Mo Ehsani, president of QuakeWrap, outlines how fiber-reinforced polymers can enhance the axial and flexural capacity of concrete columns. Ehsani offers a look at the history of FRP as a concrete strengthener as well as design examples.




An Economical Solution for Non-Ductile Frames

Many older buildings include columns that require strengthening. Several scenarios could cause this. In coastal regions and aggressive environments, for example, the corrosion of reinforcing steel results in loss of capacity of the columns.

In other cases, the poor quality control during the original construction may have resulted in low compressive strength in the concrete.

The author has been personally involved with the retrofit of two such buildings in Florida, where the concrete compressive strength has been below 1500 psi, only a fraction of the strength specified in the design documents. Some of the collapsed Champlain Tower investigations in Surfside, Florida, have also mentioned the “powder-like” concrete in the columns as a potential contributing factor to that failure.

In yet another scenario, before the late 1970s, concrete frames were commonly designed with the beams being stronger than the columns. When subjected to lateral forces, for example, during an earthquake, plastic hinges can form at the ends of such columns.

In the worst case of weak columns, flexural yielding can occur at both ends of all columns in a given story, leading to the column sway mechanism and collapse of the building. This is shown with the dashed line in Figure 1.

In contrast, when the flexural capacity of the columns exceeds that of the beams, the failure of the frame is more ductile (beam sway mechanism), as shown with the solid line in Figure 1. A large number of plastic hinges that can form at the ends of the beams dissipate significant energy, leading to a more desirable ductile failure.

In 1983, in recognition of this behavior, ACI-318 required the ratio of the sum of the flexural capacities of the columns to those of the beams to be larger than 1.2. It is well recognized that keeping this ratio even larger than this specified minimum improves the frame’s overall performance.