For the nerd herd: Open Web Floor Joist strength questions

[DaveB]

I'm going to be buying a place in Chicago soon, and obviously a big piece of the decision making process is just how big a tank it'll support.

It just so happens that I know the builder, though, which is handy. We currently rent from him.

In our current place, which was built 11 years ago, he used IGF joists 16" on center, and in the bedroom they span 18 feet. With his blessing, I have a 125 on a solid oak dresser running perpendicular and about 5 feet out from the load bearing wall (there's a closet behind it). These joists also support the king size tempur-pedic bed (weight borne by 9 points on the frame) and 500lbs of nightly mass (me, fiance, 200lbs worth of dog). This is all fine. Elsewhere in the house we had a 5' 110g directly on a load bearing wall and a 75 on the opposite end, actually running parallel to those same joists. Also with no problem (we got rid of the 110 a while ago though so now it's just the 75g alone now).

The new places he's building use open web trusses, which are apparently quite a bit stronger. Again, though, much of this strength is used up by the fact that in Chicago architecture they're asked to span 18-20 feet. Anyway, two of the units we're considering are penthouses (4th floor) and another is a duplex down unit with concrete on the very bottom... but the living areas are both actually on the floor above that.

I just want to see if I'm understanding a few things correctly, because I'm not an engineer.

1) Minimum allowable strength is 40psf. But you can go higher than that for a tank, provided those joists aren't being asked to support >40 every single foot of their spans.

2) IGF and Open web trusses can actually support way more than that.

3) Thus, in theory I shouldn't really have any problem at all putting a 96x24 or even 96x30 tank against a load bearing wall perpendicular to the joists, provided I don't overload those joists elsewhere in the room with a tub or waterbed or another tank.


Assuming those 3 points are valid (and I'm not afraid of criticism if you disagree), I then have these questions:

1) I've been unable to find out just how much stronger these open web trusses really are. Are there any legitimate figures for them? Can they take 60psf as a reliable minimum? 80? Stick with 40 for the math anyway?

2) Does the triangular nature of these joists mean that there are certain points that are more able to handle a static load than others, based on the span from triangular point to triangular point? And does that mean that you can't just say "well it'll take 40 psf and there are 18 feet of that, so that joist is good for 720" because of that variation?

3) This is just me being curious: I assume that if a joist terminates at a set of stairs, it's going to end up being quite a bit weaker since that end doesn't go to a load bearing wall. So I wouldn't want to put a tank opposite the staircase headed down. But what bears that weight, then? Is there a perpendicular joist at the top of the stairs that these main ones are mounted to, which then shifts the load to the full length joists on either side of the stairs? (I know I didn't describe this well. In the photo below, though, opposite the bar shown is a great open spot that'd hold a 6 foot tank. But the stairs going down mean that those joists under that bar terminate right there where the railing is... so I assume they can't bear a full load... right?)


I have a general idea of what can and can't be done (and have obviously ruled out what would be the perfect spot, looks-wise, for a nice 8 or 10 foot tank in the one 2-story unit, since it'd run parallel to the joists AND butt up into the stairs) and obviously still would be talking to the builder about it some more... but I wanted to run it by people here to get some thoughts.


(For reference, open web trusses are what are pictured in this thread: )
http://www.monsterfishkeepers.com/forums/showthread.php?t=129517

 

[sikoko]

holy $#!& wat type of dog do you have a horse

 

[DaveB]

Sort of. Rottweiler and a shepherd.

 

[Laosy Fish]

Sort of. Rottweiler and a shepherd.

i take it you sleeep on the floor or the couch?

 

[dawnmarie]

The photo you posted doesn't show clearly so I would see if the wall between the stairwell and the edge of the floor continues down uninterupted to the foundation. If it does it could be considered load bearing. Certainly the final word should be the builder.

 

[DaveB]

In that particular case it does not.

The part about the stairs is mostly just me thinking out loud and trying to understand this stuff. That area isn't the #1 spot I'd want to use. I just want to know if anyone knows what the true load capacity is for these trusses. I've heard as high as 10x the minimum but I wouldn't want to rely on that.

 

[DaveB]

The other #1 choice, which we saw today, has an ideal spot, visually, that is in line with the stairs. In a stroke of luck, however, there is a big open laundry room/utility room directly below it. This is pretty great, because it means I could use giant jacks to support the floor. (It also rules because all the hospital and fry and growout tanks could fit in this utility room.)

This house is on 18" open web trusses. The downside to these, though, is that they're strong enough that they advertise that people should build as much as 24" apart on center, instead of 16". Which I would assume probably happened here. If I bought it, I'd punch out the ceiling in the utility room and find out.

I'm assuming that even if they're 16" on center, I couldn't put an 8 or 10 foot tank parallel to the joists even if jacked up, though. That'd be just an absurd amount of weight on 2 joists.

Or would that be OK if reinforced from below?

Because that would be incredible to have a ten foot wall of tank in the dining room.

 

[JakeH]

I spent many years as a concert lighting engineer & master stage rigger building & hanging all kinds of temporary load-bearing structures for national tours, so hopefully I can clear up a bit about the truss concept... Truss structures (wood, steel, aluminum, etc.) do not behave like solid boards/beams or joists. They are rated in several ways & its up to you to adjust your loading for your intended application.

Basically the truss section itself (say a 10 ft stick of your wood truss) will have a huge load rating labeled as "UDL" or uniformly distributed load. For example, a UDL would be like placing a 10ft tank ( or 2000 lb weight) parallel & directly on top of the joist as it sits totally supported on a concrete floor. This situation never really happens, so throw that number out. Besides, the connections to the load bearing walls are the real "x-factor" & would give out long before you hit that mark. Next would be a "single-point load" or "center-point load". This is like supporting the stick in one point in the center, then seeing how much weight you can load on each end before it snaps in half. This is the polar opposite of the UDL & is usually the lowest rating. However, this is what you should be looking at because you intend to put a massive weight between the two supporting points on the ends, in effect just turning the test upside-down...

If you plan to use jacks to support the load from below, you friggin' ROCK & the following applies:
Now it gets more complicated... There will be "third-point load", "quarter-point load", etc. & will probably go all the way up to "eighth-point load". For example, a "third-point load" means that the stick was tested with 2 supporting points (evenly-distributed from the center, cutting the stick into thirds) & the load was applied at the center between the points. It goes that way for the other tests too: a "quarter-point load" will have 3 supporting points, cutting the stick into quarters, and on down the line. In these tests, the weight is always placed between the 2 outermost supporting points & evenly distributed across that distance. However, load cells show that the "even" load is actually not evenly supported in cases of fifth-points or more, due to the flex of the structure itself. These get up there because you're distributing the load across several supporting points.

Bottom line: If it were me, I would make sure that tank is crossing as many seperate joists as possible (joists perpendicular to length of tank) to spread the load evenly across multiple joists, then "acquire" another piece of that trussing (or a steel I-beam) & position it below & perpendicular to the other joists (parallel to the length of the tank), on as many jacks (supporting points) as possible. This will spread the tank's weight across the primary joists, which will be evenly supported from below by your secondary truss, which will be supported by as many jack points as possible to reduce the load on each single jack point, forming a solid platform independent of the primary joist's wall connections. That way you're using the wood's best traits:compressive strength.
If you're planning on relying only on the primary joists & the guys that connected them to the load-bearing wall, I wish you good luck & I will stand well back, patiently awaiting the show.

 

[fox3]

I have had to repair those truss joists in the past. When you say can the truss support this weight the major factor is the span. It could be 80psf rated at 12' span and fail at 19' span under 40psf 12" centers.

Basically throw out the WFCM woodframe construction manual as it is guidlines and go by experience. When sizing trusses they usually can support more than what is necessary they support. But they have bounce. Its not the down that you feel but the reaction back up so once the room is furnished you do not feel this reaction. But the floor diaphragm is ponded (sunken).

When we size truss fl jsts we use L/480 deflection even though L/360 is what is called for since we won't have to put up with complaints from the homeowner about bouncy floors. Increases the construction costs.

As the previous poster suggested bracing is an option. You can also box beam them by having ply fastened to one or both sides in the area that you feel might be stressed.

 

[DaveB]

Yeah, box beaming them seems like an easy way to get some piece of mind for me. I know they're supposed to be very strong but I can't get over the fact that there are stretches in the truss in between the top points of triangles that would totally sag if that was the spot I chose to put the tank above. (Likewise, my uneducated ass assumed that if jacking I would do so only at the bottom points of the triangle or else it'd be worthless.) And if I have access from below, would there be any benefit to sistering? Either with other trusses (but whose triangle points are spaced differently) or with a different kind of joist or wood beam? All under the load bearing I beam as well of course.

I don't particularly care about bounce. Only making sure nothing falls through the floor.

My thinking about jacking was only if the tank happened to sit above that one room. And in that case it'd likely only fit parallel to the joists, which would rule it out no matter what, right? As I mentioned, I have a 75g sitting parallel on an IGF joist spanning 18' in the current house, but 75 is not 240+. I'd be hoping for 240 at a minimum. This is why I initially started looking at only duplex downs that have concrete slab basement living areas. So of course it figures I finally win the battle with the woman (she wanted only penthouses with roof decks) and of course it's a duplex with both living spaces upstairs and the bedrooms downstairs. A third bedroom could be a fish room for sure, but that's not optimally located for actually enjoying it.

Anyway, the only spot where jacking is realistic is in that utility room, and the only spot above that that works is the parallel way, I think, unless I block a window. I could be wrong. So that could all be out the window anyway.

Jake, assuming they're 16" on center, what would you consider a safe load perpendicular to the joists and up against the load bearing wall?