Concrete formwork: Opinions and advice

What do formwork contractors and suppliers want structural engineers to know? What could builders learn from the design challenges faced by structural engineers? To start a dialogue around these questions, Structural Engineer hosted a roundtable discussion in Las Vegas on Jan. 24, 2007. To focus the dialogue, several experts participated—including several practicing structural engineers, formwork contractors and manufacturers, builders, and others allied to the industry. (See "The Panel" below for a list of participants.)

The group shared their opinions and offered some advice regarding concrete construction as it relates to formwork; excerpts from the discussion are contained in this article.

Opinions on cost

COOK: My initial thought was that we could simplify the presence of concrete on the jobsite into a cost circle, a pie-chart. And assign, according to the wishes of those assembled and their experience, different percentages of the costs to different features.

For example, the American Concrete Institute (ACI) says that formwork is 50 percent of the cost of the concrete. Others would say 60, from their experience, and architectural may even be higher than that. Others that come from the precast or tilt-up firms would say 5 percent of the concrete cost is formwork. So there’s very great variance.

But if we stay with structural, in-place concrete for the purpose of building and giving an owner a completed project, not the architectural finish and other features, would it be reasonable for the house to say that half the cost of the in-place concrete is formwork? Would anybody care to take issue with that, or is that a valid number?

GIORGI: Not to take issue with it, but say that it would vary between 40 and 60 percent of the cost of the concrete structure.

RICHARDSON: What are you including in those costs?

COOK: All of the things it takes to get the formwork up on the building and back down again, including the purchase of it, the layout, the surveying, all of the things, every feature of having the concrete left in place to stand by itself, unaided, and acceptable to the owner’s standards, too.

CLAPP: Is that regardless of columns supported or full table support by the levels below?

COOK: I’d say yes.

BERNDT: It still varies within that range.

SUTHERLAND: This is interesting from a manufacturer’s point of view; if we look at this piece of the pie that we’ve assigned to cost of formwork—say 40 to 60 percent, or call it 50 percent. How would that be broken down a little bit further in terms of materials versus labor? Could you break that piece of the pie?

TULIS: You could, but it’s difficult because the breakdown really becomes structure-dependent: how many floors and how many uses. Certainly the fewer the uses the more material costs may come into play.

SUTHERLAND: So form material with greater re-use would provide good savings.

TULIS: Again, it depends on the job. If you’ve got a material that can go more than 100 uses, but you’re on a building where you’re only going to use it 10 times, you’re paying a premium for that material. Is it worth it?

DYKHOFF: That’s right. You have a different system that requires less labor to move per square foot, but the system itself costs more. For example, a column-mounted system is more expensive to invest in, but the return is on the labor side.

BERNDT: One of the things commented on is all the same size columns, with the strength of concrete changing throughout the height of the building. Obviously, that’s going to be on the plus side as far as savings go, with formwork being 50 percent of the cost. If there are only eight forms and they’re all the same size and you re-use them 150 times, that’s much cheaper than changing column sizes every two or three or four levels, all things being equal.

COOK: Accuracy is up too. Workers are getting better attending to it, plumbing things happen quicker. All of these things go along together, but somebody has to be willing to possibly pay the premium for high-dollar, high-strength concrete on lower floors. It’s not free.

GIORGI: That’s going back to what Mark was saying about the ratio of labor to formwork. If you were saving labor, then it can readily offset the cost of the higher-priced material, or the extra material, that’s involved.

TULIS: In fact, you may not even save on material or on labor if you keep the columns the same, but you may save on time, and time is a savings, too. So you may have the same material costs with one column size, but as Ramon said, if the crew is doing the same operation over and over again, you can get efficiencies in installation.

Advice on cost

CLAPP: Let me throw a wrinkle into this. When we are going to get a lot of compression in concrete columns, such as with one of our high-rises here in town, we design the formwork to be built upward at some of the columns to allow for the column compression, ensuring that the floor will become flat later on. Does that add to a lot of cost for you guys?

RAMERTH: Are you talking about camber?

CLAPP: No. We are physically raising the formwork at certain column locations, sloping it up.

SULLIVAN: The reason for this is because there is significant column shortening in high-rise buildings. When we intentionally detail the floor slab to slope up from the core, for example, ultimately that portion will come down flat as the rest of the mass of the building is added through the remaining construction.

TULIS: I would think that placing and finishing costs might go up a little bit as a result of that.

BERNDT: And, if you’re talking two-way camber, or sloping, it can cost more.

Opinions on tolerances

BERNDT: To me, tolerances is one of the most critical issues that needs to be discussed up front because there are lots of conflicts between what can be achieved in concrete and what other components, such as curtain walls or other systems, may require. And I would say that more often than not it is a problem because it is not addressed up front adequately.

GOUPIL: Ken, do you mean flatness tolerances?

BERNDT: Yes, flatness can be a problem. Things like railings on balconies. There’s just a whole myriad of things that need to be coordinated all the way through the project, which are often not coordinated.

TULIS: And, in my opinion, it’s up to the architect and engineer of record to look at all of them and make them all work with each other and spell that out up front for the contractor. The one thing that this would do is put every bidder on the same level, bidding the same job. Otherwise, you will get someone who will bid it without understanding the implications and others who will just disregard the conflicts. Then, when you go to build it and things don’t fit, somebody has to pay for it and it won’t be the owner.

Opinions on technology

DYKHOFF: I’d like to ask a question. When I’ve visited Chicago, I notice that the fastest-cycle projects are using the down-pour process technique for placing columns. My question is, is there a structural concern regarding that?

TULIS: You’re talking about structural concerns about the way they are built?

DYKHOFF: Yes. They are pouring columns at the same time they pour the deck: down-pour.

BERNDT: That’s been done there for years.

SULLIVAN: I don’t know that I’d have a problem with that. The major issue any time you pour a column—whether it gets done the same time as the pour, whether it gets done separately—is that you’re just not dropping the concrete, where it’s going to separate where it hits the rebar and the formwork on the way down to the bottom. So, I wouldn’t seem to have a problem with that method.

CLAPP: Seems like there’s going to be more shoring involved in a system like that.

BERNDT: The major issue that we’ve run into with this method is that we’re not told about it until we build gang forms for our columns, get them underneath the deck, and then we can’t get them out. It’s got to be a hand-set column situation and then you’re a couple floors behind with your column forms.

TULIS: You also have to be concerned about the lateral stability of the formwork deck itself because now you no longer have in-place concrete columns to help provide lateral stability. But, as Ken says, if you know about it up front, great.

Advice on documentation

TULIS: I’d like to touch back on the issue of shoring and reshoring, because reshoring is one of my favorite topics anyway. But you asked us, Jennifer, about what can the structural engineer do to help the formwork guy and the contractor? The principle item I need to design the shoring and reshoring is the net design loads used to design the building. I do not want to know that it was designed for 100 or 75 pounds per square foot (psf) and that these loads are reduced as the code allows. I don’t want to chase that. I don’t know what code you used. I just want to know what the net design load is.

For example, I had an office tower designed for 100 psf, 30-foot by 30-foot bays with a 9-inch, two-way flat slab. The engineer took a live load reduction on the 100 psf and the net design load was 47 psf. I found this out after we were on about the sixth floor. The long-term deflections on those floors were awful. It was awful. Unfortunately, nobody told us anything about the load. The stated design load was 100 psf with no mention of a live load reduction.

My comment is driven by the target audience for this article: the structural engineers. I want them to know that this is information that’s valuable to the owner, it’s valuable to the contractor, and it’s especially valuable to the guy who is going to design the shoring and the reshoring. Put the design loads on the drawings.

GOUPIL: Is there a response from the engineers?

RAMERTH: I guess what I’m hearing loud and clear is that on applicable jobs, we should be giving very clear direction on the reshoring requirements—the actual net design loads.

TULIS: Also provide the superimposed dead loads. Knowing those is a tremendous advantage in designing reshoring.

COOK: Because they’re not present yet?

TULIS: That’s it, exactly. You can utilize that strength in a floor system to carry the construction loads.

SULLIVAN: I really appreciate what you’re saying and I think it’s a great idea. I also think that these questions can come at the beginning of the construction process through RFIs, and through having pre-construction meetings. This can be obtained well before the formwork system and reshoring is designed. There are other means to come to that same end.

COOK: You have to have the information available before the bidding process in order to be valuable to the project and owner. That’s the only time.

TULIS: This is because it puts all of the bidders on the same page.

SULLIVAN: Does that really affect cost?

TULIS: Over all, yes. It affects both cost and construction time.

SULLIVAN: I guess you’re saying that by the time you find out you could need more reshores?

BERNDT: Yes, and if you have a lower capacity, you may need more levels of reshores.

TULIS: Correct. Or you’re going to pay more for remedial work later. You’re going to be putting floor toppings on. And contractors love to do that. Believe me, I have no problem selling lots of topping, but that’s not what I want to see happen.

COOK: Nor does the owner.

CLAPP: Actually, all we put on our notes is the live load of 100 pounds, or 40 pounds psf. Then, behind it we put reducible.

TULIS: I want to know the number, though. I don’t necessarily know the code you are designing to, or what assumptions you made.

CLAPP: We have no problem discussing what we design for and giving actual loads. We’ve done that to quite a few shoring subcontractors. I don’t know if we need to put that in our drawings, though, because then we get into a means-and-methods situation.

COOK: But, you’re describing the characteristics of the as-built structure, not the means and methods.

TULIS: If you’re designing for 75 pounds psf reduced to per code to 48 psf, all you need to do is state the reduced value.

CLAPP: You just want the reduced value?

TULIS: I just want to know the reduced value because that’s what I can use to design the reshoring. And, if there’s a superimposed dead load that you used, tell me that, too, because I can utilize it.

COOK: A ceiling is a good example because there’s no ceiling or mechanical load on the job when we’re doing shoring and reshoring. The reshoring design can take advantage of the strength the building has to support these structures, which are not there yet.

TULIS: And there’s a flip side to this, too. If you have a floor that is capable of carrying the construction load without reshoring, let me build it that way. Don’t insist on reshoring just because your specifications require reshoring. If your design loads will allow construction without it, I’ll tell you there isn’t a contractor out there who wouldn’t jump on that.

BERNDT: The opposite side of that is, so often there will be a requirement in the specification for two levels of reshores, but two levels may not be enough. Specifications should never dictate the number of levels of reshores that are going to be required, but, unfortunately, that is often the case.

TULIS: Additionally, the arbitrary requirement to strip forms at 75-percent strength is also very dangerous.

RAMERTH: Explain.

TULIS: I have done the calculations and found that very often floors, based on our design loads, need to reach 78 percent, or 82 percent of strength, before you can safely remove shores and support whatever construction loads exist on top.

SULLIVAN: That’s another issue to discuss here. What is a reasonable construction load? I think typically we assume that it’s approximately 20 pounds psf. But oftentimes the contractor could stack drywall, brick, or block in concentrated areas. I think those are areas that need to be discussed in pre-construction. But should the design team be assuming a heavier construction load?

COOK: No. A formwork designer is responsible for that. Within the construction processes he’s got to tell the builder where he can or cannot stack block, brick, and drywall. And his job is to tell the builder that he can or cannot pile material here while I’m reshoring. It’s very important to know those things, but it is the constructor’s job to protect the building while it’s in those green stages.

TULIS: Remember, too, the envelope that the formwork contractor is involved with and responsible for: the floor under construction and the floors that are tied together by reshoring. Once all of that has moved up the building, from then on down it’s no longer the formwork contractor’s issue, it’s the general contractor’s. Also, my reshoring notes always cover stockpiling material.

RAMERTH: That’s good to hear.

Conclusions

SULLIVAN: I have two conclusions. One, I think that success comes down to knowledge and communication. Second, I think we can all say that in the different markets there are different standards. You’ve got to know that and design your structural system to what’s typical and local.

TULIS: My final comment is regarding the quality expectation of the owner. A smooth form finish does allow for bug holes in concrete. But the painter or the plasterer may not find that acceptable. And usually it’s the formwork contractor or the general contractor who gets to pay the bill. So I think quality expectations and a clear understanding or communication of that is essential, too. It will save a lot of grief later and a lot of expense for someone. Also, I would advise engineers to try to not affect the bottom slab elevation.

RAMERTH: Sure. Form drops are expensive.

TULIS: Every time we change elevation, it costs money and it interrupts systems.

DYKHOFF: In general terms, I believe it’s the alignment of the entire team that has to start with the owner, the engineers, the architect and the major players, contractors all have that alignment early on and understand each other’s expectations to design a system that will work for everybody’s end benefit. That improves the quality.

SCULTHORPE: With insulating concrete form (ICF) wall systems, the biggest contribution that structural engineers can make to save cost is to design the rebar that readily accommodates the dimensions of the form systems that are being used. For example, a horizontal rebar on center spacing that works conveniently with the form system can save a lot of labor. Frequently you get calls from contractors when they have to deal with this issue because the rebar placement is awkward. So, if you know you’re designing an insulated concrete form wall, study the dimensions of the ICF before you design the rebar. It would make a significant difference in labor costs.

SUTHERLAND: We talked about productivity and quality of the product. From a formwork and plywood manufacturer’s point of view, there are an awful lot of products out there, and maybe not necessarily an understanding of the quality of the concrete or the productivity that different panels can provide for the formwork system. I would encourage structural engineers to educate themselves on the options available because there are certainly many different products and solutions out there for the structural engineer to understand.

RICHARDSON: I believe there is an excellent opportunity for the engineering community to work with those involved in creating stay-in-place, concrete forms with a focus on structural issues. Let’s look at cure rates, project objectives, and time targets. Work with us to develop the knowledge that is necessary to meet the building codes. We’ve got some very interesting technology throughout the industry that offers some answers to the problems you have.

COOK: I want to close by emphasizing that since much of our work is on hard bid jobs, the structural engineer can really help level the playing field by placing appropriate information in the contract documents. That way, all of the bidders get to read and evaluate in-line with their own supplies, their own equipment, their own styles, and they can negotiate changes if they have to, but everybody is on the same page when the bid goes out.

TULIS: I couldn’t agree more.


Structural Engineer would like to thank the Portland Cement Association—especially Ed Alsamsam, Ph.D., P.E., S.E., and Amy Trygestad, P.E.—for its help in organizing the roundtable.

THE PANEL

Kenneth L. Berndt, P.E., is vice president of equipment resources for Ceco Concrete Construction, LLC. He has been with Ceco for nearly 40 years in engineering, sales, project management, and he now runs the equipment group where his team buys, designs, or manufactures the various formwork systems that Ceco uses on its projects around the country.

Gregory L. Clapp, S.E., is a principal with John A. Martin & Associates of Nevada, a local structural engineering consulting firm. He has more than 25 years experience in structural design.




Ramon J. Cook, P.E., is principal and sole proprietor of Independent Consulting Engineer For Constructors. He has been in concrete construction since 1948 and works primarily with the constructor to preserve the structure according to what the owner is paying.



Tom Dykhoff is vice president of field operations with Adolfson & Peterson Construction—a general contractor with four offices: Minneapolis, Denver, Phoenix, and Dallas. He is responsible for field operations and self-performed concrete work in the Minneapolis market.


Bill Giorgi, P.E., is presently senior group engineering supervisor for Symons Forming Products. He has been with the company for 25 years and also designed formwork with his own company for about 10 years.




Mike Ramerth, P.E., is a principal of Meyer, Borgman, and Johnson, Inc., a structural engineering consulting firm headquartered in Minneapolis.





Dave Richardson is the president of Vancouver, British Columbia-based Octaform Systems, Inc. The company manufactures stay-in-place, PVC concrete forms.





Robert Sculthorpe, P.Eng., is an engineer with 10 years of experience in the concrete form industry. He is presently working for Formtech International, a manufacturer of insulated concrete forms based in Aurora, Ontario, Canada.



Andrew Sullivan, P.E., S.E., is the principal and managing director of the Las Vegas office of Walter P Moore—a national structural engineering consulting firm with nine regional offices.




Mark Sutherland is the sales manager for industrial and engineered wood products for Ainsworth Engineer Canada. The company manufactures concrete forming plywood and is based in Vancouver, British Columbia, Canada.



Ralph H. Tulis, P.E., is the corporate engineer at A.H. Harris & Sons, Inc., where he manages their Engineering Group. He has been involved in the concrete construction industry for 38 years. The company is located in Newington, Conn., and is a distributor of concrete construction products.





HOST
Jennifer Goupil, P.E., is a structural engineer and the editor of Structural Engineer magazine.

Photo credit for all images: Event Photography