Eldon Tipping, P.E., FACI
Level suspended floors don't just happen; they are the result of a collaborative effort between the contractor and the engineer. Each member of the design/construction team has knowledge and information that are necessary components of a successful solution.
This year, millions of dollars will needlessly be spent by Contractors or Owners to improve the levelness of suspended floors which do not meet the needs or expectations of the Owner. The cause...a failure by the design/construction team to anticipate and to overcome differences between anticipated and actual deflections. Floors that are not level enough to meet tenant needs must be corrected by use of a self-leveling underlayment or by installation of a topping slab. Implementation of either of these expensive solutions can be avoided by incorporating a pro-active strategy to produce level suspended concrete floors.
The keys to a pro-active strategy are-
- Identify factors that result in level deflected concrete floors,
- Develop construction techniques that successfully address each of the factors,
- Collaboratively agree on the approach prior to construction, and
- Implement a monitoring program that provides real-time feedback to the contractor.
Factors and solutions vary with the structural system utilized by the engineer, but the final result should not. Properly executed suspended slab construction typically results in deflected floors with 80% or more of the surface inside a ¾" deep horizontal envelope.
It has been said that concrete floor design and construction is more art than science. This statement is especially true when applied to concrete floors that move during or after construction. Although buildings come in all sizes and shapes, with a wide variety of column spacing and structural solutions, the challenges presented by each combination of building geometry and structural solution can be economically addressed by implementation of a pro-active strategy.
Cast-inPlace Concrete Floors
Constructing a level deflected cast-in-place concrete floor incorporates four elements. In order of occurrence, they are 1) implementing a design process to anticipate movement of the concrete floor, 2) producing an initial surface profile that is a complement to future movement, 3) eliminating significant movement of the concrete floor during the form-removal/form-stripping process, and 4) monitoring actual movement to provide the Contractor with real-time feedback on system behavior.
After supports are removed and the structure assumes its final deflected shape, it is difficult to determine the primary cause of a deflected floor that is not level. A time study focusing on important elements of the flooring system is necessary to identify movement after key phases of construction. A rod and level can be used to collect elevation data at columns, mid-point between columns, and at mid-bay. Data samples should be taken 1) while the floor is still supported, 2) after forms have been stripped and reshores are in place, and 3) after all supports are removed and the unloaded structure has relaxed into its final deflected shape. Comparison of sequential samples allows the design/construction team to quickly identify and respond to local instances of unexpected behavior with modifications in camber or construction technique.
These additional measures can be used to improve the levelness of deflected cast-in-place concrete floor systems:
- Limit the elevation of floor forms to design grade plus ¼ inch. If form elevations are too high, the use of pre-fabricated supports, pre-fabricated reinforcing, and minimum requirements for concrete cover can preclude production of a floor surface that is at design grade.
- Intentionally hold floor forms slightly low at areas of reinforcing congestion such as beam/column intersections. Reinforcing separation that appears adequate on paper may prove to be inadequate in the field. An additional fraction of an inch of vertical space provides the contractor with sufficient "wiggle room" to install all the floor reinforcing without compromising design grade for the floor in question.
- Use forming systems that limit movement of the cast concrete floor during the form stripping process. Form stripping procedures that require the new concrete to perform as it will in service result in deflections that are significantly larger than those typically anticipated by the design engineer. Two separate European manufacturers provide forming systems such as that described. Each system allows floor form panels to be removed while supporting shores remain in place.
Concrete Floors on Metal Deck
Several specific elements are crucial to constructing a level unshored slab-on-metal deck composite concrete floor. In order of occurrence, they are
- Implementing a design process to anticipate and limit movement of the supporting steel frame during construction,
- Producing an erected steel frame with level beam-to-column connections,
- Controlling movement of structural steel floor members during concrete placement, and
- Monitoring behavior and providing timely feedback to the contractor.
Multiple challenges face the design/construction team that contemplates construction of an unshored composite steel floor system:
- Available design approaches and grades of structural steel encourage the design engineer to use light, shallow shapes.
- The American Institute of Steel Construction (AISC) Code of Standard Practice does not impose tolerances other than base place elevation on levelness of the erected structural steel frame.
- Structural steel members may or may not be cambered to offset anticipated movement when subjected to the weight of fresh concrete.
The use of Load and Resistance Factor Design (LRFD) procedures and American Society for Testing & Materials-Grade 50 (ASTM - 50 ksi) steel can lighten the floor framing system by as much as one pound per square foot over working stress design procedures. Unfortunately, potential savings in steel tonnage come at a price to the contractor; deflection of the lighter, shallower structural members may be three times that of solutions using working stress procedures and lower grades of steel. To take advantage of the savings afforded by LRFD and Grade 50, while limiting movement of the floor frame under weight of fresh concrete, we recommend limiting maximum dead-load deflection of floor members to member span divided by 240.
The AISC Code of Standard Practice is an industry-generated document that protects steel fabricators and erectors by addressing fabrication tolerances while avoiding the issue of erection tolerances. Certainly accurate fabrication is a necessary component of a level erected structural steel frame, but details of the erection process are just as critical, if not more so. Column splices are fabricated to fit flush, but necessary plumbing of vertical members during the erection process can result in gaps that contribute to differential elevations at beam-to-column connections.
To improve the levelness of beam-to-column connections:
- Survey column splices during erection to establish relative elevations, and
- Use column splice details that allow the contractor to adjust relative splice elevations during erection.
All unshored structural steel floor members deflect under the weight of fresh concrete. When the calculated deflection is sufficient, the engineer requires that the member in question be fabricated with a camber to offset part of the anticipated deflection. Normal engineering practice is to require fabricated camber equal to 70%-to-80% of the calculated dead load deflection. The fabrication tolerance for camber is minus 0" and plus ½" for members up to 50 feet in length. When all these factors are considered, it should be no surprise that unshored structural steel floors are never perfectly level. To control movement of structural steel members, use "loose shores" at mid-span of members to stabilize beams and girders after they deflect as desired under weight of fresh concrete. Removal of shores after the concrete hardens results in a negligible additional composite deflection.
It's all about choices. In a competitive market, a pro-active strategy is the factor that separates the successful designers and builders from those who are plagued with call-backs and claims. It's your choice.
ABOUT THE AUTHOR:
Eldon Tipping is President of Structural Services, Incorporated (SSI). A registered professional engineer in Texas and Louisiana and a Fellow in the American Concrete Institute, he has twenty-eight years experience in the design and construction of concrete floors.
Mr. Tipping pioneered the development of new placement, finishing, and monitoring procedures that make construction of superior on-ground and suspended slabs both possible and practical. As a leading authority on tolerances and construction of suspended floors, he works with owners, design teams, contractors, and testing laboratories to produce surfaces that meet or exceed end-user needs. Tipping is currently a member of ACI Committees 117 (Standard Specification for Tolerances for Concrete Construction and Materials), 302 (Guide for Concrete Floor and Slab Construction-Current Chairman), 360 (Design of Slabs on Grade), and 544 (Fiber Reinforced Concrete).
Mr. Tipping can be reached at: Structural Services, Inc. © 2002 L&M Construction Chemicals, Inc. | ConcreteNews Spring 2002.
© 2002 L&M Construction Chemicals, Inc. | ConcreteNews Spring 2002.