Cable Rails, ASCE 7, 4.5.1?

[Jim Z]

OK - first timer here. In interest of full disclosure, I'm an architect, but there was no category for such on joining info, but hope I'm still allowed! A local building inspector is concerned, I think rightly so, about the cables in a cable rail system a developer is using on a building we've designed. The issue is that the bldg code requires limitation of passage of a 4" sphere. The cables will deflect enough to allow passage of that sphere, if pushed hard enough. The code for intermediate rails says they shall be designed to resist a concentrated load of 50# in accordance with Section 4.5.1 of ASCE 7. Havent been able to get a look at that yet. However, I'm thinking that the cables will certainly withstand 50#, but will stretch in doing so. Any help with how ASCE 7 4.5.1 is applied here? Thanks!

 

[dauwerda]

See below for the section from ASCE 7-10

4.5.1 Loads on Handrail and Guardrail Systems
All handrail and guardrail systems shall be
designed to resist a single concentrated load of 200 lb
(0.89 kN) applied in any direction at any point on the
handrail or top rail and to transfer this load through
the supports to the structure to produce the maximum
load effect on the element being considered.
Further, all handrail and guardrail systems shall
be designed to resist a load of 50 lb/ft (pound-force
per linear foot) (0.73 kN/m) applied in any direction
along the handrail or top rail. This load need not be
assumed to act concurrently with the load specifi ed in
the preceding paragraph, and this load need not be
considered for the following occupancies:
1. One- and two-family dwellings.
2. Factory, industrial, and storage occupancies, in
areas that are not accessible to the public and that
serve an occupant load not greater than 50.

Intermediate rails (all those except the handrail),
and panel fillers shall be designed to withstand a
horizontally applied normal load of 50 lb (0.22 kN)
on an area not to exceed 12 in. by 12 in. (305 mm by
305 mm) including openings and space between rails
and located so as to produce the maximum load
effects. Reactions due to this loading are not required
to be superimposed with the loads specifi ed in either
preceding paragraph.

I would say that ASCE 7 is really only concerned about the strength (not so much the deflection).
That said, the building code requirements to prevent a 4" sphere from passing through is so that a child's head cannot fit between the rails (and possibly get stuck). If the cables can be easily be deflected to allow this I would say it is not meeting the intent of the code. Imagine an adult standing next to the rail on the phone with one foot on one of the cables causing it to deflect, his toddler is board and decides to stick his head through the gap, the rest of his body follows or foot is removed, head is stuck, panic ensues...

Put the cables close enough together and tighten them up enough that they will always maintain no more than 4" separation under reasonable loads (perhaps the weight of a person standing on one?)

 

[Deker]

With the right combination of cable spacing and pretension, the intermediate railing can be designed to prevent sphere pass-through. Special consideration needs to be given to the end posts where intermediate rails are anchored and the pretension in each cable is additive. These documents should help: Link #1, Link #2.