Beam member connection

[Sifu]

From time to time I have seen the members of a header or beam with a pieces of plywood or osb sandwiched in between them as spacers. I typically don't worry much about this for most beams. However when a span or a particular load begins to become excessive it gives me pause. Now I am seeing it more and more and I am starting to see other materials, such as foam between the beam members. I have always been taught that the members of a beam should be fastened together in order for them to work as a single force resisting member, otherwise they will work less as a built up unit and more as individual members. I have recently asked for some input on a few jobs from the designers but have so far been met with the sound of crickets. I would like to know your opinions. Has anybody seen this, have issues with it?

 

[steveray]

That is how we used to do all of our headers, 2 2x and 1/2" ply= 3-1/2" wall.....Did have one inspector that called us on it once, he said the nails were not getting full penetration and the sheathing had no strength....The next job we did in that town had 22' I-joists...His head may have exploded......For the record we did use 3.5" spikes....then we had to angle them more so that they didn't rip you apart on the backside....

From time to time I have seen the members of a header or beam with a pieces of plywood or osb sandwiched in between them as spacers. I typically don't worry much about this for most beams. However when a span or a particular load begins to become excessive it gives me pause. Now I am seeing it more and more and I am starting to see other materials, such as foam between the beam members. I have always been taught that the members of a beam should be fastened together in order for them to work as a single force resisting member, otherwise they will work less as a built up unit and more as individual members. I have recently asked for some input on a few jobs from the designers but have so far been met with the sound of crickets. I would like to know your opinions. Has anybody seen this, have issues with it?

 

[Sifu]

I don't deny that in some cases the plywood may add to the strength of the beam. I certainly don't deny that myself and everyone else has built them that way for decades. What I am talking about is not full length plywood "flitch plates", it is pieces of plywood or foam or in the case of one big R2 job currently in progress, osb pieced in on exterior deck girders. My questions is where the foam, or plyood pieces or the osb that will deteriorate does not add to the beam strength and provides no nailing or transfer of force and may even compress or disappear all together, how will the beam strength be affected if the layer in between is not structural.

 

[Mark K]

The first rule should be what to the construction documents require. If the drawings list a 4x12 2-2x12's do not work.

If the load is applied equally to both joists there is probably not a concern. On the other hand if most or all of the load is applied to one side then it is important that the members share load. This will not happen if there is foam between them.

 

[RJJ]

Agree! It no longer works as a beam

 

[TheCommish]

as a header that is top loaded i would have no problem wiht foam or plywood between the plys of a buit up header, if it is side loaded that is another matter

 

[DRP]

For a typical header in a 2x6 wall I'll have a 2-1/2" gap between the header members with a 2x6 above and below flatways, a hollow. insulation filled, tube. Top loaded and no problem, can still take the repetitive member adjustment as the members are spaced closer than 24".

If a built up girder is side loaded the prescriptive nailing is inadequate.

 

[MASSDRIVER]

I've never understood the penchant for built up headers. The time to make them offset any cost in lumber, and there is no advantage. Sure, if you are slack in materials, have the lumber laying around and need to whip one up, I get it.

But a whole project? No thanks. Not when I can cut my whole header package off the load in 30 minutes or less. Not to mention getting the guys to properly nail into the end from the kingstud so it hits the middle of the 2x4 each time.

Brent.

 

[jar546]

What about for 2x6 exterior wall construction, using a double 2x10 header for example and placing it flush to the exterior then filling in the 2-1/2" with foam board on the inside for insulation?

Does the offset of the header make a difference since the top plate is technically hanging off 2-1/2"?

 

[Sifu]

I tend to agree with most everybody but I wonder where the 24" limitation comes from DRP. Is that a code or standard somewhere that could be referenced? Is this where it comes from?

Design values for structural lumber are published in the

supplement to the National Design Specification (NDS)

for Wood Construction. The tabulated design values are

for normal load duration under the moisture conditions

specified. Bending design values, Fb, for dimension

lumber may be multiplied by the repetitive member factor,

Cr = 1.15, when such members are used as joists,

truss chords, rafters, studs, planks, decking or for similar

members that are in contact or spaced not more than

24 inches (610 mm) o.c., are not less than three in number

and are joined by floor, roof or other load-distributing

elements adequate to support the design load.

 

[Sifu]

Because to be perfectly honest I am not sure I understand what that is telling me. Is that intending that in some cases, 3 joists run 24"o.c. with a load laid across the top of all 3 of them will get an increase in capacity? If so an increase in capacity from what? It is not saying that they would be the same as a 3 ply beam with members in contact with each other is it?

 

[DRP]

Ah, I was going from memory, the NDS increase from base design value is for 3 or more members and yes it applies to built up beams. I'd have to dig in old texts to find where it was applied at 2 or more members, the NDS is being more conservative, which is good. The increase is due to load sharing and distribution of defects. One way to look at it is rather than an increase, as stated, there is a rightful conservativism for a single member. We severely ding a single member because we assume it has the worst allowable defect in the worst possible place. Typically, or according to grading rules, that piece is still 2.1 times stronger than the design value and in practice generally many times stronger. Begin multiplying the members and there is a legitimate reason to allow an increase in the design value.

The typical adjustments applied to base design values in the NDS are for;

Duration of load

Repetitive member

Size

Wet service

Incising

Less commonly;

Flat use

Temperature

Fire retardant

Column stability

Bearing area

Buckling stiffness

In our hip exercise the other day the NDS base design value for Fb was around 850psi but we were working with an adjusted Fb' (Eff of bee prime (or adjusted)) design value of around 1200 psi. Properly using the adjustments is huge.

 

[Sifu]

So, working off of our hip exercise the other day, one of the problems I see with a 2x10 or 2 x 12 hip or valley "beam" is the huge seat cut. The propensity for the member to split near the inside face of the bearing wall, say 3 1/2" from the end of the member. I have been told that is why the WFCM doesn't really get into those sizes unless they are multi-ply. Given the current thread topic, if we were to lay a flat member across 3 CJ to support the seat cut of a hip/valley beam it might work without too much worry. Kind of an empirical thing but something I see all the time.

 

[DRP]

If you look at the reactions on that bottom end that will usually work and I've done that. A dragon beam of one type or another is the "correct" way to handle that detail.