Concrete is one of the most widely used and readily available building materials on the planet. Cast-in-place concrete, unlike any other building material, arrives at the construction site in an unfinished state and is subsequently formed into the desired shape. As such, concrete designers and builders have ultimate control in determining how a structure is built. As project cost and schedule often determine the fate of a given structure, concrete builders are constantly searching for ways to build faster with less or more efficient use of labor and equipment. For the concrete designer, constructability is not often at the forefront of thought during the initial phases of a project as other challenges typically overshadow it. However, it is during this time when the concrete designer has the greatest opportunity to influence project cost by considering not only the design of the structure, but how to build it.
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Cost-effective formwork
In the United States and other developed nations, formwork often represents as much as 50 percent of the total cost of cast-in-place concrete buildings. In addition, it is frequently observed that the combined total cost of concrete formwork can surpass the combined total cost of concrete reinforcement materials and labor. It is therefore critically important that any effort to reduce cost of a cast-in-place concrete structure include the consideration of formwork. The most common barriers to cost savings in regards to concrete formwork can be attributed to the following:
Seeking cost savings by minimizing concrete volume — Based on the age old adage of “less is best,” this approach overlooks two of the key characteristics in formwork efficiency: repetition and simplicity.
Focus on individual elements rather than an entire system — Losing sight of the big picture can lead to adding cost rather than reducing it; repetition and simplicity must not be limited to framing member sizes and should be applied to reinforcing size and spacing where appropriate.
Not understanding formwork and structural framing compatibility — Disregarding formwork systems until the construction phase can be an expensive proposition for both designers and builders, as it can require a customized forming system or involve considerable modification to the design to accommodate a more economical forming system.
Lack of attention during conceptual and initial project phases — The tight design schedules of today’s projects, particularly for buildings where the structural frame must be submitted for permit ahead of all other disciplines, can lead to inadequate constructability reviews from the start. More often than not, the design process moves so fast that the structural engineer has no opportunity to consider forming systems.
Private versus public projects — Designers unfortunately do not always have the opportunity to access contractors or concrete subcontractors for constructability input, particularly when designs must be complete before they are released for bid. As such, designers should be aware that early participation from the actual builder may not be available and that seeking out potential builders for insight into the construction practices of the region may prove invaluable.
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Design and construction synergies
The successful outcome of a cast-in-place concrete building can be guided by the designer by simply being aware of the synergy between structural design and the anticipated forming system. Cost savings can be had in the following several ways:
Preplanning — Determine if the structural system chosen is compatible with both the architectural and MEP layout and the anticipated forming system; slight shifts in column, wall, or opening locations can be managed with minimal impact to other disciplines provided they are coordinated early. Effective collaboration with other disciplines is critical in providing economy to the structure.
Consistency and uniformity — Limit fine-tuning of member sizes, shapes, and reinforcing; select member sizes that work with the anticipated forming system, and keep the big picture in mind.
Work with the builder — Early discussions with the general or concrete contractor are essential when establishing the compatibility between formwork and structure. However, in some cases the general contractor may not have the ability or resources to provide input or access or a general contractor may simply not be available. In these cases it may be necessary to consult with one or several concrete subcontractors to gain valuable insight as to the type of forming systems that are best suited for the project. It is important to note that differences may occur between one subcontractor and another based on available resources and local preferences.
Because of the relative size and volume of formwork required for a given floor framing system when compared with other building elements (such as columns and walls), the choice of the floor framing type is one of the earliest and most important decisions on a project. It has the greatest potential for cost savings, particularly for low- to mid-rise buildings where horizontal framing costs can claim as much as 80 percent of the total cost of the concrete frame.
Many factors can influence the choice of a particular floor framing system, such as building occupancy, span, depth, deflection, vibration control, future modification considerations, and architectural layout. From a concrete formwork perspective, the most economical system is a flat plate or flat slab system where the formwork has very limited interruptions and can be assembled and removed with relative ease; these systems require the least amount of time and labor to build and use the most straightforward formwork and reinforcing layout. Other systems include one-way joist and slab, one way banded slab, waffle slab, and one way beam and slab. Each of these systems has its associated cost savings advantages and disadvantages that must be evaluated during the initial stages of a project, considering both structural and forming efficiencies to determine the most economical option.
Designing a structural frame around a concrete formwork system is commonly used in design-build type project delivery systems. Close collaboration between owner, contractor, and designer can lead to efficiencies in design and construction, where both contractor and designer are joined at the hip to capture improved productivity rates available through the selection of the most appropriate framing and formwork systems. Examples include tunnel form systems where concrete walls and floors are cast all at once, core wall self-climbing systems where the central core of a building is advanced ahead of floor framing to remove it from the critical path, and column-hung forming systems where all floor formwork is supported by the building columns.
Proven techniques to consider when designing for cost savings in cast-in-place concrete structures are listed on page 32. These techniques can provide significant savings in rebar placement, formwork labor, and forming materials with the added benefit of potentially improving construction schedule and reducing field errors.
Conclusion
More than ever, concrete designers are faced with designing structures that not only meet their required functions, but do so at an economical cost. Understanding the synergy between structure and construction has become an integral part of the design process where concrete designers that are mindful of the price of complexity and cost of formwork can contribute significantly to the ultimate success of a project.
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Designing for cost savings — proven techniques and tips
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Martin Maingot, P.E., S.E., is a project manager and associate with Cary Kopczynski & Company, located in Bellevue, Wash. He has more than 10 years of experience in the analysis, design, and review of high-rise and mixed-use projects, hotels, hospitals, office buildings, sports facilities, and parking structures. He is also chair of the newly formed PTI Sustainability Committee and can be reached via e-mail at martinm@ckcps.com.
