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RE: UBC: How Do You Interpret 1997 UBC P

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The UBC requires one to design the slabs for a live load of 50psf with two
distribution patterns: uniform everywhere and uniform skipped so as to
produce highest moments and shears.  The supporting structure, beams,
columns and foundations may be designed taking into consideration any live
load reductions per the UBC.  Continuous beams should also be designed with
the two load distributions.

Bear in mind that the live load is factored up 70% and the dead load by 40%,
part of that reflects the uncertainty in the actual loads.  As a garage
designer, one knows with a fair certainty what the dead loads are, there
will not be additional partitions, permanent heavy equipment, book stacks
and the like added down the road so the 1.4 factor for dead load is very
conservative. If the slab has been designed for 50psf then it should perform
well as there will be reserve capacity provided by the conservatively
factored dead loads and the large factor for live load.

There are many issues beyond actual design that affect the performance of a
slab.  Some of the issues are related to design practices such as location
of shearwalls, detailing for shortening and so on.  Other items are
construction related such as curing practices, possible problems with the
concrete mix design (poor aggregates, high shrinkage mix etc), timeliness of
reshoring, distribution of construction loads etc.

I would be very surprised if the slab was showing distress due to live load
related issues, unless the vehicles using it were substantially different
from the normal everyday family vehicles, even huge SUV's, that a garage is
usually designed to accommodate.  Most garages are posted for a 6000lb
weight limit which they should handle without problems.

Note that the worst likely loading in a garage, that is clearly not
accessible to motor homes/firetrucks etc, would occur when a tow truck was
called in.  Most tow trucks exceed the weight distribution of a normal
family car once they have a car hooked up.  With the factored loads that are
used for design, most slabs will perform fine, although there is the
possiblity of local overload which might induce some localized cracking.
This should not create a problem such that the owner felt the engineer had
under-designed the building to the point that a lawsuit was brought.

Questions to ask would be:

What use was the garage intended to provide and how was it really used.  If
the owner leased it to a towing company(even partially), the garage could be
underdesigned if it was designed for pleasure vehicles only.

Another possible use that may not have been considered would be valet
parking.  In this case the density of parked cars is significantly higher
than for owner parked vehicles and might require a more conservative
approach.

What were the contractor's construction practices, did they cure the
concrete properly, how were construction loads distributed, did they use
equipment that overloaded the deck, anything over about 5000lbs including
load can cause harm and this rules out the use of most items other than
small forklifts.

Was the concrete made with quality materials that have been shown to provide
good performance in past use.  Note that shortening in garages is a bane
that must be detailed for.

Is the garage a multi-level affair or is it a single deck.  Single decks
suffer much more from daily thermal variations than multi-level garages and
accommodation should be made in the design.

Are there detailing issues that have not been looked at, trapped partition
walls, poorly located shearwalls, large reentrant corners, were pour strips
used to mitigate some of the shortening problems?

Did the engineer make any site visits during construction to see how the
contractors were doing their jobs and note any items of concern.

What time of year was the project built.  If the temperatures were low,
special precautions should have been taken otherwise you could have
durability problems etc.

For your reading audience: what appears to be the problem that live load or
loading in general is being possibly fingered, if you are allowed to tell
us?

Nick Blackburn, PE



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