A formwork formula: Tips for success


    Click here to view this article in the Structural Engineer e-zine.

    Formwork—the temporary or permanent molds used to hold wet concrete until it cures—is a crucial element in concrete construction. Just as important is selecting the right type of formwork since that greatly affects the schedule, labor requirements, quality and total cost of a project.

    Over the years, formwork molds have evolved from traditional job-built timber to pre-engineered systems composed of a combination of steel, aluminum, manufactured timber, plywood and plastics. These advancements in formwork molds have led to increased jobsite production and safety, with less labor, while producing a better finished product.

    Formwork trends
    Fewer than 15 years ago, approximately a dozen major formwork systems were readily available in the United States. However, over that short time period, several European forming companies have entered the domestic market, more than doubling the number of systems available.

    The increase in competition is pushing innovations to a rate previously unseen in the industry. Thirty-year-old systems that have enjoyed wide use and popularity are being supplanted by new, modern systems that offer greater productivity and a higher quality results.

    Walls—Presently, the most prevalent system in use for handset wall forming are steel-framed, wood-faced panels that require consumable ties at 2-feet-on-center and one connection per square foot. These are being replaced with larger, two-person handset systems that require less labor and eliminate consumable purchases because of reusable taper ties.

    Gang forming has completely changed over the past 10 years. Older systems of steel-framed wood or steel-faced panels with double channel stiffbacks that connect with bolts/pins have been overtaken by clamp connection forms with wood or plastic form faces that provide tremendous labor savings in assembly and use. Assembly and reconfiguration of these standard systems to meet changing structure dimension happens very quickly, and also provides a consistent concrete finish.

    —The use of fixed or adjustable wood posts, stringers and joists is still the most common method of shoring of slabs in the United States. This method ― passed down from generation to generation ― requires substantial labor. Because the posts are placed as close as 2-feet-on-center, construction sites become very congested.

    A new construction method featuring engineered lumber and metal posts increases post spacing up to 5 feet by 10 feet and offers components that are systematic and reusable. This increased spacing allows for less material on site to form the same slab area. Less material means reduced handling requirements, less labor to set up and strip the formwork, lower transport costs, and an increase in overall job site productivity.

    The current method for gang-forming slabs uses trusses or structural decks, which require a substantial amount of time for assembly and disassembly. Moreover, this method consumes an enormous amount of crane time, thus increasing the time for resetting a standard operating procedure. The customer must also purchase the plywood facing and sometimes replace it multiple times on the same project.

    Because of the expense of setup and takedown, gang-forming slabs are used mostly on structures taller than 15 stories high. Smaller tables ― delivered to the job site fully assembled with plywood ― are becoming a better solution to these systems, especially for mid-rise buildings where gang-forming was previously not economical.

    Another innovation that reduces job site crane time and formwork labor requirements is formwork-lifting elevators that mount to the exterior of a building, allowing all formwork to be cycled from floor to floor without the need for a crane. These table-lifting systems are used in conjunction with the smaller table method and also allow for other construction material including handset shoring, vertical formwork and reshores from below to cycle from floor to floor with a crane.

    What the structural engineer needs to know
    In the design process of a concrete structure, it is the structural engineer’s responsibility to the owner that cost and economy be considered in the design. During this value engineering process, it is extremely important to consider not only economies in permanent materials of the structure, but also economies in the total construction process. In reality, reducing element sizes to meet exact requirements in each area of the structure in order to reduce permanent material cost can very often increase the total frame cost of the structure. Instead, it is more economical to build consistencies into the structure that provide for repetitious construction. Keeping member sizes consistent saves on labor and time in the construction process.

    With this in mind, and understanding that formwork can be the largest cost component of a concrete building’s structural frame, it is to the engineer’s advantage, when considering total cost, to thoroughly evaluate the entire construction process. This includes assessing temporary formwork material plus labor, time and equipment requirements. The result will be construction of the most cost-effective structure possible.

    With new and innovative products constantly entering the marketplace, learning about them and how they affect the construction process is an ongoing challenge. For example, a fairly new product being used in the construction process on high-rise structures is protection screens. These systems fully enclose a building’s slab edge, providing a completely contained working environment that is safe and more productive. The systems anchor to a slab edge around the perimeter of the building. Therefore, if they are going to be used in the construction process then the structural engineer needs to be aware of the loads they impose on the structure and consider this during design of the slab edge area.

    Another example in high-rise structures is the increased use of self-climbing formwork used on cores and shear walls. When these systems are used, rebar design needs to be considered because anchoring the formwork becomes a critical factor.

    Selecting the right formwork
    Structural engineers should realize that concrete contractors perform a detailed analysis of available formwork solutions to help select the best system for the construction project. Because conditions vary for each individual project, there is no simple formula for choosing the right formwork supplier or system.

    Formwork typically accounts for 40- to 60-percent of the total cost of a building’s structural concrete frame. For concrete walls, the cost can be in the 50- to 60-percent range. These percentages include the cost of material and labor, with the largest cost being for labor. It is important to analyze labor costs thoroughly since it is the largest expenditure and reducing that amount will have a much greater impact on the bottom line.

    To determine the most efficient solution for a project, a contractor will evaluate several forming systems. As the shortage of available and capable labor continues, it has become even more important to select the right forming system. Simply stated, a contractor has two choices: an inexpensive forming material that is labor-intensive or a forming system that may cost more but provide high productivity.

    Other factors the contactor will evaluate when choosing a formwork system include:
    Is the required material readily available? Does the supplier manufacture the material or do they purchase it from another company?
    Can the supplier pre-assemble some or all of the formwork prior to delivery? This can reduce rental cost, save labor requirements, and minimize assembly area requirements.
    Does the supplier provide on-site field service to train and reduce the learning curve of the formwork crew?
    How safe is the system to install, use and dismantle? Can the forms easily be climbed and are tie-off points built into the system where required?
    What experience does the firm have with your type of project?
    Does the supplier offer engineering services? Will the supplier provide generic drawings of the system or specific assembly drawings for the formwork for the project?

    Whether to purchase or rent a system is another consideration. This decision should be based on the duration of the project and the overall strategy of the construction company. Typically, if a form system has to be rented for more than 8 to 10months, purchasing the system might be more economical, even after figuring in the required maintenance and storage costs. Another advantage is that some formwork companies offer services for customers who purchase their equipment.

    The quality of the product also must be considered in the decision-making process. Steel-framed wall formwork with standard plywood facing requires more maintenance and repair throughout the life of the form than does hot-dipped galvanized steel frames with specially manufactured plywood designed for longer life.

    Forming a relationship
    In order to consider formwork in the overall design, it is important for the structural engineer to have a good handle on the standard products available in the marketplace. This can be accomplished by contacting the major formwork suppliers. They are more than eager to provide assistance to educate the industry on product options and overall costs specific to the design of a structure.

    Formwork suppliers should be contacted in the very early design stage of the project. This ensures that the much and best information available is incorporated into the bid documents, providing a more accurate cost for the owner. Formwork suppliers can advise on sizing structural concrete members to meet standard form dimensions. And, because major form suppliers typically are involved in a large number of projects in a variety of construction markets, they can draw upon their resources to suggest formwork means and methods.

    Concrete forming contractors should also be involved early in the design stage of the structure. Since they are ultimately the builders of the structure, they can provide a tremendous amount of knowledge on the most economical means and methods.

    Among the many considerations when designing a concrete structure, the choice of formwork is of prime importance. Product quality, material and labor costs, and worker safety are all part of the total cost of forming. With better, faster and safer formwork available today, designing for the formwork is a crucial component to ensuring the success of an economical concrete construction project.

    Michael Schaeffer, head of product management for Doka USA Ltd., has been in the industry for more than 15 years. He is a formwork professional who has also served in the areas of engineering, sales, operations and regional management. Today, Schaeffer’s responsibilities include company-wide sales/product training, product development, new product rollout and work as liaison with marketing for all trade show planning and execution. He can be reached at 877-365-2872 or at Michael.Schaeffer@doka.com.