How to Make Earthquake Resistant Concrete?

Closely spaced hoops and stirrups improve ductility.

Closely Spaced Ties and Widely Spaced Ties in Columns

Find some reinforcing bars, bend them into a cage, and place a canary inside. If it flies south for the winter, you need more reinforcing. The canary test is a joking reference to the large amounts of transverse reinforcement needed for reinforced concrete beams and columns in earthquake regions. This seemingly excessive reinforcement is necessary, however, to prevent concrete buildings from collapsing during an earthquake.

Designing for Ductility

 

Without extensive reinforcing, reinforced concrete structural members can behave in a brittle manner. A brittle beam or column deforms only a small amount before fracturing and losing its load-carrying ability. Ductile beams or columns carry loads even after undergoing large deformations (Figure 1). The area under a load-deflection curve indicates the structural member’s ability to absorb and dissipate energy. Earthquakes generate energy that moves from the ground into the structure. If the beams and columns can absorb and dissipate this energy, then the structure can be designed for lower earthquake forces than might otherwise be needed.

These two columns from the same building show the remarkable difference transverse confining steel makes. The column on the left, with widely spaced ties, failed during an earthquake. The column on the right, with closely spaced spiral reinforcement, continued to carry loads even after  an 18-inch lateral deflection.

Seismic design codes allow for the ability of reinforced concrete to absorb and dissipate energy. The static lateral design loads prescribed by building codes for ductile structures are one-third to one-fifth of the earthquake loads to which a brittle structure must be designed. The engineer is presented with two choices:

1. Use lower earthquake loads and design the structure for ductility,
2. Use three to five times the earthquake loads prescribed by codes and don’t design for ductility.

It is more economical to make the structure ductile. Structural ductility is usually assured by making individual beams and columns ductile.

Figure 1. Ductile members continue to carr y loads even after they’ve deformed significantly. Brittle members fracture suddenly and without warning. The area under the load-deflection curve indicates the member’s ability to absorb and dissipate energy.

Bending or flexural ductility is obtained by using hoops or cross ties in columns and closely spaced stirrups in beams. When the beams or columns are overloaded, this transverse reinforcement applies a confining pressure to the concrete core, enabling it to dissipate more energy. Tests have shown that the strength and ductility of the confined core increases with increasing amounts of transverse steel (Figure 2). Circular hoops or spirals apply a more uniform confining pressure than square hoops. If square hoops are substituted for circular hoops or spirals, they must be spaced more closely to provide equivalent confinement.

Figure 2. Hoops come into play when the column is overloaded. They confine concrete in the column core and prevent a sudden collapse. The closer the hoop spacing, the more effective the confinement.

Overcoming Onsite Placing Problems

Engineers tend to be conservative when detailing members for ductility because of the potential loss of life in a collapse. This conservatism results in heavily reinforced sections that often cause a twofold problem for contractors. Not all of the rebar specified by the designer may fit in the space available. And even if it does, the concrete may be difficult to place. To avoid the first problem, designers should make scale drawings of beam-column joint details. The Concrete Reinforcing Steel Institute (CRSI) provides a rebar template
that is useful in making the drawings.
The template shows standard hook details for bar sizes up to #11 at a scale of 1.5 inches equals 1 foot. Using the template, designers can draw each rebar in the joint detail to check for interference among bars and to ensure all the rebar will fit. Even after the beam-column joint details have been inspected and interferences eliminated, concrete placement may still be difficult because of rebar congestion (Figure 3).

Figure 3. Beam-column joints are very congested in earthquake resistant structures. The design engineer should make scale drawings of joint details to ensure that bar interferences are eliminated and that the design is buildable.

However, field personnel should not arbitrarily move ties, hoops, or stirrups to make concrete placement easier. Changing the spacing shown on placing drawings may alter structural behavior of the member. Generally, if bars have to be moved more than one bar diameter, or enough to exceed specification tolerances, the resulting bar arrangement should be approved by the engineer.
Problems with placing concrete in heavily reinforced members can be minimized by proportioning the concrete for ease of placement. For example, these steps may help:

• Reduce maximum coarse aggregate size.
• Use superplasticizers to increase slump.
• Adjust mix proportions to improve workability.

Combined effectively, concrete and reinforcing steel have produced structures capable of resisting severe earthquakes. Proper attention to member ductility while minimizing rebar and concrete placement problems ensure safe, economical reinforced concrete structures in severe earthquake regions.