We're making plans to repair a concrete beam that's been damaged by corrosion. The 100-year-old, nine-story building being repaired has a C-shaped footprint that extends through the first eight floors. At the ninth floor a beam spans the open gap in the C. The beam supports masonry walls for the ninth floor, some of the roof loads, and a masonry parapet on the roof. The beam spans 24 feet and is 5 feet deep and 18 inches wide. Much of the concrete has spalled off the underside of the reinforcing steel because of corrosion. We need to remove the bad concrete, clean up the steel, add new steel where needed, and apply a repair concrete. We think we'll have to remove concrete to a height of about an inch above the reinforcing steel for the full length of the beam. Here's the problem. Because the beam is 100 feet high, temporary shoring during repairs would be costly. However, removing all the concrete surrounding the steel may cause beam failure because of a loss of anchorage. The beam ends rest on brick piers, with 20 inches of bearing length at each end. Concrete above the piers is sound, and a cover meter indicates that the tension steel extends through the bearing area. We think we could safely remove and replace small portions of the concrete by starting at each end and working toward the middle. The 20 inches appears to be enough anchorage to permit this approach. But we're also looking for other options. Is there a way to support the beam without using 100-foot-tall towers or shores?
You might try external post-tensioning. The method is sometimes used for permanent repairs but it could also possibly provide temporary support during repair. If there are suitable anchorage points at each end of the beam you could attach prestressing cables at the ends and place a compression member in the middle. Tensioning the cables applies an upward force at the beam midspan. A second approach could be used if there isn't room for a compression member or saddle beneath the beam and if you have access to both sides of the beam. A hole is drilled above the tension steel at the center of the beam. A pipe is inserted in the hole, projecting from both sides of the beam. Ken Bondy of Seneca Construction Systems Inc. in Canoga Park, CA, has repaired several structures using the external post-tensioning method. He says the method could be used for temporary support too. But he suggests that if it's a feasible approach for temporary use, it might be more cost-effective to consider a permanent repair using the method. At perhaps a lower cost than repairing the beam, a facade could be built to hide the cables and pipe or compression member while leaving them permanently in place.