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RE: Garage Cable Guardrails (was 200# Load on Cable Guardrail)
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Garage Cable Guardrails (was 200# Load on Cable Guardrail)
- From: "Greg Meyer" <gmeyer(--nospam--at)scahouston.com>
- Date: Fri, 25 Jul 2003 13:19:09 -0500
We often use barrier cables in parking garages. I use an article that was
published in Concrete International "Designing Prestressed Barrier
Cables" May, 1989 by Hoshang Presswalla.
leads to another question. How many people still design parking garages
with cable guardrails? If so, how do you justify the impact
loading? There is a big difference between 200# and and the impact
Salt Lake City, UT
have required guards to be designed for a 200 lb concentrated load applied
in any direction along the top rail and a 50 lb load applied to the
intermediate supports and openings. This is consistent with section
1607.7.1.1 aand 1607.7.1.2 of the 2000 IBC. The 1997 UBC table
16-B category 9 and footnotes 8 and 9 apply to guardrails (no mention of 200
lbs) and require 50, 20, or 25 lb depending on occupant load and
or components. Category 11 and footnote 11 appy to handrail and you'll
see the 200 lb loading requirement. Our inspectors will apply the lean
test to the cable rail systems...They take their 4" sphere and lean into the
openings...if it pushes thru they'll tell you to tighten the turn
buckle...hopefully you won't get one of our big
We haven't, but the interpretation by the City is correctly applied
as far as the 4" sphere test goes since it is a guardrail. We have
seen the requirement before and have usually opted to provide a midspan
(i.e. between the columns) vertical support to reduce the length over
which the cable spans vertically. This support is in no way able to
take car impact loads so the horizontal span remains the column clear
spacing +/-. We usually provide the support as the cables have a
tendency to relax reducing the preload and allowing larger deflections to
That said, if your cables are at 4"oc then they are 3 1/2"
clr. If my math is correct, you need a 270lb load or so for a 1/2"
deflection assuming 20" wide columns at 15'oc and a total cable length of
120', a 4000lb preload and 270ksi uncoated PT strand. So you could
make the argument that they meet the test. Note that the elongation
calc is sensitive to preload and total cable length, i.e. less preload or
more length reduces the vertical load required to deflect the cable
1/2". You could also do an empirical test on an existing
structure if the City will buy into that method.
Nick Blackburn, PE
We have been asked by the city to provide a
letter stating that the cable guardrail system in a parking garage can
support a 200# load and still not allow a 4" sphere to pass
through. According to the UBC, the 200# load requirement is
applicable to "handrails" not "guardrails". Cable guardrail
systems are fairly common here in Texas. Typically, these cables
are spaced at 4" on center with a 4000# prestress force in the
cable. The supports are at 15'-0" on center. You cannot
reasonably maintain a maximum clear distance of 4" when a 200#
load (placed anywhere) is applied to the cable.
Has anyone else been asked to provide
such a letter?
All comments and suggestions are