×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

handrail wire rope infill - design question
4

handrail wire rope infill - design question

handrail wire rope infill - design question

(OP)
I am designing a handrail that has a 5mm wire rope infill. The wire ropes will be pre-tensioned. The variables are the rope spacing and the span of the rope between the vertical posts.

I have been told that I have to design the wire rope to be sufficiently taut so that a 4" diameter sphere can't be pushed between two ropes. What is the force that I am applying to push this theoretical sphere through the rope? The harder I have to push it, the larger the required rope pretension force will be to prevent the sphere from passing through.

RE: handrail wire rope infill - design question

Wouldn't it make sense for it to be the 200lb concentrated load they make you design the handrail for?

RE: handrail wire rope infill - design question

2
What direction are your cables running?

Typically, the IBC requires a 50# load over a 1 square foot area.... so a 4" dia ball would have approx 4.4lbs of force behind it. Not much, but it's not an easy problem to solve. I found the information here to be helpful:

http://thecableconnection.com/data-on-cable-flex.h...

Good luck

RE: handrail wire rope infill - design question

FYI, the 50# is the load IBC requires for infill, which is what you have here.

RE: handrail wire rope infill - design question

Yes, as stated, it is a theoretically complex problem. I would tend to approach it from a deflection standpoint.
Say, I space the wires 3" apart and design for a max opening of 3.5".......this would be a 1/4" deflection in each wire with the other 1/2" as a safety factor.Depending on the span of the wire this could require a significant pretension in the wire and a resulting moment in the last post in the system.This would be an iterative process to get to a pretension one can live with.
The next step is to take that design deflection and translate it into a point load on the wire and then check that point load with the 50# requirement from the codes....using the 3.0" sp as an example,........P=50x3/12=12.5#...so maybe the required pretension will not be as bad as first assumed.

RE: handrail wire rope infill - design question

(OP)
The cables are 3" apart and they are running horizontally. The vertical posts through which the cables are threaded are about 5' apart.

RE: handrail wire rope infill - design question

I interpret the 4" sphere idea as a purely geometrical constraint, not related to strength or stiffness of the handrail.

RE: handrail wire rope infill - design question

NO!

The 4 inch "rule" is a bureaucratic interpretation of the very real need to prevent a baby's head from being trapped between the bars of stairway railing ... Theory is: If a baby can't get her through the bars, the baby's head can't get stuck, the baby won't choke - nor can it get its body through the bars and fall downstairs. A second part of the requirement "against" horizontal bars (wire in your case) is the lesser-know rule that the horizontals can't be used as a ladder by a baby/infant/toddler/small child. Why 4 inch sphere? Its also an approximation: no real head is shaped like that at any age!

So, you need to make a reasonable estimate of how hard a baby can push. Add a conservative factor to that force, then use it as your force. Keep your notes. The code (by itself) is not smart enough to have figured this out already.

RE: handrail wire rope infill - design question

(OP)
The link that SteelPE provided is excellent. (Thanks Steel PE - and thank you everyone else...)

The force applied to the 4" sphere is about 9#.

The procedure is straightforward but intense.

RE: handrail wire rope infill - design question

It is a nonsense problem. There is no starting point. Without a starting force, you cannot select wire spacing, pretension or anchor spacing.

There must be a starting point.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

RE: handrail wire rope infill - design question

I always show "cable rail by others" on my drawing so the question does not get asked to me.
Don't forget to check deflection in the end posts as they may give enough to be significant.

RE: handrail wire rope infill - design question

Looking at SteelPE's post, they are assuming 3-400lbs of tension on each cable or about 2400 lbs of constant tension on the end posts if all cables are tightened to that value. A 6x6 will creep over time under that load which will slacken up the cables. Hopefully, tube steel will be used. I have never seen anyone crack the cables down that tight, however.

RE: handrail wire rope infill - design question

Excel, the formulas can be changed to allow for less of a pretension force by reducing the spacing of the cables, or reducing the spacing of the posts, or increasing the size of the cable.

While the method may be intense, it is really simple once you write a MathCAD spreadsheet.

RE: handrail wire rope infill - design question

(OP)
I missed the complexity of this when we gave our fee for designing these cables. Yes. In the future we will say "cable rail by others". Yes, the end posts are steel, and yes, we will make sure that they can resist the significant tension force from the cables.

I wish I could find some sort of chart that would make this easier.

Thanks again everyone.

RE: handrail wire rope infill - design question

A 5-foot spacing on the posts presents a large moment for the posts (50 plf x 5 ft x 42 inches height....10,500 lb-in). Be careful with that.

As for the cables, I use the approach that Sail3 described.

RE: handrail wire rope infill - design question

Interesting post. In the article cited, they mention that the codes do not specify any load associated with the passing of the sphere, and that's why I've interpreted it as a geometrical constraint only (note that I don't deal with cable railings). But I note that in the discussion above, we have suggestions to use 200 lbs, 4.4 lbs, 50 lbs, and 9 lbs as the appropriate load to be applied. My guess is that this was originally written assuming solid railings and thus the issue wasn't addressed at all. And come to think of it, do any of you ever check solid railings for deflection from spheres being forced through at specified loads?

If I remember right, that sphere size changes to 12" for areas not accessible to the public and to 21" for industrial areas. I would assume in those cases that any specified railing loads were applied directly to the rail and not to a frictionless sphere being forced between the rails.

In certain instances, OSHA allows wire rope toprails, and googling around on the OSHA site shows some requirements of a maximum 3" deflection with the design load in those cases.

RE: handrail wire rope infill - design question

SPE... excellent link. Information, galore... a well deserved star...

Dik

RE: handrail wire rope infill - design question

I am on the side that the Code requires 50 psf loading AND a 4" sphere shall not pass. They are two individual requirements not linked.

RE: handrail wire rope infill - design question

Around here we are allowed 5 1/2" spacing for horizontal members and 4" for vertical. Has something changed.

RE: handrail wire rope infill - design question

oh, maybe that is for residential.

RE: handrail wire rope infill - design question

That is IBC-2012, Section 1013.4, and I think it was similar in the last several editions.

RE: handrail wire rope infill - design question

"I missed the complexity of this when we gave our fee for designing these cables."

So did I when I was asked to look at a similar project where "cable rails by others" would not fly. In this instance, I passed on the project because I could not do the analysis but became intrigued by the problem. I came across the link above and programed their equations into MathCAD. In all took about 1-1/2 days to do everything (research to programming) of which I was paid for none (I did have to tweak their analysis to get it to work).

Recently I was asked to help another engineer solve this problem. I gave him the link and my MathCAD spreadsheet (we help each other out from time to time) and told him to make sure he charges the appropriate rate to solve the complex problem because someone should be getting paid for the effort that went in.

RE: handrail wire rope infill - design question

Not to derail the thread too much but what SteelPE said brings up something that drives me nuts.

"I missed the complexity of this when we gave our fee for designing these cables."
"So did I when I was asked to look at a similar project where "cable rails by others" would not fly."

That is exactly why I HATE "design by others" so much.

No offense to anyone here (okay, maybe a little if you're guilty of doing this) but if some is hired to design something, DESIGN IT! Passing it onto "others" (usually the contractor) is great for your fee or deadlines but only hurts everyone else. "Others" usually means the contractor who then has to hire another engineer to design something.

If the contractor is smart he'll catch this "by others" bit and drop a big fat question mark into his bid which raises the cost of the project. If not, he then has to scrounge to fit an engineer's fee and design into his costs and time schedule (or he farms it out for cheap and gives a substandard product). THEN if it turns out that the design is much more complicated than anyone anticipates the contractor gets pissed at the "by others" engineer who gets pissed at the original EOR engineer and the client is pissed at everyone for either cost overruns or delays.

A while back we got hired to do a design for some steel pipe handrails for a post office. REALLY basic stuff but it was in the drawings as "handrail design by others" and the EOR required a stamped calculation submittable for them. It was only a few hundred bucks of design fees IIRC but the contractor hadn't budgeted for it and only caught the "by others" note last minute so they had a day of delays. DON'T DO THIS!

Then who becomes responsible for the design? For example if the EOR specified horizontal wires and told you to do the structural design only you would have to fill your design full of caveats that you're not responsible for anything but the structural scope. Legal battles would be fun for those I'm sure; lots of finger pointing. We had some concrete stairs to design the other day for a commercial building. 7.5" rise (vs 7" maximum allowed by IBC). We gave them lots of notes regarding how that's wrong but we're not responsible. Fun. I'm pretty sure it went in at 7.5" rise. DON'T SPECIFY "BY OTHERS"!

...

Sorry, huge pet peeve of mine. Interesting topic here.

Maine EIT, Civil/Structural.

RE: handrail wire rope infill - design question

Huh, I design metal studs for commercial building occasionally. It is pretty typical for the EOR to call out "design by others" for metal studs.
Go ahead and design everything on a building yourself while your competition charges the same and "calls out design by others" :>

RE: handrail wire rope infill - design question

Well, allow me to caveat my rant with that there are plenty of good times to call out "by others". Metal studs, connections, etc. Things with very defined scopes that don't add giant question marks to a contractors bid and aren't critical to the original design. Of course I've called out "by others" tons of times. If you weren't hired or expected to do that part of the project then obviously you're not going to. I'm just saying, I see an alarming amount of work come through our office that should have been designed by the original engineer but they pass it off because it's time consuming and expensive for them to do.

For example, say you're hired to design a multimillion dollar underground concrete sewage structure where probably about 1/4 of the design work is structural and your fee is based on being hired to do that work. You should not be dumping that onto the contractor who may or may not have read all 3,000 pages of your specification with a magnifying glass during the bidding phase. A specification which has multiple "hidden" sections that relate to the structural design spread throughout the spec. A specification that has a clause stating that after 3 reviews by the EOR they can start charging for reviews. A clause which can get very abused and cost the contractor almost $100,000 and who still hasn't been paid for this work.

My boss and everyone in our office provide fair, honest, no BS work for the jobs we're bidding on and are hired to do. Sure we lose out, I know of a couple of jobs I'm sure it cost us the bid. Doesn't matter, it's not right and sooner or later clients will (hopefully) wise up to the fact that this can and does happen because, in the end, they're the ones who really lose.

Maine EIT, Civil/Structural.

RE: handrail wire rope infill - design question

Anyway, I didn't mean to hijack the thread which it looks like I might be doing. If people want to discuss my rant I'll create a separate topic.

Maine EIT, Civil/Structural.

RE: handrail wire rope infill - design question

Actually, IMHO, I think metal studs SHOULD be designed by the EOR or at least subbed out and designed prior to sending out the construction set. Too many times I have worked on jobs where the EOR made little accommodation for proper stud connections or allowed the arch. to design the building around a 6" stud when 10" studs were req'd., or showing metal stud headers to span 30 ft. etc. I am then left in the position of trying to get their crap to work while staying in the metal stud contractor's budget.

RE: handrail wire rope infill - design question

In may case, I'm the "other" that gets to design some of this stuff, so that concept is good for job security. My pet peeve is when it is ambiguous who is to design. The typical situation is to show every last detail on the drawings, stamp them with a PE seal, then put a clause in the specifications that design is by the contractor, then elsewhere require it to be built per the plans. In those cases, it works out about 50/50 whether the intent is to build it as shown or whether the design shown is just "typical" and to be ignored. Which is kind of scary.

But back to the original post: Consider this loading: Little Kid A weighs 40 lbs and stands on that bottom cable. Littler Kid B then sticks his 4" head between the cables. That is probably much more likely to happen than an actual infant forcing his frictionless head through the cables with 9 lbs force.

RE: handrail wire rope infill - design question

I also thought that the horizontals were verboten (sp?) due to the fact that they can be climbed like a ladder. I don't have a code handy but any architect should know.

If they were vertical I'd look at placing them 3 1/2" on center with a reasonable pretension and see what the force would be cause 1/2" deflection. I bet you will get more than the 200# and you'd be in the free and clear.

Horizontal rails seem like a bad idea to me. Period.


RE: handrail wire rope infill - design question

Yeah, racookpe1978 mentioned no horizontals allowed above.

Maine EIT, Civil/Structural.

RE: handrail wire rope infill - design question

Which Code prohibits the horizontal only type of guardrail infill? I don't think IBC or IRC have it.

RE: handrail wire rope infill - design question

Most codes prohibit it not directly but they say that your guards must not be climbable. Horizontal infill basically makes the guard into a ladder. I don't have a code reference from either of those two but I could give it to you from the NBCC if you would like.

RE: handrail wire rope infill - design question

for reference in the NBCC it is clause 3.3.1.18.(3) that prohibits any member that could be used for climbing between 7.5" and 36"

RE: handrail wire rope infill - design question

Jayrod12,
It seems like a very logical requirement, but I don't recall seeing a similar requirement in the US Codes.

RE: handrail wire rope infill - design question

Good point. I just quickly went through the guards sections of both of those codes and couldn't find anything.

And off topic but honestly how do you guys even use those codes, the NBCC is full of the legal mumbo jumbo stuff but at least I can find useful information relatively easily. Both of the IBC and IRC are a hell of a lot tougher to manage than the NBCC. Granted I'm biased because I solely use the NBCC so I have lots of experience reading through the junk to get to the meat.

RE: handrail wire rope infill - design question

HEADS UP!

The Michigan LARA (Building Codes) regulators have changed their horizontal cable rails guidelines1

http://www.michigancodes.com/module11.html

Quote (LARA MI)

This is a link to their on-line training video, and the text that precedes it.
Introduction: Guardrails
Guardrails provide critical safeguards, particularly for children, so proper design and installation are important. Guardrails are required anytime a porch, balcony or deck surface is positioned more than 30 inches above the adjacent floor or grade. Guardrails are also required along stairs with a total rise of more than 30 inches.

Previous versions of the Michigan Residential and Building codes restricted the use of guardrail infill constructed with horizontal rails or other ornamental patterns that resulted in a possible ladder effect. However, the basis for this restriction was thoroughly reevaluated and consequently, the wording was revised. As a result, horizontal intermediate rails are now allowed in the 2003 Michigan Residential Code.

One increasingly common type of guardrail infill system uses low-profile, stainless steel cables instead of the more traditional wood or steel balusters. Tempered glass panels can also be used as infill. Both designs improve aesthetics where views are important (such as along balconies or decks) while maintaining safety.

It does appear that the "words" they use do seem to be a sales guide to the railing cables, but ....


RE: handrail wire rope infill - design question

Jayrod,

Like anything else you get familiar with it. I find the IBC ok, but the IRC is a disaster.

RE: handrail wire rope infill - design question

dcarr upon a second cursory glance I would tend to agree with you on that. The IRC was much more difficult to comprehend than the IBC. But for the time being I will stick to my code.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources