This article refers to the design and construction of reinforced concrete transfer slabs which I designed over the past year. The design of these slabs involved a very thorough analysis procedure as the concrete element must be designed to withstand both the large bending moment forces as well as the very large shear forces created due to the large line loads present along the slab.
It would be informative to start this article by perhaps explaining the way these slabs are designed and constructed in Malta. Construction works in Malta perhaps differ to those as carried out in many other European countries. In most European countries, apartment blocks are often constructed using a frame construction. The frame is usually designed to withstand both lateral and vertical load although the construction of a service core usually helps with the lateral loads. The positions of the columns are usually so as to allow the space in the lower basement levels to function as a parking space and those at the upper levels to function as residential areas.
In Malta, construction methodologies are usually based on a masonry design. The upper storeys are usually designed as apartment units and are constructed using a combination of hollow concrete block work and limestone blocks. The lower levels are usually designed to house parking spaces and are usually constructed using a reinforced concrete frame construction. The slabs at all levels are normally built using reinforced concrete.
As can be appreciated, the basement slab will thus need to support a large load stemming from the heavy masonry walls above as well as the reinforced concrete slabs. This transfer slab is usually designed as a thick element which is supported using the frame construction at parking levels. The transfer slab is usually the costliest element in a traditional Maltese apartment block and thus a meticulous design will save a developer a large sum of money.
When carrying out my design, these slabs could be designed using 1 way or 2 way strips according to BS8110 or EC2 but due to the lack of continuity as afforded by these methods it was felt that this type of analysis would not have been very efficient.
The transfer slabs I worked on were analysed using finite element analysis. The programme used was STAAD Pro 2007 and the slabs were modeled using plate elements. The masonry walls above were modeled using non stiff line elements so as to attract too much load and so as not to allow the slab in those regions to act like stiffer beams. The supports were modeled according the type of support possible on site and within the design. Walls were usually modeled using a pin or fixed connection. The fixed support obviously helped to decrease the deflection of the slab though was obviously harder to achieve onsite. The columns were modeled using vertical beam element and the junction with slab was designed as a thicker region to achieve a more elastic interpretation of the expected construction.
The analysis was carried out using a linear analysis and the long term deflection was carried out in various areas using the EC 2 method to carefully understand the expected behavior of the slab due to creep and shrinkage. The shear forces were analysed and designed in both directions and as expected a large amount of shear reinforcement was required. These slabs are very weak in shear and it was thus important to carry out a meticulous shear capacity design. Through finite element analysis, great savings were carried out as large amounts of steel were only placed in the areas required and thus the slab was more efficient.
David Grima CEng MICE, ICE Member in Malta