Wood Floor Truss question ....

[xtal01]

Hello,

I was looking at wood floor trusses and noticed something odd .. well at least to me.

Most floor trusses are built with the wood (2 x 3 or 2 x 4) on flat.

Why?

I know they are calculated to take the correct load but every roof truss I have ever used has been built with the wood "standing up".

I did a search and did see a few floor trusses built with the wood vertical but almost all were built with the wood flat.

Even the bearing area floor the mending plates is much less when built this way.

The only advantage I can see it to the builder who how has a much larger area to hit when putting down the plywood sheets ( did some carpentry "back when" and remember having to be careful the floor sheets landed the center of a floor joist and even that only gave you 3/4" for nailing).

I just thought it would be much stronger with wood in the vertical.

Thanks!

 

[JoshPlumSE]

For a truss, the AREA of the chord is what provides the greatest flexural resistance, not the orientation. So, the orientation of the 2x4 top or bottom isn't really that much of a deal. It likely comes down to what's easiest for assembly and construction.

It's much easier to nail into a 2x4 flat than vertical. You're much less likely to split it. It's easier to walk on. It's probably easier to connect to the OSB web or such as well.

 

[jayrod12]

It also depends on what the fabricator has in terms of equipment capability. Also to me it depends on the depth of the member. If you're doing a 24" deep truss, with lumber on flat your lever arm is 22.5". For on edge, it's 20.5". 10% more strength essentially just by flipping them. Also for shallower trusses, it maximizes the available open space for ducting.

 

[KootK]

I agree with all that the other gentlemen have contributed and will add the following.

 

[xtal01]

You guys are awesome!

I figure there had to be a good reason ... engineers don't do things "for the fun of it"

Now .... is there such a thing as a "free" truss design program?

I need a simple floor truss for a workshop attic.

Looked at all the you tube videos (I know ... dangerous) .... I think I can save about 2/3's the cost by building them myself.

This is no small about ...about $4K over wood I's ... and $6K over floor trusses.

I live in a rural area .... no inspections ... no engineering stamp required .... I already have the permit (just had to put a square on the plot map where the building is going.

I know this may not seem like much but to me it is a huge amount (I am building on a super tight budget).

I know no engineer is going to design one for me ... the liability of how I would build it ... quality of the wood ... how well I would build the joints .....

I did my homework ... even talked with engineers at the adhesive manufacturer.

Loading is simple distributed load (no point loads) .... simple support at both ends (two supporting walls).

I have looked at many truss manufactures sites and literature ... I can "copy" one ... but I hate to "guess"

I realize I just don't have the knowledge to design one without software (pratt ... warren .... k-wed .... I don't know when to use each or why)

There are just too many roof trusses and they are much more complicated so I will purchase them.

Any suggestions on any "free" software I could use to design a simple floor truss.

Thanks ..... Mike

 

[KootK]

Any suggestions on any "free" software I could use to design a simple floor truss.

No such thing. At best, you could use a general purpose 2D analysis package like RISA but you'd still be stuck with:

1) Needing to resort to Truss Plate Institute standards to figure out how to model the thing properly and;

2) Trying to figure out some way to design the plated joints manually which is not easy.

If you're going with the 4x2 orientation, don't even think about plywood gusset plates. You'd be better off just sheathing the entire thing with plywood. If you do go with toothed metal plates pressed on in the field, buy the plates from a local truss shop and have them supply you with an engineered design. They'll probably be able to run it for you in 15 min.

 

[xtal01]

I read ... and maybe it is not true ... that I should not use those plates.

They are pressed in and can not be guaranteed to work correctly if you hammer them in or press them in with something like a C clamp.

Is that wrong?

The adhesive manufacture said there glue is indeed stronger than the wood but there is a problem with creep over long time periods. This is why they recommended using plywood gussets nailed over each joint.

I might be stuck ... just looking for options.

 

[KootK]

They are pressed in and can not be guaranteed to work correctly if you hammer them in or press them in with something like a C clamp.

They need to be pressed, not hammered. Many fabrication plants use something akin to a C-clamp in fabrication. That said, you need to have confidence that you know how to do the install properly. If you don't, seek competent professional assistance.

The adhesive manufacture said there glue is indeed stronger than the wood but there is a problem with creep over long time periods. This is why they recommended using plywood gussets nailed over each joint.

Most engineers that do this kind of thing ON 2X ORIENTED TRUSSES will design the connections with traditional fasteners through gusset plates and then add the adhesive as a backup connection mechanism intended to primarily improve connection stiffness. The two big difficulties associated with adhesives are:

1) Quality control in the field application of them and;

2) They may exhibit brittle failure modes and are thus unforgiving of imperfection.

What are you hoping to achieve here with respect to span, load, and truss depth?

Given that you're already ordering a roof truss package, I'd be surprised if you couldn't also get the floor trusses for cheaper than you could buy the lumber to make them yourself. The truss plants buy their inputs in bulk, like you buy toilet paper at Costco.

 

[JoshPlumSE]

Some thoughts on "manufacturing" these sorts of wood joists / trusses yourself.

a) This might be more challenging than you think. It's a very competitive market and there are many suppliers. Their profit margins are likely not incredibly high in this competitive of an industry.

b) You don't have access to the same glue and connectors and such. Therefore, you'd have to go a more conservative route. Heck, you probably want to go a conservative route anyway. Just because it will be easier to build. And, your labor costs will quickly add up.

c) I worked on a project where we were "replacing" a series of wood trusses that were not much deeper than SJI type joists. This was for the roof of a "double-wide" type home. I think I ended up using 2x wood members as the web verticals. Then 2x4's sandwiched around them for the top and bottom chords. Almost double the chord area of the original truss. But, it was crazy easy to build on site. I also connected the double 2x4 chords (using 2x spacers) periodically between web locations.

I'd be willing to bet that my job could have been done cheaper with SJI (or similar) joists if they would have worked for the space.

 

[JoshPlumSE]

Heck, you might create a version of it using plywood on either side of 2x4 top and bottom chords (oriented vertically). That's double the web material, but the connections are crazy easy.

 

[xtal01]

Thanks guys ... I will double check my #'s but the building industry has gone crazy over the last year!

It's a long story but I started building my house almost 7 years ago (another long story .. built an accessible house since my wife was paralyzed from the shoulders down in a diving accident when she was 12) ... pour a pad for a 3500 sq ft workshop ...16 ft walls ... 1000 sq ft of storage on a second floor (about 50 x 20).

Don't cringe too much but I even built an elevator out of the mast from a forklift ... $40K to purchase ... $2K to build

Since the pad is in ... and I really need the space (I am a machinist and work out of the house) ... the size of the building is fixed.

Problem ... when I planned the project 7 years ago, I got a quote (I subbed out all the work and acted as my own GC) for all the materials to build the shell ... $35K

Two years ago I got ready to start ... about $55K in material

In the middle of last summer ... as they started lifting the lock down in our area .. I got new quotes ... $110K for materials and that didn't include roofing ... windows ... doors ....

So now I am struggling to cut every corner I can just so I can build the place.

Changed my outside material from Hardie or LP to vinyl ... took out some windows and going vinyl ..... not going to meet the current VT energy standard (would require 2" of foam on the outside of sheathed walls), I can do this as I am the CG and it is for my use ... OSB sheathing rather than plywood ....

I am cutting every corner I can.

I looked at 2 X 12's (at 12", structural grade "just" work according to span charts) ... last year $51 each ...called today $86 each

Wood I's ... need 16" deep ones .... $140 each

The floor trusses I had quoted last year were already $160 each ... I am sure they will be over $200

I did a quick estimate .... wood for the truss will cost me about $45 ... plus some glue ...say $50 each

I don't count labor as I would make them myself.

Anyway ... I appreciate all the advice.

I will double check my #'s and get some fresh quotes on trusses.

Thanks .... Mike

 

[gte447f]

You have mentioned glue/adhesive several times. You need to scrap that idea. Commercially available wood floor trusses are not manufactured with adhesive. They are called "metal plate connected wood trusses" in the industry for a reason... they are connected at the joints using toothed metal plates that are pressed onto the sides of the trusses at each joint. No glue/adhesive is used. If you are going to try to fabricate these yourself, you basically have 3 options for the connection plates: 1) the same toothed connection plates used by the professional manufacturers, 2) non-toothed metal plates that are nailed on instead of pressed on like the real thing, and 3) plywood gusset plates that are nailed on, not glued on. For the nailed on metal plates, don't expect to get anywhere near the capacity of the toothed plates, which means they probably won't work for a flat (i.e. shallow), heavily loaded (i.e. storage/work shop) floor truss spanning 20 feet. For plywood gusset plates, expect them to be huge at each joint, which means they will basically cover the entire sides of the truss like KootK already pointed out. That leads me to another point; you might be better off to consider site-built, plywood box beams, instead of site-built floor trusses. I don't know if they would work, but I think they have a better chance of working than your site-built trusses, and there are some old references out there that have information about nailing patterns, spans, and capacities. Another idea, put a row of columns down the center for a center floor girder to split your joist span in half and use solid dimensional lumber floor joists. I know this idea will compromise your open space on the ground floor, but that open space comes with a price.

 

[xtal01]

Thanks ...

I know how commercially available trusses are built ... but why would plywood gussets that are nailed and glue not work?

If glue joints are stronger than the wood itself (I should say, I am looking at Titebond III wood glue) and nailing the guest would take care of any creep (talked with the engineer at Titebond) ... they what am I missing?

Box beam ... interesting idea.

Still a lot of joints. If I use a 2 x 4 top and bottom ... they come in 16' long max ... I need 22 ft. Sheets if 4 x 8 would have at least two joints. I would think I need to nail and glue all these.

I think (and will check) but the price of material might be approaching the price of a wood I beam or a wood truss.

A 16" TJI 560 X 22 ft costs $150 each

If I can't build something for say $100 or less, they not enough savings to worry about.

My original thinking was I could build a truss for about $50 ... now that is a big savings.

 

[phamENG]

Trying to beat the prefab guys at their game is folly. At least, it is if you want something that even comes close to approaching the strength and reliability they offer (and if it were my property, that's what I'd be shooting for.)

These guys buy the material by the train car(s) and mass produce the end product. They've also optimized and tested their systems. Trying to do a one off, untested truss or beam is going to require more material than their system, driving the relative cost up even more. So it's a question of engineering efficiency, but most of all it's a question of economics. They have economies of scale that you could never dream of replicating. That's how they can buy the materials, make their product, sell, and ship it for about what it takes you to buy the raw materials at home depot (remember, you're paying for the same shipping and a much higher markup at home depot or your local yard).

 

[JoshPlumSE]

phamENG -

Good post! Your argument is succinct and compelling.

 

[phamENG]

Thanks, Josh. I went through the thought experiment on it a few months ago while doing the initial planning for a large carport-style addition to my barn. I wanted to see what it would take to do my own trusses - in my case I may still as getting a truck with prefabs to the site will be nearly impossible, but it won't be cheaper than buying trusses by a long shot.

 

[Ron247]

The top chord spans a much shorter distance between diagonal supports on a floor truss. It is capable of spanning this distance flat. So it can take advantage of the Ix increase by being flat and still perform its "beam function" between diagonals. Roof trusses on the other hand, must span much farther. A 2x4 roof member can span 6' to 7' for LL type designs when turned strong side up. I am not sure about Snow Load, I do not work in a snow area. For roof trusses, 6' to 7' is a common span for a rafter between diagonals.

 

[xtal01]

Thanks again guys .... I truly appreciate all the input!

I asked for quotes from three lumber yards for wood I beams and wood trusses.

I will have a lot to thinking to do.

Thanks !!!!!!!!!