Building and Construction - Truss Calculations again...different issue

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Truss Calculations again...different issue

Alan

2006-08-12, 3:25 am

Ok, my 10,000lb reactions per point load on the garage dormer trusses
are known to require a 3-1/2 cubed footing of concrete to support the
load. So, I have the tough job of cutting into the garage floor,
digging a big hole around and under the existing perimeter foundation
footer of the garage, and pouring a lot of concrete and rebar. It's
been suggested that I go ahead and consider cutting through the
existing footer to make the job easier.

But, now I'm interested whether or not the two 8' long 4x12 garage door
headers (plus double top plate across both) can support the point load
of the trusses. I can't figure out how to convert reactions numbers
into load or forces down on the header. Span tables aren't of any
apparent help. Obviously, I want minimum deflection and no structural
breakage for these headers.

I have been told by my truss designer that my 2x4 stud wall will hold
the point loads, so it seems possible, but I'm still concerned about
whether the headers can. To clarify, I have two 8' wide garage doors
with 4x12 headers, and a central 8" wide post of several 2x4s (I may
swap this out for a 3" steel post, swapping headers is more difficult).

In any case, each door header takes the load of one of the heavy truss
point loads, and the point loads of lighter trusses, all spaced 24"OC.
I'm trying to solve this while my engineer finds time to fit me into
his schedule. Previous help was interesting, so I'd like to understand
more about how the trusses will affect my existing headers.

Italian

2006-08-12, 1:25 pm

> Ok, my 10,000lb reactions per point load on the garage dormer trusses
> are known to require a 3-1/2 cubed footing of concrete to support the
> load. So, I have the tough job of cutting into the garage floor,
> digging a big hole around and under the existing perimeter foundation
> footer of the garage, and pouring a lot of concrete and rebar. It's
> been suggested that I go ahead and consider cutting through the
> existing footer to make the job easier.
>
I cant speak to the other issues other than it sounds like regardless
of what you use the footing needs to be increased. If you have access
to the entire perimiter of the garage from the outside you do not need
to do this from the inside. You can sister on the additional footing
next to the existing footing from the outside. In general this involves
drilling in to the existing footing at specified intervals epoxying
rebar to tie the 2 together plus a bunch more crap. Since you have an
engineer he can make up a detail based on the existing footing and what
modification you need to make to it work.....that is if you can get him
to take your calls.....been there...

> I'm trying to solve this while my engineer finds time to fit me into
> his schedule.
Yea and your paying him..dont you love it...

Alan wrote:
> Ok, my 10,000lb reactions per point load on the garage dormer trusses
> are known to require a 3-1/2 cubed footing of concrete to support the
> load. So, I have the tough job of cutting into the garage floor,
> digging a big hole around and under the existing perimeter foundation
> footer of the garage, and pouring a lot of concrete and rebar. It's
> been suggested that I go ahead and consider cutting through the
> existing footer to make the job easier.
>
> But, now I'm interested whether or not the two 8' long 4x12 garage door
> headers (plus double top plate across both) can support the point load
> of the trusses. I can't figure out how to convert reactions numbers
> into load or forces down on the header. Span tables aren't of any
> apparent help. Obviously, I want minimum deflection and no structural
> breakage for these headers.
>
> I have been told by my truss designer that my 2x4 stud wall will hold
> the point loads, so it seems possible, but I'm still concerned about
> whether the headers can. To clarify, I have two 8' wide garage doors
> with 4x12 headers, and a central 8" wide post of several 2x4s (I may
> swap this out for a 3" steel post, swapping headers is more difficult).
>
> In any case, each door header takes the load of one of the heavy truss
> point loads, and the point loads of lighter trusses, all spaced 24"OC.
> I'm trying to solve this while my engineer finds time to fit me into
> his schedule. Previous help was interesting, so I'd like to understand
> more about how the trusses will affect my existing headers.

Bobk207

2006-08-13, 3:25 am

Alan wrote:
> Ok, my 10,000lb reactions per point load on the garage dormer trusses
> are known to require a 3-1/2 cubed footing of concrete to support the
> load. So, I have the tough job of cutting into the garage floor,
> digging a big hole around and under the existing perimeter foundation
> footer of the garage, and pouring a lot of concrete and rebar. It's
> been suggested that I go ahead and consider cutting through the
> existing footer to make the job easier.
>
> But, now I'm interested whether or not the two 8' long 4x12 garage door
> headers (plus double top plate across both) can support the point load
> of the trusses. I can't figure out how to convert reactions numbers
> into load or forces down on the header. Span tables aren't of any
> apparent help. Obviously, I want minimum deflection and no structural
> breakage for these headers.
>
> I have been told by my truss designer that my 2x4 stud wall will hold
> the point loads, so it seems possible, but I'm still concerned about
> whether the headers can. To clarify, I have two 8' wide garage doors
> with 4x12 headers, and a central 8" wide post of several 2x4s (I may
> swap this out for a 3" steel post, swapping headers is more difficult).
>
> In any case, each door header takes the load of one of the heavy truss
> point loads, and the point loads of lighter trusses, all spaced 24"OC.
> I'm trying to solve this while my engineer finds time to fit me into
> his schedule. Previous help was interesting, so I'd like to understand
> more about how the trusses will affect my existing headers.

Alan-

If you're looking for an answer you'll need to provide the size &
location of the truss loads into the 4x12

example.

I have got a 4x12 with an 8 foot clear span; 1 kip at 2 ft, 10 kips
at 4 ft and 3 kips at 6 ft.....can the 4x12 take this load?

this might be of some help

http://www.umass.edu/bmatwt/publica..._and_beams.html

cheers
Bob

Alan

2006-08-13, 8:25 pm

Thanks for the reply. Of four engineers that I talked to, I think I
have one that has actually begun to work. He came to the house,
examined the situation, took measurements, and left after explaining a
clear plan of further investigation and an analysis. Nevertheless, he
might like some help, because he was himself concerned about the beam
capability. This header is actually two rough 2x12 doug fir members,
providing a true 4x12 size. It's not completely knot free wood, but
it's not full of knots and cracked either. In general the old wood
looks good, but he wasn't sure how to grade it.

>
> If you're looking for an answer you'll need to provide the size &
> location of the truss loads into the 4x12
>
> example.
>
> I have got a 4x12 with an 8 foot clear span; 1 kip at 2 ft, 10 kips
> at 4 ft and 3 kips at 6 ft.....can the 4x12 take this load?
>

OK, here's the information needed to calculate just one of the 8" clear
spans. If we can understand one, then the rest can be determined
pretty quickly I think. Starting at the left side edge of the open
span, 18", T1=2,170lbs, 24", T2=10,952, then I have an open span to the
center post. The lbs are the "reactions" values provided for the
calculating the footing--are these the same as the combined dead/live
load numbers?

Then, if the lumber isn't strong enough, do I replace with an LVL or
other cost effective engineered lumber, steel or what? If I stay
within the 4x12 (plus double 2x4 top plate) dimension, then swapping
headers would be real easy.

> this might be of some help
>
> http://www.umass.edu/bmatwt/publica..._and_beams.html
>
Yes, I'm familiar with the U of Mass articles, and I've read them
through with great interest. But they didn't provide information that
I needed for this calculation, if I understood this article correctly.
It doesn't talk about wood beam carrying capacity, grades of lumber,
etc., but for instructional purposes this is a great website. I'd like
to see other universities in the nation producing this sort of
instruction on-line for DIYer's.

Alan

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