] [Thread] [Date] [Author]

[dennis pantazis <dpantazis(--nospam--at)yahoo.com>]

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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