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Connections between steel and concrete

Connections between steel and concrete must account for differences in strength and accuracy between the two materials.

Connections between steel and concrete are fundamentally different from normal steelwork connections for a number of reasons:

  • Safe working stresses in concrete are typically in the order 1/10 to 1/15 of those for steel;
  • The accuracy with which in-situ concrete can be formed is much less than for factory fabricated steel;
  • Concrete has negligible tensile strength, and so any tensile forces applied to a concrete element must usually be carried back into the reinforcement.

The connection between a column base and concrete foundation is detailed to distribute the stresses in the concrete and provide adjustment on site for inevitable inaccuracies.

A very common column base connection involves casting holding down bolts into the concrete foundation. The concrete base is poured under typical site conditions, and a high degree of precision is unrealistic. After the concrete has hardened, the top level of the base can be checked, and shims placed so that when the column is erected, it will be placed at the correct level. The bolt boxes allow the bolts to be adjusted sideways to fit into the holes in the baseplate. The bolts serve to locate the column and provide some stability until the frame is braced. They may also be required to resist uplift or bending of the column base, if the building design requires this. Once the column position has been checked, the space under the baseplate can be grouted, and the baseplate can then distribute the high stresses in the steel section over a large area of concrete in bending.

Typical column base connection detail.

Tensile or shear forces in steel-concrete connections can be accommodated by using expanding bolts or resin anchor bolts.

A different type of connection occurs when tension or shear is to be applied to a concrete slab or wall. Expanding bolts can be used to transfer loads into concrete. These bolts have to work at lower stresses and be at much larger centres and edge distances than bolts for connecting steel. Such connections can therefore become very complicated. If fire resistance is not a problem, resin anchors can be used as an alternative to expanding bolts.

  

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