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RE: Wind uplift on a 3-Story wood structure from Dennis Pantazis, PE

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Dennis,
To be honest, I got lost in your explanation of the existing conditions.
With that said, the obvious is that you have an 80-mph wind load (not
excessive) and a 6' overhang (which I would consider excessive). The
tendency is for the eave to whip up and down so you need to consider the
uplift potential on the eave alone. This assumes that the structure is not
an open structure like a hanger or storage lean-to where you would need to
apply the uplift uniformly to the whole underside of the roof.

If I understand the condition correctly, only the eave is exposed to the
potential uplift. The weight of the building should be sufficient to resist
uplift - lateral forces are another issue.  If you consider the exterior
walls to be adequately tied from level to level, then the structure should
be sufficiently heavy to compensate for the uplift on the eave. This means
that you only need to make a positive connection of the roof trusses to the
top plate of the wall to prevent uplift - something that hurricane clips
should do very well.

Lateral forces are another issue. The lateral force distribution to each
level is cumulative except when distributing force into the diaphragms at
each level. The resisting walls, however, must accumulate loads from the
upper level so that by the time you get to the foundation, you have
considered the cumulative lateral load minus the dead-load of the materials
and then some.

If you are worried about the possible failure of the eave above openings,
you can strap the trimmers on each side of the opening which supports the
header to secure the continuity of the framing above and below the opening -
although a king stud is generally provided which is continuous from plate to
plate at the end of the header with the trimmer-studs below the header
nailed into the King stud. The King-stud then acts as a drag.

At each floor you need to consider the uplift on the shear panels and make
sure that the tension is tied from level to level. Simpson makes a number of
Floor-2-Floor ties and straps to help tie the levels.

If this does not answer you question, please write back and be more
specific. If you would like, I would be willing to post a drawing in PDF
format (I'll create the PDF file for you) on the Structuralist.Net
discussion forum so that others who are willing to offer you some help will
be able to view the conditions that you are concerned about. Just email me
at the address below if you want me to do this.

Regards,
Dennis S. Wish, PE
The Structuralist Administrator for:
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-----Original Message-----
From: dennis pantazis [mailto:dpantazis(--nospam--at)yahoo.com]
Sent: Wednesday, January 03, 2001 12:11 PM
To: seaint(--nospam--at)seaint.org
Subject:


i am working on a 3 story wood building, hip roof, h~
45' . boca exposure b, v=80 mph. roof truss spans ~60
feet with a 6 foot overhang (~72' oal), 4:12 pitch.
floor trusses are only ~30' in span with interior
bearing walls. lightweight gypcrete topping on the
subfloor (some DL). its a very light structrure
overall, wood sheating, gyp board, insulation. because
effecive wind trib area on single trusses is less than
700 sq ft, MWFRS loads actually are defined as
components and cladding.  for walls that are longer
than ~15', the area is above the limit and i can use
MWFRS loads for uplift.

either way, because of the geometries, i am finding
that i have potentially SIGNIFICANT wind uplift, its a
combination of: overhangs, almost no superimposed DL,
long truss spans. ~1000 lbs of NET uplift at the
bearing locations (500lbs/foot)!  the exterior walls
are being used as shearwalls, but between all the
window openings and being so lightly loaded that they
have chord uplift and moderately sized hold downs
already. i am going to specify hurricane clips/straps
to transfer the uplift to the walls/headers.
thing is now i have uplift in the walls! the most
obvious thing to do is cut the spans in half, and have
roof trusses that are 2 independant right triangels to
form the overall roof section. end result is the same,
i have uplift in the walls, only now not as much but
in 4 instead of 2.

it is easy to visualise a discrete loadpath ie:
headers need to strapped to the jacks & kings, etc...
to carry the uplifts down to the next level and
strapping across the floor system to the levels below.

how does one design/detail for this "distrubuted"
condition of net uplift along a wall's length? do i
need to just consider the whole shearwall/drag
strut system resolving the forces => leading to
greater overturning => bigger shears per foor and
larger hold downs? mohr's circle to yeild greatest
diaphram shears or simply create tension continuity
between floors only with sheathing? typically we
specify that the wall sheathing splices at each level
over the rimjoist, and lap over the sill plate.  i am
trying to rationalize utilizing the sheathing spanning
over the wall and acting as a tension tie between
floors but i am getting nowhere.

i am interested in how others would approach this
design & detailing issue.

as always, thanks in advance.

dennis pantazis, pe



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