Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
1
Lecture two
Control of Deflection
Three structural systems will be discussed in the course topics: slab-beam, flat plate,
and flat slab systems.
Flat plate is slab without beams, and sometimes its strength is improved by adding drop
panel and/or column capital. Drop panel is increasing the slab depth to have better
flexural strength, and column capital is increasing the column dimensions to reduce the
slab span and have more punching shear resistance.
Flat Plate
Flat Slab
Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
2
The drop panel shall satisfy the limitations of ACI 8.2.4 and 8.2.5 as follows:
When beams support slab, a portion of slab will be included within the beam.
Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
3
Control of deflection is important to choose proper dimensions for the structural system,
such as slab thickness and beam section. For slabs without beams, the minimum slab
thickness to control deflection is found from ACI 8.3.1.1.
Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
4
The use of edge beam in slabs without beams will reduce the minimum thickness to
control deflection by 20%. It should be noted that edge beam is effective if α
f
is not less
than 0.8.
If α
f
≥ 0.8, the edge beam is effective, and
if α
f
< 0.8, the edge beam is ignored.
α
f
is the ratio between beam and slab stiffnesses, and calculated from ACI 8.10.2.7:
For slab with beams, the minimum thickness to control deflection is found from:
Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
5
The edge beam in slab-beam system shall have enough stiffness to use Tab. (8.3.1.2),
otherwise increase the thickness by 10% as stated by ACI 8.3.1.2.1.
Example:
Concrete Design Ali N. Attiyah, Ph.D.
Fourth Year 2015-2016
6