It Shrinks?!

February 20, 2009

It Shrinks?!

Although we may think of concrete as being very rigid, hard and solid, substantial changes occur throughout the life of a concrete element. The curing process is actually the result of a chemical reaction which continues long after the concrete is cast and the structure is placed in service. In practical terms, this means that concrete members in compression will shorten over time, even if no new load is applied. These effects are called creep and shrinkage.

Lately, I’ve been digging deep into the subject. Estimating the shortening of various parts of a building is important, because we would like the floors in the building to be level at some point down the road. If the walls and columns are erected to the same elevation, creep and shrinkage of the wall will quickly mean that all the floors slope toward the center of the building. Therefore walls are typically cast at a higher elevation. We have to be careful though, because overcompensation will mean that the floors all slope to the outside.

Typical for commercial buildings in Chicago. At the core of the building is a rectangular concrete wall that is designed to resist most of the wind force applied laterally to the building. For efficiency, the perimeter columns and floor framing are steel.

Steel doesn’t creep or shrink. It does, however, shorten elastically due to applied loads. The term elastic means that if the loads were removed, the column would spring back to its original length. I was involved in a project a couple of years ago in which we investigated a sudden snapping of several bolts at the top of a building. There seemed to be no explanation for the sudden failure until we learned that a major tenant had moved out of the building over the weekend – taking tons of paper files with them. We surmised that they removed enough load from the building that some of the columns rebounded up to ½”, enough to shear the bolts in question.

Concrete also shortens elastically, but because core walls are so large, this effect is much less pronounced. Creep and shrinkage effects, on the other hand, can result in approximately 1/8” shortening per floor after ten years.

Estimating creep and shrinkage requires some fairly complicated algorithms. I used a spreadsheet developed over time and adjusted based on observations during previous projects. Concrete properties, the construction sequence, and even the relative humidity affect the outcome of the calculation. To handle the time dependent loops required in the programming, the spreadsheet developers had to create Visual Basic macros. Given the complexity of the spreadsheet, we decided that it would be necessary to compare results with several similar buildings that had been designed using similar methods.

One of the other buildings in our database was notable because the estimated analytical values for creep and shrinkage were compared against field measured data. While addressing some maintenance concerns, the engineers observed scrapes in the elevator guide rails in this building. The scrapes were caused by brackets that were mounted into the floor slabs. The elevator guide rails were supported directly on the foundation, so they would not shorten over time. The scrape indicated the distance that the wall shortened. There was a satisfactory correlation between the observed shortening and the analytical model. This gave us a fair amount of confidence in the calculations.

At the end of the day, we compared the shortening of the steel columns to the shortening of the concrete core wall at various times in the building’s life. Eventually the building will reach equilibrium. With that information in mind, the tenants can gauge what accommodations, if any, need to be made in building-out their interiors.

Yes. It shrinks, and that’s completely normal.

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