×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Flitch Plate Beam Question

Flitch Plate Beam Question

Flitch Plate Beam Question

(OP)
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.

RE: Flitch Plate Beam Question

You are going to add a 5" high plate (on each side) to the bottom of a 11 7/8" high LVL, is that correct?  The first question I would ask is why are you doing this?  Was the LVL not designed right to begin with, can it not take the load?  

RE: Flitch Plate Beam Question

You will need to check both the stresses in the steel and those in the LVL.  If the stresses in the LVL are low, it is because the steel is taking more of the load.

I think the more difficult part of this task is getting this beam to act as composite.  What type of connections would you use to ensure the members are deflecting together?  Typically when a flitch plate is designed, the wood on either side of the plate is ignored and the steel is assumed to take all the load.  The wood acts only to brace the steel.

RE: Flitch Plate Beam Question

Agree with enginerding.  Calculating a composite section is one thing.  Providing the connection is entirely something else.  I would treat the steel as being at the neutral axis of the LVL and simply add the transformed  moments of inertia.

RE: Flitch Plate Beam Question

Not sure I completely understand your problem - the 5'' thing does not make sense.  Anyway - most flitch beam calcs will show that the wood carries very little load -- wood deflection too high versus the steel.  A lot of times I just assume that the steel will need to carry all the load and the wood is used to stabilize the steel (Lc and Lu).  Makes calcs much easier and is conservative but not overly.

Beamchek.com has software that will analyze that - if you want.

RE: Flitch Plate Beam Question

Why not use 2 channels (are 5" channels available in the US)? You get greater stiffness per weight of steel.

I also agree with the others. Forget about composite action and assume the steel is taking all the load.

RE: Flitch Plate Beam Question

(OP)
Thanks for all the comments. Unless I am missing something here, I have two 12" deep LVLs that can carry over 500 plf. Why should I disregard them? If I convert the steel to the equivalent wood section, the neutral axis comes down but the over all section strength increases. I can even jack the beam for the amount of dead load and then attach the steel plates or the channels.

Anyway, if I disregard the wood the steel won’t work for the 18’ span and over 800 plf. Based my calcs, it will work as a composite section. I just wanted to make sure I was doing it right. Any input is greatly appreciated.

RE: Flitch Plate Beam Question

kxa- Of course you can analyze the section as composite.  The material is there, why not use it?  Plus, it sounds like it's not as simple as a steel plate on the same neutral axis as the lvls.

The other suggestions of ensuring composite action occurs are valid though.  And your idea of jacking the beam is exactly right.  Otherwise the existing lvl would be stressed with the existing dead load, then the new live load would add stresses proportional to the new transformed section.

Regarding the stress check, you are correct in transforming the section and calculating I and Stop, Sbot.  You can transform the section twice, once to equiv LVL and once to equiv steel to check the stress in each material.  I find this easier than keeping track of a modular ratio.  Deflections are easy.  You should get the same deflection whether you choose equiv steel or lvl.

RE: Flitch Plate Beam Question

In order to provide composite action between the wood and the steel, you need to ensure no slip between the two materials.  I don't see how you can do that.  I there a reference in the literature that shows how to make the connection?

RE: Flitch Plate Beam Question

jmiec- Correct me if I am wrong, but I think you are picturing this beam similar to composite action as one would have with a steel beam and conc slab.  In that case, you are correct, there cannot be slip between the materials.  However, flitch beams are not composite in that sense.  They are essentially three beams bolted together side by side.  kxa's example is slightly different in that the NAs do not line up, but that just means a more complicated calc of I and both an Stop & Sbot.  Search this site for "flitch" you'll find numerous forums on this topic.

RE: Flitch Plate Beam Question

To ensure composite action you would need to calculate the shear flow and ensure that there are enough connections (probably bolts) between the LVL and the plates. You should use a timber design manual to calculate the capacity since the timber component of connection will probably govern as opposed to the steel component.

RE: Flitch Plate Beam Question

Alternatively, you can design the beam without considering composite action and design based on deflection compatibility (relative stiffness). Since it is just a plate and only 5 " tall the stiffness will not be that great but might enough to take the balance of load that the lvls can't handle.

RE: Flitch Plate Beam Question

ataman is right.  Forget the composite action and use deflection compatibility.  Place bolts (or lag screws, etc.) as needed to brace the plates.  Hopefully this will be stiff enough to take the balance of the load.  Remember to check the resulting stresses in both the steel and the wood when you have distributed the loads among the beams.

RE: Flitch Plate Beam Question

PMR06-

You are right, if kxa is simply adding the transformed Is (see my original post), one does not need to calculate the shear flow as in VQ/It.

However, kxa pointed out that the plates were "on the bottom".  This implies that the difference in neutral axis is a consideration.  Further, you stated "the NAs do not line up, just means a more complicated calc of I", and both an Stop and Sbot.

If one is simply adding the transformed Is, then it doesn't matter that the NAs do not line up, and the calculation of I is not effected, and Stop=Sbot.

Anytime one locates a composite neutral axis and then starts adding A*d squared terms to the transformed Is, there can be no slip between the two materials.


RE: Flitch Plate Beam Question

(OP)
Maybe I wasn’t that clear. I am not adding the steel plates to the bottom of the LVLs. The plates will go on the sides of the LVLs right below the floor joists. There is about 5” clear area.

This should not make the clac’s too complicated. Once the new neutral axis is calculated which will be below the LVL’s N.A., Sx top and bottom are figured out and stresses are determined.

RE: Flitch Plate Beam Question

Doesn't matter whether the plates are on the bottom or on the sides. Once you start adding A*d^2 terms to determine your total moment of inertia, you have to prevent slip between the two materials so they act as a unit.  I expect the connection would be some variation on VQ/I.  Is there an example in a textbook or anywhere else that shows how to do this?

RE: Flitch Plate Beam Question

Flitch beams are commonly assessed based on deflection compatibility. Load taken by timber/plate is based on their stiffness. In calculating load taken by steel plate its best to use the minimum timber E for maximum load on plate. Conversely, timber is checked based on its mean E value. You need enough bolts throughout the span to transfer the load from timber to plate, and bolts at ends for load to be transferred back to timber if only timber is bearing at supports. ( as would be by making plate slightly shorter than timber beam to allow for shrinkage)

RE: Flitch Plate Beam Question

Please tell me if I am interpreting this problem correctly or not.
Don't you have to ensure composite action for this to work?  Even if you put the top of plate directly under the joists framing into the lvl, you can't count on bearing to transfer the load into the plates because the bearing distance will be too small.  Can you use a 5" channel to achieve a greater bearing distance?

RE: Flitch Plate Beam Question

Myerges,

In terms of looking at it as sharing the load through bearing transfer , no I dont think you are interpreting it correctly. The thickness of the plate is therefore not the factor as to how much of the load the plate will take. Transfer of load to the plate is via bolting of the timber/plates and the amount of load taken calculated on deflection compatibility.

RE: Flitch Plate Beam Question

If you are transforming the steel into an equivalent amount of wood (or vice versa) and calculating a new neutral axis location, then this is called composite and adequate connections have to be made to account for shear flow. This is the most efficient use of the beam and plate material.

Otherwise, the steel and wood will share load relative to their E*I's, each material having its own neutral axis location. It is possible to make it work either way.

RE: Flitch Plate Beam Question

(OP)
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.
 

RE: Flitch Plate Beam Question

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.

RE: Flitch Plate Beam Question

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

RE: Flitch Plate Beam Question

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.

RE: Flitch Plate Beam Question

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/

RE: Flitch Plate Beam Question

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?

RE: Flitch Plate Beam Question

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.

RE: Flitch Plate Beam Question

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.

RE: Flitch Plate Beam Question

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.

RE: Flitch Plate Beam Question

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.

RE: Flitch Plate Beam Question

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.

RE: Flitch Plate Beam Question

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.

RE: Flitch Plate Beam Question

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.  

RE: Flitch Plate Beam Question

(OP)
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

RE: Flitch Plate Beam Question

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources