Repair of a laminated wood beam 6

[YukonOne]

I looked at a late 1960's vintage building that has 40' long vertically laminated wood beams at 12' oc. The beams are made up of vertical plys as follows: (1) ply of 5/8" plywood; (2) plys 1-1/2" lumber; (1) ply of 5/8" plywood;(2) plys 1-1/2" lumber; (1) ply of 5/8" plywood. The total beam depth is 23-1/2"; the total width is 7-7/8". There is a 39" wide x 6"high notch @ the bottom of the beam approximately in the center of the 40'span.

Here's the questions:
1) Has anyone seen laminated beams constructed like this before where the plys are vertical?
2) Any thoughts on possible repairs?
a. Steel side plates 6' to 8' long??
b. Wood side plate 6' to 8' long??
c. Steel plate lag bolted to the bottom of the beam 6' to 8' long???
c. Other ideas????

 

[JAE]

I would look at using a steel "U" plate around the bottom and sides and extend it well past the notch.

Then use bolts or lag screws to fasten the plate assembly to the beam and ensure that the fasteners can transfer horizontal shear forces to the plate to engage it adequately.

 

[Althalus]

It sounds a lot like LVLs. But those weren't commonly used back in the 1960s. Are you sure they are from that same era and not a remodel?

The simplest fix would be to size out two new Microlam beams to be placed on either side. You can sandwich the existing beam without worrying about the end shear (the original beam will take care of that). But I'd extend the new members to the middle 1/3 of the span. Be sure to glue and screw sufficiently.

 

[Sawbux]

It's a plywood box box. Not too common, but the American Plywood Association had a design guide for them back then. Basically, you will need to replace the cut/missing chord members and fasten the patch material to transfer the chord force. Microlam or LVL sounds like a good option. You will likely need to glue and through bolt to develop the needed tension forces. I wouldn't recommend lag bolts.

 

[hokie66]

Sawbux has it. The box beam will be hollow between the chords. The bottom chord has been cut, so you need to replace the capacity of those four members, as well as the three plywood webs at the bottom. You could use steel side members, through bolted to develop the tension capacity required.

 

[dhengr]

YukonOne:
And, what indications are you seeing that it really needs to be fixed? It’s been carrying the loads for forty + years, and we’ve had some pretty good snows and winds in MN over that period of time. Is this a fairly new notch? Are you adding new loads, is it just starting to show distress? Those beams must be a real testament to a good nailing and glue job, because they would be a real miss-mash of vert. end butt joints of pieces in any one vert. ply layer and horiz. glue joints btwn. 2x pieces to make up the depth in a ply layer. Do you have any idea of the make-up of these types of joints, finger jointed ends, random nature of these joints? Can you see open end butt joints in the 2x plys at the bottom edge of the beam? Same question for plywd. butt joints.

From your description, I never thought of a plywood box beam until Sawbux mentioned it, but that certainly is a likely candidate from that era. Then the bottom 2x members must be 2x8's or 2x10's or you’d see the void above at your 6" high notch. Can you see a plywood nailing pattern which shows vert. blocking at 16 or 24" o/c and particularly at plywood butt joints? Can you hear the hollow void btwn. the vert. blocking? Since you already have the 6" high by 34" long notch at about mid span, they have effectively reduced that beam depth by 6" +, but left a perfect pocket for a turnbuckle. If you can get at the ends of the beams to apply bearing plates (some bearing assembly) and nuts and washers; route out 3/4" of the bottom of the middle plywood web and install tie rods with a turnbuckle to reestablish the lost bottom edge tension. Then just put a U shaped plywood cover under the turnbuckle, to fill the notch void. You could also apply a couple 3" wide straps to the bottom edge, length to be determined. Shore the beam to get some camber and unload the bottom edge, and nail one end of the strap. Apply a tension force to the other end of the strap and nail it off, and cut off the gripper waste.

 

[YukonOne]

dhengr -

Currently the beam is shored with posts on either side of the notch - it has been that way for quite a while. The owner wants to get rid of the posts.

I agree that its testament to the quality of the original construction.

Michael J Preston PE
MJP Associates, ltd.