Steel Cable Infill for Stair Guardrail 1

[Banks]

I have a situation where the steel fabricator is using 1/4" diameter steel cables for the infill area of a guardrail for a stair. I have to check the infill for a load of 200 lbs on 1 sq. ft. of area so that people don't fall out the side of the stairs. I'm wondering the best way to check this because it's not like the cable is rigid. I basically don't understand how I'm going to get the side load into a stress in the cable. Off hand it doesn't sound like an easy thing to do.

Thanks.

 

[SlideRuleEra]

For any reasonable tension in the cable, the cable will deflect some when the load is applied. Then this becomes a statics problem. Here is a hypothetical example:

1. Say that an 8 ft. length of cable is restrained from horizontal movement at each end (a continuous cable passing though holes in steel post 8 ft o.c., pehaps).

2. A 200 lb. horizontal load is applied to the center of this section of the cable; say that this case say the cable deflects 2 in. (horizontally) because of this load.

3. Since the only way that the 200 lb horizontal load can be resisted is by tension in the cable, draw the force triangle with 200 lb being the horizontal component. Using the Pythagorean theorm, tension in the cable has to be just over 24.02 x 200 lb (or approximately 4804 lb.)

 

[SlideRuleEra]

Oops! Tension would be half of what I said - the cable tension resists the 200 lb force on both sides of the load. Corrected cable tension approximately 2402 lb.

Of course these calcs are much too precise for the "real world problem". They are offered only to show the geometry for this situation.

 

[dbuzz]

Roark's has a formula for exactly this type of problem: ends pinned to rigid supports, uniformly distributed transverse load on entire span, beam is flexible like a cable or chain and has an unstretched length L.

Variables
E, modulus of elasticity
A, cross-sectional area
L, unstretched length of cable
w, uniform transverse load
y, maximum transverse deflection
P, tensile force in cable

Calculate y:
y = L*[(3*w*L)/(64*E*A)]^(1/3)

Calculate P:
P = (w*L^2)/(8*y)

 

[GregLocock]

Incidentally, in Australia at least, there is a requirement that a child can't stick their head between two of the wires. I think they test this with a 6 inch diameter ball and a load of 50 lbs, from very unreliable memory. Most of our tests are based on US or UK practice, you might want to check that out.

Cheers

Greg Locock

 

[Ron]

Your issue is not one of load resistance but one of code compliance. A 1/4-inch diameter cable, properly anchored will withstand the load you want to apply; however, the issue is one of Life Safety and Code Compliance as GregLocock referred.

I'm not sure where you are located, so I'll give you the IBC 2003 requirements.....

Your setup would not comply with code unless you could assure that the cables are so closely spaced and so taught under load that they would not allow a 4-inch diameter sphere to pass through the space between cables, whether placed horizontally or vertically. This is virtually impossible with cables.

The rules and assessment requirements for handrails were written under the assumption of solid balusters and rails.

The same or similar requirements exist under both the International Building Code and the Life Safety Code (NFPA 101) in the US.

 

[pba]

Also - Don't forget that the cables need to be fixed to something rigid to take the high tension loads. You will probably need raking props at the ends.

 

[swBausch]

I think the code challenge is properly stated.

However, these folks may be able to offer some insight:

http://www.allformwelding.com/allform_026.htm

 

[swBausch]

Judging from client list and catalog offerings, this firm may be the authority on cable railings.

http://www.secosouth.com

If you took a look at the allform site, practically none of their portfolio meets code for infill.

 

[Banks]

Thanks for all of the replies. I am familiar with the code issues of a 4" diameter ball passing between the ballusters and rails. I do have another question that's loosely based on that. One of the guys I work with had a building inspector raise concerns regarding rails because they were horizontal and could be climbed. Now I've see quite a few horizontal rails in my time and I've never heard of this before this incidence and only one time since then. I'm just wondering if anybody else has heard of this and possibly knows where it came from.

Thanks.

 

[swBausch]

Until the code explicitly forbids horizontal infill any challenges should be successfully appealed.

It probably comes from the inspector's childhood.

 

[GregLocock]

Nice ethical response.

Seriously.

"Oh, this guard rail is OK, admittedly there is a 100 ft drop the other side of it, but the code doesn't explicitly forbid horizontal rails in that situation."



Cheers

Greg Locock

 

[dicksewerrat]

Is this for the OSHA requirements during construction or occupancy? If occupancy, what was designed by the EOR? And is the steel fabricator going to post a few million in an escrow account forever?

 

[Ron]

Except for the curb rail, the code does not allow horizontal rails within 17 inches of the floor. This is to prevent small children from climbing the rail; however, the code requires a curb rail not more than 4 inches above the floor and allows a horizontal rail at 17 inches, then the climbing step is only net 13 inches. Most any ingenious little rug rat could figure that one out!

Hmmm...would that mean that parents are actually supposed to supervise the little buggers?