Composite Continuous Beam: Loads and Deflections (Eurocode 4)
Composite construction methods are extremely important to take into consideration when going about designing composite beams. The two types of construction methods that need to be considered are either fully propped or unpropped/partially propped constructions.
A fully propped construction method means that the concrete is cast in situ and that the supporting props along the length of the beam are kept in place until sufficient curing (Usually 28 days or until at least 75% of the total concrete strength has been obtained) of the concrete has occurred. An unpropped or partially propped construction method means that either some or none of the props along the length of the beam are kept in place and that the permanent formwork is used to allow the concrete to cure.
The two different types of construction methods have various advantages and disadvantages. Both methods are extensively used in the industry, and it is important to consider the chosen construction method when designing composite beams. When a construction method has been chosen, the following different types of loading need to be considered for the different stages in the design:
Combination of actions for serviceability limit states
If the steel beam at the construction stage is unpropped, the deflection calculation is performed separately for the construction stage and the composite stage. The construction stage deflection is calculated as follows:
If the steel beam is fully propped, the total deflection of the composite beam is obtained by summing the following deflections: