Flitch Plate Beam Question 1

[kxa]

I have a situation where the beams (2-11 7/8" LVLs) are already in place and there is about 5" of clear height on the bottom right below the connecting floor joists on each side. I am thinking of adding two 5" wide by 1/2" thick plates on the bottom. One on each side to create a composite section. I am using a transformed section and calculating the top and bottom Sx based on the total Ix.

Should I just calculate the top and bottom stresses and compare them to the LVL's allowable stresses. For some reason I get very small stresses.

 

[kxa]

haynewp,

I agree. That is the most efficient use of the wood beam and the steel plates. That was my intension from the start. I was even going to jack the LVL’s for the amount of the DL deflection before attaching the steel plates.

The question is how to connect these side plates so that they all act as one beam. The load is being transferred from the LVL’s since the 2x8 floor joists are attached to them. The 4 1/2” plates will be placed under the joists on each side of the LVL’s.

 

[haynewp]

You will have to use mechanics of materials to figure out the amount of attachments required. Then, I suggest thru bolting the plates. I may be able to help you tomorrow if you can't figure out the shear flow.

 

[xxpegasus11]

If you are working on load sharing relative to their EI, as haynewp also said, then its simple to calculate the load taken by the plates. Bolts drilled through the plates will have to be provided throughout the span to transfer the proportion of load to the plates, which since you already have the load again is simple to work out how many you need.
As mentioned previously I would then provide enough bolts at the supports to transfer the load back to the LVL's

 

[Lion06]

Let me go through a thought process here and someone please help me to see where I am going wrong.
We have this lvl with beams framing into it from both sides. There is a 4 1/2" plate on both sides of the lvl just below the bottom of the joists that are framing into it. The plates are bolted to the lvl in order to get them to deflect together, but not act compositely (is that even possible in this case?).
Now, I am picturing this beam being fully loaded and the holes start to get sloppy. Now the load is taken off and loaded again, but with a smaller load than the initial full loading. I can see the lvl deflecting much more than the steel plates now because of the sloppy holes that are demanding the steel plate to bend with it. That being the case, the lvl could be overloaded.
Just because the members are joined doesn't mean they are automatically going to deflect together. The load needs some means to get into the steel plate.

 

[ironmon]

When connecting steel flitch plate to wood, i recommend using Teco Shear Plates, they come in 2 5/8" and 4" diameter. They come with custom drill bit and instructions, very easy to install. I have used successfully many times to reinforce wood beams in place.

http://www.clevelandsteel.com/

 

[haynewp]

Myerges

You can't bolt the plates to the LVL without it acting compositely, at least at first until there is failure in the transfer somewhere. That is why you have to design the connection to develop the required capacity. And figuring the connection requirements for allowed deformation around the bolts in the LVL is part of the design process.

Glulam trusses are fabricated by thru bolting steel plates all the time. If the bolt bearing is within the wood code requirements, I don't see cyclic loading eventually causing a failure of the LVL through deformation around the bolts. Does anyone else?

 

[Lion06]

haynewp-

I agree 100%. Previous posts in this thread talked about sharing the load based on compatible deflection WITHOUT them acting compositely. I don't see how that is possible for a plate on the side of the lvl. I see how it can happen if the plate is on top of or on bottom of the lvl, but not the sides.

 

[Lion06]

I guess I should qualify that last statement with, "in this particular situation".
It would obviously happen (deflect together without necessarily being compositely connected) if the plate was the same height as the lvl and bearing on the support.

 

[apsix]

MYerges, haynewp:

I don't agree 100%.
"You can't bolt the plates to the LVL without it acting compositely, at least at first until there is failure in the transfer somewhere." That's true to an extent, but I believe ensuring transfer is more difficult than you state.
As I see it you will invariably get slip due to clearance in the holes. For a bolted truss this is of no great consequence, it defects a little more but strength is unaffected.
Slip when resisting shear flow will result in loss of some or all of the composite action, leaving you with deflection compatibility (or pretty close to it).
If you can ensure a no-slip connection, which the builder will not get wrong, then, of course, composite action can be used.
As for the the steel plate requiring bearing on the support, surely all you need is capacity of the bolts to transfer the plates' reaction to the LVL.

 

[haynewp]

You may be borderlining on having the top of the steel plates in compression, ie. watch for compression buckling of the top of the steel plates between fasteners.

 

[Lion06]

Apsix-

Let me take your idea to the extreme. You have these plates bolted to the lvl with (3) bolts - (1) on each end and (1) in the middle. Now you fully load the beam and the center hole elongates in the wood.
Now unload the beam. Now load the beam again (but less than the initial full loading), and I think that the elongation of the hole will cause the center bolt to transfer very little little load to the steel plate and that they WILL NOT deflect together.
I understand that you will have more than the (3) bolts which will cut down on the difference in deflection. I am just having a hard time wrapping my head around how you get them to deflect together without acting compositely when the ONLY means of transferring load from the lvl to the plate is via the fasteners.

 

[haynewp]

There is some validity to the idea that if they oversize the holes in the wood, then there would be deflection of the wood beam required prior to engaging the steel plates.

I don't know what else to tell you, except look into using a whole lot of lag bolts or something else if you are still wanting to get this whole thing to act compositely and avoid any slip. I can't find any other references on doing this, just plain old flitch beams where the neutral axes line up and everything is elementary. Maybe this would be a good application for FRP, but you seem set on using these steel plates.

 

[Lion06]

I did not originate this post, it just caught my attention. My steel text talks about flitch beams also, but as in yours everything lines up so there is not an issue of whether it needs to act composite or not.

 

[kxa]

haynewp,

This is what I am thinking. The LVLs seem to carry the shear. The jacking of the beam for the DL deflection puts a negative moment into the beam which will reduce the overall moment. As a result, when the jack is removed, the steel goes into tension. For this reason, the ends of the steel plates need to be bolted to the LVL’s such that bearing stresses will be OK.

As far as the intermittent bolt spacing is concerned, if the plates never go into compression, then a bolt ever couple of feet would be OK. However, if parts of the plates go into compression, then I have to make sure that they (steel plates) won’t buckle

 

[apsix]

MYerges

Continuing with the hypothetical 3 bolt beam:

If the centre hole elongates, the bolt is too small for the shear force. As with any properly designed bolted connection in timber, elongation should not occur.

Regarding non-composite action; if there was a single point load acting at the center bolt it is obvious that the LVL and steel will act together (assuming no elongation). However I believe you would be hard pressed to show that it acts compositely.