Case Study

Slab and Column Design for the Great Western Hospital, Swindon

• Analysis of large rectangular in-situ concrete floor slabs

• Reinforcement bar contour plots

• Natural frequency checks

TPS Consult used LUSAS Civil & Structural for the design and analysis of reinforced concrete floor slabs and columns for the new Diagnostic and Treatment Centre at Great Western Hospital in Swindon, UK. Static analysis using the reinforced concrete slab/wall design utility provided reinforcement contour plots for the slabs and also provided design moments and horizontal shears for columns. Dynamic analysis permitted checking of modal frequencies of vibration for slab elements against allowable design code values.

To optimise the time available for analysis and results processing each floor was modelled as a separate subframe with thick beam and thick shell elements representing the columns and slabs/walls respectively. In addition to moment continuity at the interface between slab and supporting elements, the remote ends of columns and walls were typically fixed in translation and rotation, with the upper ends freed in the vertical direction.

In addition to the self weight, partition loads of 1kN/m², services loads of 1kN/m² and live loads of 3kN/m² had to be modelled. The top floor, on which plant rooms are present, required a 7.5 kN/m² live load, with a supplement of 15 kN/m² in two locations where water tanks are located. A single loadcase of 1.4 dead load plus 1.6 live load with all spans fully loaded was adequate for the typical floors, according to UK design code BS 8110:1997. The plant room floor slab however, with its relatively high live load meant that, for this, loading of alternate spans had to be modelled. Full and empty water tanks were also modelled.

As the subframe model consisted of only two storeys, a sensitivity study was conducted by freeing the bases in rotation. This had the effect of concentrating the moments at the slab interface and provided an element of conservatism in the analysis. LUSAS analysis data for the columns was then compared with calculated values. Each column was considered in turn and the most onerous values were used for the detailed design. Walls were designed separately assuming eccentricity moments resulting from the slab interface.

Using LUSAS, Wood Armer reinforcement contour plots were produced for each floor. Contour levels were adjusted to reflect combinations of 12, 16, 20 and 25mm bars with a 10% increase to account for bars being cut when drilling for services installation. Appropriate redistribution was applied to the contour plots to reflect actual concrete behaviour under loading.

With minimum additional effort a modal analysis was performed and highlighted the locations where dynamic effects would be the most adverse. Alistair Mugford, project engineer at TPS Consult explains: "We did this by adding a mass equivalent to the load per unit area and then ran a standard eigenvalue analysis to obtain the natural frequencies which we compared with the appropriate limits for each floor". Conservative vibration frequency limits were obtained from published data – 8Hz for operational floors and 5Hz for the plant room floor. The natural frequencies for the building exceeded the vibration limits in every case – proving the design to be satisfactory.

Alistair Mugford, Project Engineer, TPS Consult

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