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Re: Design of Top Plates

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*warning -- long post*

> 1. The loads are not concentrated at one level but accumulate from each
> level.

Actually, that is misleading.  Yes, the loads accumulate at each level, but
look at how the loads get from the second floor joists to the first floors
joists.

First, all of the load is in the studs in column action.  The upper joists
bear directly on the top plate, which bears directly on the floor sheathing.
For an exterior wall, this plate is mostly supported by the rim joist with
additional support by the joists.  The composite action with the plate,
flooring, exterior sheathing, joists and top plate of the floor below should
be able to support almost any wall load.

Now look at an interior load-bearing wall, assuming typical construction
with 16" stud spacing and 24" joist spacing (manufactured joists, like
trusses or I-joists).  The load is in the studs, to the bottom plate, to the
floor sheathing.  If the upper stud bears mid-way between joists, can a
single bottom plate carry the load in bending, even using composite action
with the floor sheathing?  Assuming that it can, *all* of the load is then
transferred to the floor joists, which then transfer this load and the load
from that floor as a point load to the top plate of the wall below.  Trusses
are designed with vertical web members at bearing points, and I-joists
required web-bracing, so they can handle the load.  The top plate, however,
cannot.

The only solutions I know of are to either install blocking (which changes
the point loads to a continuous line load), add additional studs when the
joists are offset from the main studs by more than a few inches, or install
the floor joists at 16" o.c. and line them up with the studs.  The last
option is usually cheaper.


> 4. The safety factor included in wood design.

Having done special inspections for several years, I reserve any and all
"safety factors in wood design" for taking care of contractor screw ups.


> I think the fault of the rationale used is simply the failure to identify
> the structural elements as a system and to recognize then you have
> created a built-up section (double plate, blocking and single top
> plate with plywood sheathing (floor) used to tie the system from
> above).

This is true, but has the contractor properly connected all of the pieces
together so that it acts as a composite section?


> Sometimes we lose sight of the fact that conventional wood construction
> has been around for as long as there have been saw mills. There have
> been few failures in conventional construction if the historic methods
> are appropriately followed.

This is probably the most common complaint I hear from contractors, usually
phrased as something like: "I live in a house in the country that's 75 years
old, and an inspector never looked at it.  Why do you need to look at this
one?"   My standard answer is "For every house built by a competent
contractors that stands for 75 years with no problems, there are fifty
houses that are built by incompetent contractors that are torn down or have
cracked sheathing and sticky windows and doors."  And yes, I consider those
to be structural failures.

The main problem is that you can't find a contractor that "appropriately
follows" historic conventional construction.  Any ya-hoo who can hold a
hammer thinks he can build a house.

----
Jason W. Kilgore
Leigh & O'Kane, L.L.C.
jkilgore(--nospam--at)leok.com
(816) 444-3144