Dear Mr. or Mrs. or Ms. Architect and/or Structural Engineer,
I am writing this open letter in the hopes of improving coordination on the design of commercial parking structures with 7-wire Prestressed Steel Strand Barrier Cable (or “barrier cable” for short). The barrier cable system is a very important element for vehicular and pedestrian restraint in cast-in-place parking structures for airports, universities, hospitals, municipal entities and private corporations. However, based on my experience of bidding several hundred parking structures across the Midwest USA and supplying barrier cable on a majority of them, I’ve come to the conclusion that the A/E/C community has a lot of room to improve the design of the barrier cable system.
Generally, the architectural drawings show 40% of the barrier cable design, the structural drawings add on another 40% of the design and the remaining 20% is left to the “supplier’s imagination.” One of my beefs is that the architectural elevations don’t match the architectural floors plans which don’t match the structural floor plans which don’t match the structural sections. All kidding aside, the 20% gap is a potential liability for all parties involved in the design and construction of this system.
In my discussions with several structural engineers and a couple of architects, the key philosophical question comes into play: Is the barrier cable an architectural or a structural element (i.e. who takes the lead)? The architect takes the lead in the aesthetic look and feel of the structure and the design of other “safety restraint” systems such as guardrails and stair handrails. The structural engineer factors in the effect on the columns after stressing the barrier cable and designs the post-tensioning (a cousin of prestressed barrier cable). It is my strong belief that the structural drawings should be the primary reference for the barrier cable design. In order to avoid confusion for bidders, the architectural floor plans should only make reference to the structural drawings.
If I were the designer of barrier cable, then I would address the following items in my structural design:
- Standardize the nomenclature. It is barrier cable (not guardrail or barrier guards or barrier rails or cable barriers or cable rails). I’m always worried that I included or excluded a different system in my bid like a handrail. The nomenclature for anchorages should be standardized as well.
- Designate the type of barrier cable. Galvanized is the industry standard.
- Designate the type of anchorages/inserts. As opposed to post-tensioning, there are a variety of anchorages used to terminate the barrier cable runs. Furthermore, some anchorages require inserts in the concrete columns.
- Designate the location of anchorages/inserts. A simple faded line on the drawing doesn’t provide the supplier enough information. I would also add a note “Contractor to determine stressing access for barrier cable system.” This becomes very important at shear walls, stairwalls and “double-columned” expansion joints.
- Designate the number of rows of barrier cable per run. 11 rows is the industry standard. However a lower number may be needed if the barrier cable is above a parapet wall or there is a concrete wash on the slab.
- Issue a Barrier Cable specification
- IBC and Post-Tensioning Institute lay out several parameters that should be incorporated in the project specifications.
- Designate the maximum amount of deflection (Post-Tensioning Institute recommends 18”. There’s nothing worse than submitting calculations only to find out that the deflection needed to be lower in order to protect something behind the barrier cable. In order to reduce deflection, the use of intermediate anchorages would be needed.)
- Designate the minimum and maximum amount of stressing force allowed. These values have a direct impact on the lateral force on vertical columns.
- Incorporate basic stressing procedures.
- Make a requirement to back-stress the barrier cable. This is a major life-safety issue since improper or the lack of back-stressing could cause the barrier cable to sag in the long run.
- Issue Barrier Cable plans. I would create drawings independent of the post-tensioning floor plans. It would be similar to a life-safety plan.
Whereas most of these items are very basic, the goal is to convey the designer’s expectations to the construction contractor. As a result, the shop drawing submittals should improve and reduce the review time by the designer.
– Neel Khosa (Vice President)
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