Thread: A different type of floor joist

Anyone seen a floor joist like this before?
1988 1-story home with finished walk-out basement. Could only get a small view of the floor structure from a utility room. Looks like a 10" joist assembly that resembles a hollow box, presume it is a pair of 2x4's as top and bottom plates tied together with 3/8" plywood on both sides forming a box. You can see in the picture that it looks hollow where they cut holes for wiring to pass through. Based on a very limited view, we assume this forms the floor structure for the upper level of the home. The inspector didn't see any indications of structural issues, floors seem flat, no movement noted in walls.

Any thoughts on possible implications?

Similar Threads:

• Scissor joist
• Fastener type for flashing
• CPSC Recall: Floor Electrical Outlets
• Insulation type
• Joist support

Are you sure they were hollow and not I-Joists with plywood added to the sides?

Zooming in on those holes shows plywood through the holes, and it doesn't look far enough back to me to be the plywood on the other side, more like the center of an I-Joist.

What would be the point of covering the sides of the I joists even if they are inside a finished ceiling or not?

Originally Posted by Bob Elliott

What would be the point of covering the sides of the I joists even if they are inside a finished ceiling or not?

Additional strength and rigidity to the joists?

I suspect that a boxed in I-Joist is stronger and more rigid than a non-boxed in one. Whether or not it would affect its strength by the amount of plywood added, and all the time and money, I don't know, but I suspect it would definitely be stronger and more rigid with the sides boxed in too. Of course, a lot would depend on how and what fasteners were used and their spacing.

Not saying it would be allowed as it would be considered "an alteration" of the I-Joist and that would require engineering.

Didn't poke a screwdriver through the hole to verify it was hollow, but may have time tomorrow.

It approaches the concept of an inside-out I-Joist, but the one possible drawback is that 2x4 plates, even boxed, might have some opportunity to twist or cup because it is still natural wood. Whereas, the top and bottom flanges of an I-joist won't because it is engineered lumber (a glued and pressed product). Try picking perfectly straight studs out of a a bin at a lumber yard. Still, I doubt it wouldn't affect structural integrity very much.

I was mostly curious if this was unique, or if anyone had ever seen this design before. There are no residential building inspections where I do most of my work, so I doubt the joists were actually engineered, just some builders creative concept.

First thought is Sys 42s (2x4 floor trusses) esp. as seems to be 19 or 24" OC. perhaps even 32s. Can't see enough of structural from the pic. Should have stuck head up there and looked for other clues.

http://www.trusscomponents.com/pdf/F...ss%20Guide.pdf

Looks like they're home-made, on site box beams - made from a couple of 2 x 4s scabbed together with 3/8 ply. It would be nice if you could see the ends but I suspect those would be 2xs also.

Ian, I am almost positive they were home made. Here is a picture of the trusses in the attic. You can see some similar workmanship.

Just to follow up, the constructed "box" joists are on 24" o.c. with cross blocking every 12 feet or so. Yes, the 'box' joists are hollow (can poke a screwdriver through the holes that the wiring passes through). Sides of box are 3/8" exterior grade plywood, stapled to the 2x4 bottom plate (and I presume top plate) every 6 inches - did not see any construction adhesive leaking out).
- Still curious if anyone has seen anything similar.

Originally Posted by Terry Beck

Just to follow up, the constructed "box" joists are on 24" o.c. with cross blocking every 12 feet or so. Yes, the 'box' joists are hollow (can poke a screwdriver through the holes that the wiring passes through). Sides of box are 3/8" exterior grade plywood, stapled to the 2x4 bottom plate (and I presume top plate) every 6 inches - did not see any construction adhesive leaking out).
- Still curious if anyone has seen anything similar.

Interesting.
This means each joist is in reality the strength of 6/8" plywood.

Why even use the sideways 2x4's which can warp in the first place instead of just joining the 3/8 sheets together or using one 3/4" sheet ? (I suppose to stop the run from warping to the side).
Hope someone posts more info on this.

Thanks for the additional picture(s), including the attic trusses. I agree with Ian, esp. as you indicated this was late 80s construction and could be fir, unk. w/o a stamp.

Tough to see nail heads as there has been a lot of wood filler applied but I do believe I've spotted enough nail head "ghosting" suggestive of the presence of necessary "stiffners" for plywood box beams. These have been used and prescribed going back pre-APA (late 50s) back in those days it was nail-only. At 600% I see staples at both the upper and lower flanges.

I haven't had reason to consult the APA free downloads or the multitude of revisions since the turn of the last century, but IIRC the document you'd be searching for would be Z416, head on over to APA - The Engineered Wood Association and click on publications, register (have to now for free downloads), and search the publications for the document number I've referenced, hopefully I'm remembering correctly.

When properly calculated and correctly executed, use of same would make tremendous sense esp. in remote locations such as you reference. When constructed correctly these can be a very strong, comparatively lightweight, solid, site-built way to construct. If you didn't see a stamp on the ply and/or mic it, you *might* be underestimating the plywood. I am not impressed with panel joint locations or directional use of ply but am not of a mind to consider thoughtfully at the moment.

Someone wondered why ply webs/or full gussets - remote location - more time/strength/resistance to fire. Site built - remote location - no special tools.

Attaching a diagram snipped from years ago. Hope you find it helpful.

Originally Posted by H.G. Watson, Sr.

Thanks for the additional picture(s), including the attic trusses. I agree with Ian, esp. as you indicated this was late 80s construction and could be fir, unk. w/o a stamp.

Tough to see nail heads as there has been a lot of wood filler applied but I do believe I've spotted enough nail head "ghosting" suggestive of the presence of necessary "stiffners" for plywood box beams. These have been used and prescribed going back pre-APA (late 50s) back in those days it was nail-only. At 600% I see staples at both the upper and lower flanges.

I haven't had reason to consult the APA free downloads or the multitude of revisions since the turn of the last century, but IIRC the document you'd be searching for would be Z416, head on over to APA - The Engineered Wood Association and click on publications, register (have to now for free downloads), and search the publications for the document number I've referenced, hopefully I'm remembering correctly.

When properly calculated and correctly executed, use of same would make tremendous sense esp. in remote locations such as you reference. When constructed correctly these can be a very strong, comparatively lightweight, solid, site-built way to construct. If you didn't see a stamp on the ply and/or mic it, you *might* be underestimating the plywood. I am not impressed with panel joint locations or directional use of ply but am not of a mind to consider thoughtfully at the moment.

Someone wondered why ply webs/or full gussets - remote location - more time/strength/resistance to fire. Site built - remote location - no special tools.

Attaching a diagram snipped from years ago. Hope you find it helpful.

Good Job H.G.

Yes I have seen them many a times as well as the trusses in the pictures.

I've seen them before. Modular home built in a shop and set on the foundation. Many times you'll see the manufacturer's information somewhere near the electrical panel.

Adding 3/8 inch ply wood to the side of an I joist does not appreciably increase its ability to resist moment but does increase it ability to carry shear. I think that these are built on site box beams that have a section modulus of about 2 times that of 2x10.

HG - great job. Thanks,

Yes, I found the document on APA (Z46 - Panel and Lumber Beams). Not the same application as what I saw, but it does offer a good reference. Of course, without tearing one open, I can't tell if there are stiffeners inside the box joist or not.

Originally Posted by Bob Elliott

Interesting.
Why even use the sideways 2x4's which can warp in the first place instead of just joining the 3/8 sheets together or using one 3/4" sheet ?

Hope someone posts more info on this.

Bob, the flat 2 x 4 flanges are required to give each member a larger section modulus. Without the flanges, there would be little resistance to bending moment applied to the span, from both dead loads (sheathing, shingles, members themselves) and live loads (snow, wind, people, etc.).

The next time you're out driving around, look up at any steel girder bridge you happen to drive under. You'll see that the girders all have top and bottom flanges (perpendicular to the webs tying them together), serving the same function as the 2 x 4s you were wondering about.

Michael Kober, P.E.

Originally Posted by BridgeMan

Bob, the flat 2 x 4 flanges are required to give each member a larger section modulus. Without the flanges, there would be little resistance to bending moment applied to the span, from both dead loads (sheathing, shingles, members themselves) and live loads (snow, wind, people, etc.).

The next time you're out driving around, look up at any steel girder bridge you happen to drive under. You'll see that the girders all have top and bottom flanges (perpendicular to the webs tying them together), serving the same function as the 2 x 4s you were wondering about.

Michael Kober, P.E.

I can see the bottom and top 2x4's giving a larger surface area at the top and bottom however do not see that helping the major surface of each panel from bulging unless the diagram Watson provided is using those stiffeners, and I can not tell how they are fastened unless glued since there are no side fasteners visible in the original posted picture.

The Box Beam has no flanges I can relate to the bridge example so will try and look into this a little more.

Originally Posted by Terry Beck

HG - great job. Thanks,

Yes, I found the document on APA (Z46 - Panel and Lumber Beams). Not the same application as what I saw, but it does offer a good reference. Of course, without tearing one open, I can't tell if there are stiffeners inside the box joist or not.

You're welcome Terry, and thank you for the feedback.

Yes thanks for the site link however I still am not sure how the stiffeners are fastened after reading it.


Parts of a box beam are shown
in the diagram The lumber
flanges carry most of the bend
ng stresses and the structural Panel web
Panel joint
panel webs transmit the shear stresses
Lower flange
stresses. Vertical stiffeners
between the flanges act as shear
splices at panel butt joints, distribute concentrated loads and end reactions, and resist web buckling. The fasteners transfer the stresses between the lumber and panel parts

Got a helpful response to a query I had previously sent to APA - The Engineered Wood Association before I saw HG's initial reply above.

The APA support specialist said,
"Plywood box beams are (were?) somewhat common in the manufactured housing industry and occasionally used for site built structures. Site built applications seem to be mostly limited to do-it-yourselfer projects or sometimes projects in remote locations where access to large timbers is difficult."

Besides the previously referenced document on their web site (Z416), he also attached another document available at the APA,
S812- "Plywood Design Specification, Supplement #2: Design and Fabrication of Glued Plywood-Lumber Beams"

The first paragraph in that document reads similar to above,

"1.1 Beam Behavior
In plywood beams, the lumber flanges carry most of the bending, and one or more plywood webs carry the shear. Joints between them transfer stresses from one to the other.

Vertical stiffeners set between flanges distribute concentrated loads and resist web buckling. Deflection resulting from shear is usually significant, and must be added to the bending deflection. Lateral restraint is often required to maintain stability. End joints in flange laminations and webs may require splicing."

It goes into deep math detail, and does seem to require stiffeners at specific places in the beam. Again, the application I saw was joists, not beams, but can't see much different in design except size.

Terry does it say anything about fasteners,what type and where?

S812- "Plywood Design Specification, Supplement #2: Design and Fabrication of Glued Plywood-Lumber Beams"

PDF Download | APA - The Engineered Wood Association

while it does list some some specifications for nails, it focuses on using glue.

Must say I am surprised they depend on glue.
Glue fails and since covered up you would not see the failure till it failed.

Originally Posted by Bob Elliott

Must say I am surprised they depend on glue.
Glue fails and since covered up you would not see the failure till it failed.

Plywood structural panels rely on glue to hold themselves together.

I-Joists rely on glue to hold themselves together.

Glulams rely on glue to hold themselves together.

The list could go on, but I believe the point has been made that there are many structural products which rely on glue to stay together.

My dad, who was a carpenter for over 40yrs, was not a fan of floor truss's for several reasons. One was that the house felt like you were walking on a trampoline. Also, the gang nail plates that hold the web together were not installed well. They would always have to add a few nail here or there to tighten them up. So the plywood maybe there to just stiffen the truss. Another thought is from a conversation with a Fire Fighter friend. He was telling me that in a fire, the gang nail plates curl weakening the truss. So maybe the plywood was installed to satisfy local fire/building codes.