Flat Slab Floor System.

A flat slab is a one-way or two-way system with thickenings in the slab at the columns and load bearing walls called ‘drop panels’ Figure 9. Drop panels act as T-beams over the supports. They increase the shear capacity and the stiffness of the floor system under vertical loads, thus increasing the economical span range.

This form of construction has become less popular in recent years because of the limit on economical spans of about 9.5 m for reinforced slabs and about 12 m for prestressed slabs. Reinforced flat slabs may need to be sensibly pre-cambered (not overdone) to control deflection.

The plan dimensions of the drop panels are a minimum of 1/3 of the span in the direction under consideration, usually rounded to the nearest 100 mm. The overall depth of the drop panel is typically taken as 1.75 to 2 times the depth of the slab, again rounded to suit timber sizes or the nearest 25 mm.

The principal features of a flat slab floor are a flat soffit, simple formwork and easy construction. The economical span ‘L’ of a reinforced concrete flat slab is approximately D x 28 for simply supported, D x 32 for an end span and D x 36 for an interior span. Prestressing the slab increases the economical span to D x 35, D x 40 and D x 45 respectively, where D is the depth of the slab excluding the drop panel.

Advantages and Disadvantages of Flat Slabs

Advantages:

• Simple formwork
• No beams—simplifying under-floor services outside the drops
• Minimum structural depth
• Usually does not require shear reinforcement at the columns.

Disadvantages:

• Medium spans
• Generally not suitable for supporting brittle (masonry) partitions
• Drop panels may interfere with larger mechanical ducting
• Vertical penetrations need to avoid area around columns
• For reinforced flat slabs, deflection at the middle strip may be critical.