The premise behind the designs that make up the reinforced concrete structures of today actually began in 1867 when Joseph Monier exhibited reinforced concrete planters at the Paris Exhibition. Monier obtained his first patent on July 16, 1867, on iron-reinforced troughs for horticulture. In 1875, the first iron-reinforced concrete bridge was constructed at the Castle of Chazelet, France; Monier served as the designer. The first reinforced concrete skyscraper in the U.S., the Ingals Building, was built in Cincinnati in 1903 using reinforcing steel made of twisted square iron rods.
Over the decades, the design and complexities of reinforced concrete structures have evolved to where concrete structures are designed with ever increasing reinforcing and concrete design requirements. This is especially true in areas of seismic design considerations.
As building designs become more complex and the engineer looks for innovative design characteristics, it is important that both design engineer and contractors involved recognize the design and construction tolerances upon which both parties need to be cognizant of in order to prevent constructability issues.
With the use of large bar diameters, rebar hooks, and tie hooks (even with the use of T-heads to reduce congestion), the actual reinforcing geometry may be significantly more congested than what would appear on the structural and fabrication shop drawings. This type of unseen congestion not only can have an impact upon the reinforcing contractor, but also upon the concrete contractor in its ability to obtain proper concrete consolidation, even with the use of special aggregate sizes and mix designs. Even with the specified 90-degree hooks replaced with T-heads, there is still significant congestion.
American Concrete Institute (ACI) 318 Section 21.2.3 states: “In selecting member sizes for earthquake-resistant structures, it is important to consider problems related to congestion of reinforcement. The designer should ensure that all reinforcement can be assembled and placed and that concrete can be cast and consolidated properly. Use of upper limits of reinforcement ratios permitted is likely to lead to insurmountable construction problems especially at frame joints.”
The ACI standard found in ACI 315 provides standards for both the development of the structural drawings as well as the placing drawings.
ACI 315-99 Section 2.10 states: “It is important for the architect/engineer to examine the reinforcing steel layouts carefully in three dimensions and give the detailer the proper information. This examination will show congestion at beams-column joints of beam, column, and hoop reinforcement. Large scale drawings, models, or mock-ups of joint details may be worthwhile to ensure that a design can be assembled and concrete can be placed.”
Bars intersecting in three planes require adequate clearance. Concrete Reinforcing Steel Institute (CRSI) “Manual of Standard Practice” 4.6.8 states: “When beams and joints or slabs intersect in the same plane, indicate clearances required to allow passage of reinforcing bars. Bar clearances should also be indicated at intersections of beams and columns.”
The responsibility of identifying potential constructability problems often is not solely that of the architect/engineer. As the complexity of structural designs increases, the project specs often will have requirements for the detailer in Section 3200 to identify areas of congestion, thus placing some burden upon the project’s reinforcing detailer. The advent of 3-D modeling and BIM is helping to alleviate these issues, but its usage is still new and often project time constraints may prevent its full use.
Both ACI and CRSI address the importance of fabrication and installation tolerance for the architect/engineer, fabricator, and installer on the project. These ACI specifications provide the standards that the project parties are required to follow. Often these tolerances conflict with each other and early recognition will reduce potential problems.
Early identification and application of the ACI codes and CRSI standards can alleviate potential problems before they impact construction.
Robert Risser, PE, is president and CEO of the Concrete Reinforcing Steel Institute (CRSI), Schaumburg, Ill.
Michael Hoffman is principal of Construction Consulting, Lutz, Fla., and serves as a placing consultant for CRSI.