Tijdsafhankelijke vervormingen in betonnen hoogbouw: Metselwerkgevels op Nederlandse hoogbouw: een gelukkige combinatie?

The application of reinforced concrete in the load bearing structures of high-rise buildings requires awareness of the time-dependent behavior of the material. During the first few years after pouring of the concrete the elements will shorten, caused by shrinkage and creep. This may result in problems concerning non-load-bearing elements, which are connected to the load-bearing structure, such as cracks in masonry façades. The aim of the research is to identify the influence of the structural system on the time-dependent behavior of façade columns. Next the calculations will be used to check how much free scope is needed in a horizontal dilatation joint, in order to be able to cope with deformations of load-bearing structure and masonry outer leaf. To investigate the influence of the structural system on time-dependent behavior, a comparison has been made between core-systems and tube-systems. Those systems theoretically have the largest difference in compressive stresses in façade columns under permanent loads. Weight calculations have been made and the load bearing capacity of the façade columns have been determined. Next the time-dependent deformations during and following the construction phase have been calculated, based on EC2 calculations of shrinkage and creepfactors. The Age-adjusted Effective Modulus Method (AEMM) has been applied, which takes into account elastic and creepstrains, as well als shrinkage. The restraining influence of reinforcement on the deformations has been taken into account by reducing the found deformation by a separately determined reinforcement factor. Calculation of bearing capacities and the loads carried by the façade columns revealed that the theoretical distinction between ‘core’ and ‘tube’ systems is not to be consistently observed in the compressive stresses. Calculations of the time-dependent deformations show deformations (at t=2000 days) varying between 2,2 and 5,7 mm. Also a calculation method is proposed, which enables designers to make an approximation of the deformations that have already taken place at any given time during the construction phase. The choice for a specific structural system looks not decisive for the value of the concrete compressive stresses, which is an important factor in creep deformations. The advice of the KNB for horizontal joints with a free scope of 10 mm. seems enough for Dutch towers, if applied at every story.

Subject

creep
shrinkage
time-dependent behavior
concrete
high-rise
masonry facade
Age-adjusted Effective Modulus Method

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