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RE: Garage Cable Guardrails (was 200# Load on Cable Guardrail)

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There are several methods available to calc the forces in the cables under impact, you can back calculate the forces or you can use energy methods.  There was an article in the May 1989 Concrete International titled " Designing Prestressed Barrier Cables" which is very helpful.
 
Nick Blackburn 
-----Original Message-----
From: Jake Watson [mailto:jwatson(--nospam--at)utahisp.com]
Sent: Friday, July 25, 2003 10:45 AM
To: seaint(--nospam--at)seaint.org
Subject: Garage Cable Guardrails (was 200# Load on Cable Guardrail)

This 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 loading.
 
Jake Watson, P.E.
Salt Lake City, UT
-----Original Message-----
From: Schwan, Martin K. [mailto:SchwanMK(--nospam--at)ci.anchorage.ak.us]
Sent: Friday, July 25, 2003 11:04 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: 200# Load on Cable Guardrail?

I 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 inspectors.
-----Original Message-----
From: Nick Blackburn [mailto:nblackburn(--nospam--at)fdgoak.com]
Sent: Thursday, July 24, 2003 3:39 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: 200# Load on Cable Guardrail?

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 become possible.
 
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

-----Original Message-----
From: Doug Parker [mailto:dparker10(--nospam--at)austin.rr.com]
Sent: Thursday, July 24, 2003 11:19 AM
To: seaint(--nospam--at)seaint.org
Subject: 200# Load on Cable Guardrail?

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 appreciated.
 
Thank you,
Doug