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Anyone use mono trusses?

DickRussell | Posted in Construction Techniques on August 13, 2009 06:25am

I have a situation that suggests use of mono trusses as a solution. I had planned on full front-back roof trusses to handle the spans between possible support walls (up to 24 ft in one area). However, access to the site is limited, and the roads likely won’t take a truck carrying in trusses 40 ft long or so.

If I use I-joists for rafters, I would need them for the ceiling joists in that area also, and I’d need a ridge beam with supports run down to footings. The span of that ridge beam would be getting up there.

One thought is to use a piece of steel in the attic floor, with loads run down through available walls to ground footings, and pairs of mono trusses supported under the ridge by this beam. The end loads of that beam would be carried down to another piece of steel in the lower level, but that beam can have a post under it near the halfway point, so that one is easy enough to do, even though it will carry the floor and roof loads in the area above it.

So I thought I’d go on a fishing trip for comments on mono trusses. They would solve a couple of other problems as well, so unless there is a big negative to it I might like to call for their use when I have the engineer do the details and spec the beams and supports.

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  1. silvertip | Aug 14, 2009 02:58am | #1

    More specs on what spans you are dealing with and a pic would be helpful.  A truss maufacture would help you come up with something.

  2. jimAKAblue | Aug 14, 2009 04:28pm | #2

    I have set thousands of mono trusses. There are no negatives that I can think of.

    That statement assumes that they are professionally designed and installed properly.

    1. DickRussell | Aug 15, 2009 04:45pm | #3

      Thanks for that testimony. It confirms what I suspected, that they could be a useful solution for the situation, without inherent issues. The details will be engineered, and alternative solutions may come up. But I'd prefer to have something that appears to be workable to propose up front before turning loose some engineer at $x/hr.I have given thought to the beam on the attic floor. It would carry both the snow load (town code says design for 70 lb/sqft) and the weight of the ceiling sheetrock and the insulation above it. The span between possible supports down through walls is about 19 feet. As far as I can tell from the tables, my best bet would be a piece of steel a foot tall with the right cross section. I am guessing that a pair of 2x would be bolted through on each side of the web, with hangars for the corners of the mono trusses, and that the top chords of the trusses would be overhung by around 6" to provide attachment to each other with a nailing plate during erection. But you've done these - what do you do?I suppose a logical question to ask is how realistic would it be to imagine connecting a pair of mono trusses during erection so as to perform as a single truss spanning the whole distance without that beam? I suspect the answer might be "maybe" if they were designed to be erected and attached that way, and just shipped as a pair of mono trusses. What do you know? Ever do it?

      1. jimAKAblue | Aug 15, 2009 06:29pm | #4

        Everythings possible.

        Typically, one advantage of using mono trusses in a situation as you are describing is that they are smaller and easier to handle. But, if you have the equipment or manpower, you certainly can connect the parts and set them as one unit. We occasionally did that with certain trusses. We almost always did that with hipsets. 

        For instance, heres a picture of an assembled hipset with the jacks attached that will fill the roof over the master bedroom area.

      2. jimAKAblue | Aug 15, 2009 06:36pm | #5

        I cannot comment intelligently about how your beam should be set and engineered without more info. I can offer this though:

        We set many, many monos with the beam set up into the ceiling. This provided a flush ceiling situation. To accomplish this, the truss designers simply added an additional vertical strut to provide a bearing point on the truss.

        It was also quite common for the steel fabricators to weld a 1/4 x 9" plate stock to the bottom of the beam. They would hang the plate out to one side (or both if they used 11" stock) to provide a ledge to carry joists or trusses. I set many beams that spanned the garages and carried second story loads and roof/floor loads. Sometimes these beams would be much taller than the 12" you mentioned. It all depended on their goals. It all works if the engineers spec things correctly. I've done "seat of the pants" type beamwork like this on my own stuff without problems.

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