handrail wire rope infill - design question 4

[cliff234]

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.

 

[JStephen]

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

 

[SteelPE]

"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.

 

[TehMightyEngineer]

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.

 

[XR250]

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" :>

 

[TehMightyEngineer]

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.

 

[TehMightyEngineer]

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.

 

[XR250]

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.

 

[JStephen]

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.

 

[JLNJ]

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.

 

[TehMightyEngineer]

Yeah, racookpe1978 mentioned no horizontals allowed above.

Maine EIT, Civil/Structural.

 

[boo1]

look back at thread:

http://eng-tips.com/viewthread.cfm?qid=273157

 

[dcarr82775]

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

 

[jayrod12]

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.

 

[jayrod12]

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"

 

[dcarr82775]

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

 

[jayrod12]

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.

 

[racookpe1978]

HEADS UP!

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

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

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 ....

 

[dcarr82775]

Jayrod,

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

 

[jayrod12]

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.