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RE: Double Tee Parking Structure

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-----Original Message-----

I've got a single story double tee parking structure to be constructed in
Orem, Utah.  460 ft. long x 60 ft. wide.  It will be subject to pretty wide
fluctuation in temperatures year round.  I'm looking for any guidelines
regarding expansion, creep, shrinkage, etc.  Are you guys putting
expansion joints in this size structure.  I plan on anchoring one end of the
tees, and letting the other end float.  It's the long direction that I could
use some input on.  There will be a 4 in. concrete topping.

TIA.
Dan Goodrich, P.E.

Dan:

I've been involved in extensive repair work on a similar structure in
Ottawa, Ontario, with deficiencies due to thermal effects and also some
other design/construction issues.

I'm assuming the tees are spanning the 60' dimension.

Will the tees be supported on a column-beam structure, or on walls from
grade?

If you're using beams, and they are in open air, in the long direction they
will move thermally almost as much as the deck, except for differential
temperatures due to the sun on the deck vs the beams in the shade.

But if you're using walls from grade, their thermal movement in the long
direction will be restricted, and the tees will want to move laterally
relative to the walls. If you allow this movement by providing sliding
bearings you'll need a lot of bearings... costly, and an installation &
maintenance nightmare. You will also need a positive anchorage, preferably
at mid-point of each bay, to transfer lateral loads to the support
structure. But if you don't provide sliding bearings the tee legs will be
twisted as the deck moves but the wall does not - and the tee legs _will_
fail.

In the 60' direction the thermal movement can probably be accommodated by
rotation of the support structure, if the tees are fixed - but this may not
be applicable if one or both supports are, for example, the exterior wall of
a building. You may prefer to provide sliding bearings at one end. You also
somewhere need some bracing...

In the 460' length I would provide two joints, with each bay anchored at
mid-length. One possible method of anchorage would be a length of
cast-in-place wall, dowelled to the top of the beam (or wall), and to the
underside of the tee slab, that would lock in place as many tees as might be
necessary to get sufficient force transfer.

Expansion joints between parallel tees are difficult because there is
significant differential vertical deflection as a wheel load passes from one
tee to the next. This is particularly a problem when snow-plowing  - the
loaded tee deflects, the blade of the plow catches the adjacent higher tee,
and the joint can easily be damaged. We've addressed this by providing shear
transfer devices to prevent differential vertical deflection, whilst still
allowing horizontal movement.


Peter James, P.Eng


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