conservative approach. I prefer to use the wood and spread it out over all members according to their ability. BUT- The wood takes all bearing. So at a point load and uniform loads you need to transfer it into the steel as you have determined it will take and at the end/middle bearing you need to collect it out. Those load transfers are then the basis for your fastener calcs using multi ply wood and steel members. You can then use this to design lumber/LVL and Lumber/Steel and heck you got to do it just to design the side loaded fastening of a multi-ply member. Then after I show them the # of bolts and the weight of the plate steel I give them the option to install a W10x15 or something else nice and light with bolting tabs...

Also, search this thread and the internet. There are a lot design examples to be had, even if you have to buy a book or two. I know this topic is fairly frequent on this site so make sure you have exhausted resources already cited.

______________
MAP

 

[XR250]

On heavily loaded flitch beams, I usually specify a 1/4" bearing plate at the end with the vertical steel plate(s) installed so they make contact. It keeps the load in the steel and not having to transfer it back out.

 

[focuseng]

xr250, I like that. It is neat to see the different ways we do these things. Do you have the wood notched a little so it is out of the way? I imagine this helps with bearing compression on heavy reactions for the beam too.

______________
MAP

 

[hokie66]

This primer may help. But I think it applies to full length flitch plates, not a aplice which seems to be the intent here.

http://www.structuremag.org/wp-cont...-Exp-Flitch-Plates-DeStefano-pac-5-10-071.pdf

 

[XR250]

I like that. It is neat to see the different ways we do these things. Do you have the wood notched a little so it is out of the way? I imagine this helps with bearing compression on heavy reactions for the beam too.

Notching the wood 1/4" is how is is usually done. If height is not an issue, it can be constructed with the vertical steel flush to the bottom of the beam and the bearing plate just goes between the wall top plate or jacks and the beam.

 

[NS4U]

There is a paper that discusses this. It's a 1989 paper titled "Glitches in Flitch Beam Design" and it's also discussed in the 1956 "Timber Design and construction handbook"

The answer varies depending on the type of flitch beam you have. Edge distance is also matters because you want to restrain local buckling.