Truss Calculations Point Load Errors! Help

[Alan]

I provided a truss company the basics of my present garage, so that they could design a new roof for it. I told them I have 18"D by 15"W perimeter foundation, which is suitable for support of two story house in my county. I also told them that I have 16" OC 2x4 stud walls with double top plate, also capable of supporting a second story by code. Some of this information was provided in drawings and other by e-mail. The computer techie designer spent a lot of time with me on a truss system, I put half down and got engineered stamped calculations. When I managed to have a presubmisson review by the senior architect at the planning department, he told me that the point load for two multiply trusses, which support the ladder of joists supporting dormers and a deck, exceed the point load capability of the foundation. Can anybody tell me how to calculate what point load my foundation can take? Also, need help considering possibilities, other than the big hassle of jackhammering, digging, and pouring more concrete and rebar into an otherwise good foundation. I want the dormers and the patio deck. Can I spread the load from these trusses to adjacent trusses for example? Ideas wanted before I deal with the truss company again.

[RicodJour]

There are many ways to do it. You could make the first floor wall a box beam, you could install a header above the top plate and have the trusses hung off of that, you could let in a header into the first floor framing, etc.

R

[Alan]

[Alan]

Maybe I wrote too soon. Are you suggesting that if I rip off the top plate, and down the studs, say 12" or so, and install a header into the void to make-up back to the height of the original top plate, I would effectively be able to convert the point loads into weight distributed along all the studs and the axis of the entire wall foundation, or a least also whatever is the length of the header? Just to clarify, these trusses cantilever over the top plate to create a soffett eave, rather than simply have rafter ends exposed, and they are also designed to raised the height of the eave because they have a heal height of 12" at the bearing wall. Of the 7 trusses, 5 are OK in terms of point load, but each of the two center multi-ply trusses that support the dormers and 8' of floorspace between them produce about 10,000 lbs load on each end. 10,000lbs is apparently too much point load...

[DanG]

Try working with the city man. Either distribute the load as Rico says with a header or find the point load involved and the bearing capacity of the soil in question to figure out the footprint required to carry the load. Leave the existing footing as is and dig the bearing pads under the footing at the point loads.

If this is a garage, I don't see how you can create center bearing unless you have 2 garage doors with bearing in the center assuming the doors are on the gable end. ______________________________ Keep the whole world singing . . . . DanG (remove the sevens) [e-mail]

[Wayne Whitney]

A header will distribute the loads from above (usually a distributed load, but in your case a point load) into two point loads, one at each end of the header. So the advantage in your case is to take a single point load and transform it into two offset point loads half the size.

Cheers, Wayne

[Alan]

Thanks guys for the feedback and ideas. By way of clarification, the two garage doors headers will distribute point load for 5 trusses. 3 of those, the planning architect said were OK. Incidentally, I actually don't have any problem with him because he protects me from the ineptitude of the truss designers who would just as well sell anything to a DIYer. The two heavy trusses are over different garage doors, which is good. But, if I have two heavy point loads over two doors, the header distributes this to three loads, with the heaviest being on the 8" wide post between the doors. Jackhammering the garage floor slab on one side, and tearing up my foundation drainage system on the other seems like a real bummer, not to mention that I'd have to dig down at least 18" before I can undermine the foundation to shore it up. The other side of the house is also a hassle.

Isn't there a way to build an intersecting web truss system to transfer some load to the two sides of the garage foundation hardly loaded at all? Does the Mitek software used by the truss company calculate loads in a complex manner to take into account support by adjacent trusses? I've got a good solid foundation around the perimeter of this garage, so again, it seems to me the problem is how to distribute loads more equally on all four sides. But, this is my first month of experiencing the world of trusses in any meaningful sense. Last month, I was stumped by how to create dormer space without interfering webs, but now that dormer with all its bells and whistles is causing a big load problem.

Additional ideas welcomed so that I can help out my techie truss designer if I can.

Alan

Wayne Whitney wrote:

[Bob Morrison]

You could excavate for a large pad underneath the existing footing.

You could drive some pin piles and pull them underneath the existing footing.

You could use a helical anchor driven along side the footing and hooked underneath. --- this last one is my favorite.

[Bob Morrison]

Wrong!

You poured the foundation BEFORE you had a complete design and then you designed a roof shape that didn't meet the capabilities of the foundation. It is not the the truss manufacturer's fault. It's yours!

[Alan]

I definetly accept responsibility for not seeing the full world of design options available to me for this retrofit garage rebuilt project, but the truss company did have at it's disposal information about the walls and foundation AND designed trusses that would have blown out the foundation anyway. Thank goodness for government bureaucrats because they can rescue the DIY from product selling companies! Also, the software designing the trusses are only 2D, so the maxtrix of trusses as a whole aren't calculated, and so strongbacks and intersecting truss designs are apparently not yet possible for the techie using the Mitek software. This leaves the engineer to bruise his finger tips in tabulation of this information, like the good old slave days before computers---perhaps several hours work.

I do have an engineer working with me on this now, and he's confident a simple solution can work out, and that the truss company will have a chance to deliver a good product. But, Bob, could you clarify those retrofit foundation techniques again? The last one does sound interesting, but I've never heard of these before. If the installed header strategies can't overcome foundation limitations, we may yet need to do some digging...

Alan

Bob Morris> > Thanks guys for the feedback and ideas. By way of clarification, the

[Chas Hurst]

Could you put the door in the gable end?

[Bob Morrison]

Alan:

Information on the helical anchor solution can be found at