WWF support 3

[SteelPE]

I have a project that is currently under construction. The contractor is telling me that I need to tell them how to support the WWF reinforcing that we have specified for the SOG and suspended slabs. As in, what types of supports to use and how often to place said supports. In all my years, I have never been asked this question and never really gave it much though. I have told the contractor that this falls under means and methods of construction and that they need to figure out how to place the WWF as shown on the drawings.

Does anyone provide this information on their drawings?

 

[KootK]

Never. Refer them to something like the CRSI Manual of Standard Practice.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[XR250]

I had a contractor tell me they "pulled the WWF up as they went" on a job. He ended up having to cut the slab for some utilities and, lo and behold, the WWF was on the bottom.

 

[Guastavino]

Means and methods (I agree)! So no, I don't put that on my drawings. How they do it is up to them as long as the WWF ends up in the right place (I kind of laughed typing this sentence though).

Seriously though, I agree and when you step into means and methods and you take on more liability.

Now, come to think of it, I think the company I used to work for put a minimum requirement on their drawings for support of chairs (they didn't say how to do it, but they gave a minimum spacing between chair supports). I don't believe in replicating that practice though, because at what point does it end?

 

[hokie66]

I disagree. You have two choices, either specify adequate support, or spend a lot of time on site arguing and delaying concreting. They still may not do it correctly, but you won't have to argue so much if it is in the project specifications.

 

[BSVBD]

I've never specified.

I agree with Koot - Specifying applicable references to means and methods removes your liability.

XR - One contractor i deal with regularly "pulls it up" also. Never heard of an issue. But i wll pass the word on to them what you experienced. I'm surpized it would sink!

njlutzwe - i'd stay away from the minimums.

 

[Seppe]

Means and methods, all day, every day. If your contract docs show the stuff held 3/4" from the top surface of the slab, it's the contractor's obligation as part of his bid to reflect the work associated with accomplishing this. Nobody's forcing him to bid the job. Sounds extreme, I know.

That said, we all know that flimsy WWF is pretty much impossible to position reliably; it's practically chicken wire in most cases. As such, it's far too common for the stuff to just be laid on the ground and then raked up during concreting operations as previously mentioned. At that point, in principle, it's as much on us for specifying something with an unrealistic installation tolerance as it is the contractor for failing to deliver on the construction side of things.

If I'm a contractor, I bring it up during pre-bid that the chairing/bolstering component of the construction cost is going to be significant, and that typical practice is to "rake it up". There's some value in shining a light on the issue in an effort to level the playing field for bids. It's a constructibility issue that deserves attention.

Now, if I'm an engineer and I have a desire to see the reinforcement placed in a manner anywhere close to my detailed "design intent", then I probably get out in front of the matter and avoid specifying the garbage at all. Go with a bit heavier welded wire pattern (say a 6x6 W4/W4 as opposed to W1.4/W1.4 or W2.1/W2.1) and be the GC's friend from day one. Material cost is higher, but labor cost savings make up for it in spades. (The potential flaw in the logic being an assumption that labor is being accurately and honestly priced by the contractor to begin with.)

 

[SteelPE]

Thanks.

We are balancing this project a bit. The client was very specific about what they wanted as this was the 7th building they have built onsite. The reinforcing has been selected based upon the requirements of what the client has had luck with in the past and in this instance is a 6x6W4.0xW4.0 WWF.

I am beginning to wonder if there is a problem with the bid and now the contractor is trying to get out with this as an excuse. Or maybe trying to change his proposal in a sort of bait and switch tactic. We shall see.

 

[MacGruber22]

No good deed goes unpunished...


"It is imperative Cunth doesn't get his hands on those codes."

 

[Guastavino]

What I was saying is that the company I used to work for did it. I don't and disagreed with that policy. But there are some folks that do it which is what the OP was asking.

 

[Teguci]

If you don't care where the WWF is finally located in the slab (basement slab, office floor to receive carpet, etc...), then let them "rake it up." If the slab is exposed to salts, or subject to significant load cycles (road surface, parking garage, etc...), then tell them one option is to make concrete bricks (f'c = f'c of the SOG) and support the WWF at some spacing (2ft or so). Whatever they use, its their means and methods but it should be part of their submittal package for your review.

I made the mistake of dismissing a concrete form issue like this saying to myself - "Means and Methods." Everytime I drive by that retaining wall, I think how a little extra knowledge and prudence would have forced the contractor to provide adequate forms so that the final wall wouldn't look like waves on an ocean. As much as the legal requirement is the contractor's, the final product has your name on it too.

 

[Guastavino]

Tedguci,

What I love about engineering (and it used to drive me crazy) was that you ask two different engineers a question and you are likely to get opposite answers. All depends on the perspective. I think as MacGruber pointed out, it's often because "no good deed goes unpunished". In a world of litigation, it's hard to get involved in every detail. But then Teguci makes a great point.

Interestingly, and not to hijack the thread, but it seems like nearly all specifications (book specs not drawing specs) spell out many means and methods requirements. There's a line between means and methods and design, but it's a broad one. Back to point of the OP, I'm guessing you have a good mix of both. And depending on the lawyer, either could get you into trouble. But, if you never want to get sued, you won't ever do any work, and where is the fun in that.

 

[manstrom]

This is an old article that covers the topics.

Means and methods, of course. Sounds like the contractor is looking for some guidance to do it right.

I would recommend that if they can't put mesh where it is supposed to be (they never can), then use rebar instead.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[MotorCity]

We do not specify how to do it, but there is an ACI publication that covers bar placement that we reference in our concrete notes (can't remember the publication number). It basically spells out how to (or how not to) support the bars or wwf.

If you do not specify a method of support or provide some sort of guidance or reference publication, you don't have a leg to stand on when the contractor supports the WWF using a method you find unacceptable.

 

[RHTPE]

This is a conundrum for sure. If you have ever watched a crew placing concrete over WWR installed on chairs at a reasonable spacing you begin to realize how futile it seems be to get WWR in the correct location. The placing crew generally knows nothing about how WWR functions and the design requirements driving its proper location. They only see it as an impediment to placing concrete. Similar attitude towards rebar as well.

Means & Methods only? We can have that debate forever. ACI SP-66 covers the detailing of reinforcing and does have a section on reinforcing supports. I have nor reviewed this manual lately and don't know how well WWR is addressed. The Wire Reinforcement Institute has a very good Technical Publication on WWR support (see attached).

I happen to be an engineer heavily involved in the "Means & Methods" side of construction. I consult with contractors to help them to figure out the "how-tos" of construction (and sometime destruction). I do get involved in proper reinforcing support and will 'stick my neck out' to make recommendations. Problems resulting from improperly positioned reinforcing that adversely affected a concrete structure will almost always come back to the engineer of record first. Only after extensive involvement on the EoR's part can the problem be then thrown back at the contractor's failure to do his/her job.

It is my not so humble opinion that the engineer should clearly make his/her expectations know prior to bid time. If it means making a recommendation for WWR supports, so be it. But the issues resulting from poor placement techniques must also be a part of the discussion.


Ralph
Structures Consulting
Northeast USA

 

[hokie66]

This thread has primarily been about supports for mesh in slabs, which probably does very little good anyway. But the sad thing to me is how many engineers, and you all seem to be in the US, are so afraid of lawsuits that you don't want to get involved in the construction side of your design projects.

 

[flight7]

Our project specs are based on the AIA Masterspec on concrete which states "Install welded-wire reinforcement in longest practicable lengths on bar supports spaced to minimize sagging." It isn't very stringent, but it does effectively prohibit the lift-with-a-hook-and-hope-for-the-best method of construction. I'm surprised so many consider this a means and methods issue. We have support requirements for rebar rods, why not for mesh?

 

[thaidavid40]

Our standard drawing note says that WWR shall be properly supported to keep it in position and at the correct elevation at all times. It specifically disallows hooking-and-lifting as a placement method. The supports used, and their spacings, are both a part of the contractor's submittals.
Dave

Thaidavid

 

[MacGruber22]

But the sad thing to me is how many engineers, and you all seem to be in the US, are so afraid of lawsuits that you don't want to get involved in the construction side of your design projects.

It isn't always about lawsuits - it can be the loss of time arguing over the answer and the creep of scope into being the construction support engineer. They will ask for a foot after you give them an inch. Further, you will run into many contractors in the US who will complain about what you specify for mesh support until you give in to something less with some amount of given restrictions, which ends up being very muddy waters The "real" support required to keep mesh in the upper 1/3 of slabs and nearly rigid during the pour is significant, and if you beef up the mesh gage to reduce the supports you get yelled at for providing an amount of steel that the contractor has "never seen in his 30-years".

That said, a lawsuit (or at minimum a lot of complaining) will come your way with slabs requiring very good crack control, and clients are sensitive to their performance & aesthetic. At a minimum, the lost time fighting contractors is fatiguing and eats away at time better spent doing anything else.

IMHO, 99% of the time that a contractor asks the EOR to specify bar/mesh support, he/she is not looking for the real answer - rather a way to pass responsibility of a tough means and methods task to someone else. It is a lose-lose situation way too often. No thank you.

Besides, if your original contract isn't with the contractor, you need to provide a fee for construction support. You don't "need" to do anything for 1) someone who you don't have a contract with and 2) any task that isn't in the standard practice for EOR's within the building code.

"It is imperative Cunth doesn't get his hands on those codes."

 

[KootK]

It specifically disallows hooking-and-lifting as a placement method.

My own firm's specs are riddled with these statements that I'd classify as "stupid shit not to do" clauses. I feel that it's the wrong way to go about things because, unlike the list of things to do, the possible list of things not to do is limitless. And referencing some stupid things not to do almost implies that other, unlisted, stupid thing might be acceptable.

4.5.1.2 No hook and lift.
4.5.1.2 Don't allow mesh to sink to a position lower than specified.
4.5.1.3 Don't substitute woven stretched out paper clips for mesh.
4.5.1.4 Don't impregnate your biological sister.

So wait, does that mean that I should impregnate my step sister? I feel that specifications should, for the most part, prescribe rather than proscribe. As with all things, there are surely exceptions.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.