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.
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.
I don't exactly follow the ideas given. My garage door headers are
already in contact with the top plate, so if I simply put another
header on top of this, I don't see how I'm getting rid of the point
load. The local architect at the planning department says I would need
a 3-1/3 cube footer of concrete and rebar to support the 10,000lbs of
point load, and since I have two such trusses, that means four of these
huge concrete blocks, in and through an existing foundation as
described. Calculating the divided loads of a header will mean hiring
an engineer in addition to the truss company. My current thinking, to
avoid so much professional consultation, jackhammer and concrete hassle
is to have the truss company redesign the trusses to distribute load
from just the two opposing walls that are 34' apart, into three point
loads, one of which is one an engineered truss beam running down the
middle of the garage, perpendicular to the trusses, parallel to the
stressed two walls/foundations. This would put some of the point loads
on the other two walls which so far are hardly being tested by the
truss system. But, I can't figure out how many of these engineered
members I because I don't know what the max my current foundation can
take in terms of point load lbs. If you can clarify the ideas you have
in mind, I'd appreciate this....
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...
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.
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
Additional ideas welcomed so that I can help out my techie truss
designer if I can.
Wayne Whitney wrote:
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!
Bob Morrison, PE, SE
R L Morrison Engineering Co
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...
Bob Morrison wrote:
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)
You could excavate for a large pad underneath the existing footing.
You could drive some pin piles and pull them underneath the existing
You could use a helical anchor driven along side the footing and hooked
underneath. --- this last one is my favorite.
Bob Morrison, PE, SE
R L Morrison Engineering Co
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