The fundamental problem in constructing a suspended concrete floor is, as EFCO technical specialist Bob McCracken says, that concrete is very heavy. The other problem, of course, is that until it gets hard, it won’t stay where we want it to be. Therefore, we must construct formwork to keep it in place for as long as it takes to become self-supporting.
Traditionally, 4x4 posts and scaffolding were used to support plywood sheets upon which concrete could be placed. But with contractors always looking for a way to build faster and with higher quality, formwork manufacturers have come up with a variety of systems to increase efficiency and productivity in deck forms. The options can be a bit bewildering, with each formwork company proclaiming that its configuration is the best due to this or that feature. Let’s look at some of the available systems and what factors you might consider when buying or renting deck forms.
Options in deck forms
The simplest solution to suspending deck forms may be simply to erect scaffolding and place plywood—in some complex configurations that may be the best approach (see Shoring Challenge on the next page). Most major formwork manufacturers make an enhanced version of this approach with stronger steel posts to replace 4x4s and timber or aluminum beams. Peri and Doka both make timber I-beams (like TJIs) and props or shores are available with capacities up to 13.5 kips.
For many projects, however, modular systems will improve efficiency. Tom Ameel, former CEO of Peri USA, says there are five basic systems: loose framing, grid beams, panelized hand-set panels, small format tables, and large format (truss) tables.
Hand-Set Modular Grid Beams: Systems like Peri’s Beamdeck (see cover), Doka’s Dokaflex, or the Titan HV system use aluminum primary and secondary beams (to reduce weight) and steel posts (or props) that are based on a grid but designed to allow enough flexibility to meet most deck configurations. Where unusual perimeter conditions occur, these systems have techniques for infilling. Many come with drop-head posts, which allow the framing to be removed before the slab is ready to completely support its own weight, sometimes within a single day. Remember, though, that construction loads cannot be applied to the deck until it has gained enough strength to support the dead load. The props are then released to allow the slab to deflect and can then be immediately reinstalled as reshores. An advantage of this type of system is the limited number of parts—as few as five different components.
Hand-Set Panels: Similar to the hand-set grid systems, these systems come with reinforced panels that fit easily into the post and beam system. Even the largest of these panels, weighing about 50 pounds, can be moved easily by a single worker for the next placement. Meva’s MevaDec system comes with its Alkus plastic panels but the secondary beams can be flipped over to also work with plywood sheets. Most of these systems are designed such that the entire formwork assembly, including panels, can be removed and readied for the next level while leaving the drop-head post in place. Some of these systems have only two parts: panels and props.
Table Forms: Table forms allow contractors to assemble large sections of formwork on the ground and then move them from floor to floor in large sections. Peri and Doka each has systems to move small-format (20 x 30 feet) beam-and-panel tables from floor to floor using an elevator system—the tables are rolled on trolleys to the building perimeter and onto the elevator. Some of these tables have props attached that can be folded up to get past obstructions. Most other table forms are flown by the site crane. Trusses are used to support the largest table forms (see Truss Tables) that are moved floor to floor by crane. The feet have height adjustments that allow the form to be set easily and quickly to the proper elevation. Very large tables can be supported by only two trusses.
How to decide
Some factors to consider when deciding on the deck formwork system that’s right for your project include:
- Shape and size of building and the number of stories: Generally, table forms will be faster when crane access is available. The size of the tables may depend on crane capacity and access. When crane access is not readily available, hand-set forms may be the answer. “On the Burj Khalifa,” says Meva USA president Scott Fisk, “hand-set panels were used due to restricted crane access. Panels were moved floor-to-floor by hand.”
- Type of deck: Some systems work better when there are drop beams or capitals. Most of the systems can adapt but are usually configured for flat plate slabs.
- Skill and experience of labor force: “What system your crew is comfortable with is important,” says Doka’s Mike Schaeffer. “That varies around the country.” The panel systems tend to be simple to learn to assemble and require less measuring.
- Speed of construction: “In New York they may be pushing for a two- to three-day cycle,” says Schaeffer. “In Chicago, it’s four days, but in Texas it’s five to seven days.” For very rapid construction, table forms are going to speed things up. Or, if a hand-set table is used, drop heads may be essential with tight schedules.
- Panels or full-sized sheets: Panelized systems can often speed things up, but then you’re stuck with the formwork manufacturer’s facing.
- Supplier: All formwork manufacturers will rent or sell forms. When you rent or lease, the supplier’s engineering expertise is important.
Deck form systems speed up and simplify construction. Consider using them on your next project.
