Adding a 2x8 flange to 6x10 wood beam

[WmacG]

Hi everybody, this is my first post, but I have been watching for a while.

I am wondering if adding a 2x8 flange on the bottom of a 6x10 beam(actually 3-2x10s)is an acceptable way to increase the capacity and reduce the deflection of the beam.

I am considering shoring the beam to zero deflection, and gluing/screwing the 2x8 to the bottom of the beam. The 2x8 would stop just short of the supports.

This beam supports the joist (2x6) for a nearly flat roof of an older house, beam span is 17', beams spaced 8' apart, approx. dead load is 17psf. Not sure what type of wood the beam is.

Is gluing/screwing a flange to the beam an effective option?

Thanks
Mac

 

[PSlem]

You'll have a near impossible task of getting enough fasteners in it to take the shear. You should consider adding wood or metal plate to the side.

 

[UcfSE]

Try calculating what kind of fastening pattern you will need and see if it seems reasonable. If you get an outrageous answer then you know. I would use the shear flow equation f=VQ/I (results in force/length) to figure it up.

 

[WmacG]

I had checked the shear flow and found that nailing would require a spacing that is not practical.

I am thinking about using an adhesive with shear strength of around 300psi (wood is about 175psi).

I think it would work in theory (similar to gluelam beams), but am not sure if the actual results will be good due to the method of shoring, applying adhesive, screwing, humidity, temperature, creep of adhesive, etc.

I was relying on the adhesive to take all the shear stress and the screws to clamp the pieces together while the adhesive cures.

Mac

 

[SlideRuleEra]

Mac - I agree with UcfSE that your concept sounds reasonable. However, instead of screws, I would suggest considering lag bolts (maybe 3/8" dia., or so), and using them in pairs (into each outside 2x10). Also the number/spacing of the lag bolts would provide 100% of the necessary connecting strength. Still use plenty of glue, but in a secondary role.

 

[PEinc]

I would hesitate to screw 3/8" lag bolts into the 1 1/2" face of a 2x10. If you pre-drill the lag screws, it would probably reduce the strength of the 2x10. If you don't pre-drill, you will split the 2x10. Nails won't be very efficient. I agree with PSlem.

 

[WmacG]

Thanks for your replies.

The reason that I did not want to add steel or wood to the sides of the beam was that the joist were not sitting on top of the beam, but possibly joined into the sides of the beam. This would have allowed only a 4" tall board on each side of the beam, even with steel this would not help much due to low inertia of the additional boards.

I have since found that the joist are notched with 4" sitting on top of the beam and 1.5" overlaping the side of the beam.

I will add a 2x10 on each side which will increase the depth of the beam by 1.5", the same as adding the flange.

Based on the inertia of the two cross sections, a 2x8 flange is about equal to 2-2x10 side pieces (assuming you could make the connection between the pieces adequately).


Thanks again,
Mac

 

[PEinc]

If you add 2x10's to each side of the original beam, you will need to make them bear on the same column or wall that the original beam bears. The added members should not end short of the original beam's bearing. If the added pieces are not full length, then you will still have a connection problem for joining the 3 pieces together to make them act as one member. This will require a lot of nails, bolts, and or glue.

 

[tsum]

If you use glue, make sure to use epoxy and that it is a thick consistency to fill gaps. Carpenter's glue or sub-floor adhesive will not work because they will creep under load.

 

[PSlem]

This will do the trick

http://www.edgest.com/wood.html

 

[JAE]

PEinc - If the strengthening of the beam is only necessary to increase the midspan bending moment and not the end shear then the added material CAN be stopped short of the supports as long as the added material is extended beyond the point where it is no longer required.

This is analogous to a steel cover plate on a wide flange where the cover plate is used, say, in the middle third of the span.

Now if the existing beam cannot take the end shear, then yes, your statement is correct...but many times wood beams only need bending updgrades and not shear upgrades.

 

[PEinc]

JAE,
Yes, but you also need to design the fastening of the two side members securely to the original member so that the three act as one. My point was that this connection can require a lot of fasteners or fasteners and glue. If the side members are not fastened properly, they may just be adding to the dead load.

 

[JAE]

..well...yes, granted, but whether you extend the new material to the supports or not, the same number of fasteners are needed to transmit the horizontal shear forces to engage the new material along the span.

My point is that the extension of the new material to the supports is an independent issue with regard to the attachment to the existing members. Flexural capacity doesn't "know" whether the end is extended or not.

 

[WmacG]

I am planning on extending the new members over the supports if it is not unreasonably difficult.

The way I think of the connection requirements of the new members to the old is to imagine that the old beam is hanging their with a deflected shape. Now place the new members so that their center is matched up with the center of the old beam (along the side of the old beam). Assuming the new members are straight, the difference at the ends of the beam is equal to the deflection of the old beam.

The force that it would require to move the end of the new member up to the existing beam (so they both have same deflected shape) is what the connection must support.

This force would be provided by the wall if the new members were supported on the wall. I think that the force would be the total reaction force at the wall divided by 5 for each new member (because there are 5 equal size members carrying the loads, 3 old 2x10, and 2 new 2x10s)

Mac

 

[SlideRuleEra]

WmacG - You mentioned that this is an older house. Since you are planning to make some detailed measurements for deflection & close fitting, suggest that you verify dimensions of the existing members. Until the mid-1960's dressed 2x10s were 1 5/8" x 9 1/2". Since then, they have been 1 1/2" x 9 1/4". If there is a difference in the existing members and the proposed new members, of course you can compensate. However, you probably want to know about this at the start.

 

[corrosionman]

WmacG
I just had exactly your problem but it was a ceiling of 4 x2
timber which needed strengthening to be a strong floor equivalent to 7 x 2. we tried everthing without achieving anything then we simply affixed 3 x 1/4 steel flat to each side of every timber Plenty of big strong screws at close spacing The result was a massive rigid floor and it was done very easy and quick. If you enjoy the theoretic work
then examine the respective E values and link them to deflection formulae. This is the solution to your problem. Cheers

 

[FSS]

Lots of good comments on the stress issues. Aside from that, I would be very concerned about your intent to "shore the beam to zero deflection." You will crack/pop all kinds of stuff when you start jacking up the beam. Could create a whole new set of issues.

 

[WmacG]

I went to the house and saw the beams after modification.

I't looks great in my opinion, approximately .5" deflection over the 17' span with 90% of the dead loads applied.

The new members are supported at the wall.

There is no sheetrock installed (so it didn't crack), the joist don't seem to have moved any due to the shoring and the roof decking will be replaced anyway.

Thanks again for all the comments.

Mac