Understanding the Preliminary Sizing of Concrete Elements

When it comes to designing concrete structures, estimating the preliminary sizes of concrete elements is a crucial step. This involves determining the loads that the structure needs to support and calculating the dimensions of concrete members and the required reinforcement area.

Importance of Self-Weight in Design

One significant load that structures must support is their own weight. Calculating this self-weight requires the dimensions of the concrete member. The design’s purpose is to establish the geometrical dimensions and the required reinforcement area. Therefore, a rapid preliminary sizing of concrete members is essential to kickstart the design process.

Preliminary Sizing Methods for Concrete Elements

One-way Slab

• The span of the slab is obtained from the building’s plan.
• The thickness of the concrete one-way slab is assumed.
• Table-1 provides minimum thickness values, preventing the need for deflection calculations.
• Alternatively, the smaller span of the slab divided by 40 can be used, ensuring a minimum thickness of 7 cm for common floor slabs and 12 cm for slabs supporting vehicle traffic.

Table-1: Minimum one-way Solid Slab Thickness, as per ACI 318-19

Support condition	Minimum, h
Simply supported	ℓ/20
One end continuous	ℓ/24
Both ends continuous	ℓ/28
Cantilever	ℓ/10

• Note: Adjust values based on steel yield stress and concrete type.

Composite Slab

• For lightweight and normal weight concrete composite slabs, preliminary thickness is obtained by adjusting values from Table-1.

Prefabricated and Ribbed Slab

• Preliminary thickness is assumed by dividing the slab’s smaller span by 20.
• Unified Building Code (UBC) specifies the minimum thickness for ribbed slabs.

Two-way Solid Slab

• For a simply supported two-way concrete slab, a rule of thumb suggests a thickness of 4 cm per meter of span.
• Alternatively, divide the perimeter of the slab panel by 180 for a preliminary thickness.

Slab with Embedded Objects

• Minimum thickness recommendations for slabs with embedded conduits and pipes are specified by UBC and ACI 318-14.

Slab on the Ground

• UBC recommends a minimum thickness for concrete floor slabs supported directly on the ground.

Drop Panel

• Drop panels in flat slabs enhance shear strength and have specific dimensions based on span length.

Beams

• ACI 318-19 provides minimum height values for non-prestressed concrete beams.
• Beam width is assumed to be 0.3 to 0.6 times the beam height.

Minimum height of non-prestressed beam, as per ACI 318-19

Support condition	Minimum, h
Simply supported	ℓ/16
One end continuous	ℓ/18.5
Both ends continuous	ℓ/21
Cantilever	ℓ/8

• Note: Adjust values based on steel yield stress and concrete type.

Columns

• The size of the column is governed by imposed loads.
• Specify a reinforcement ratio based on minimum and maximum values.
• Equations are provided for determining column area.
• Alternatively, dimensions can be selected based on axial load.

Dimensions of a Column Based on Axial load

Equivalent axial column load with moment factor (KN)	Column size (mm)
Up to 500	230 x 230
Above 500 to 800	230 x 300
Above 800 to 1200	230 x 450
Above 1200 to 1500	230 x 600
Above 1500 to 1950	300 x 750

Shear Wall

• Shear wall thickness varies based on factors such as the number of stories and building age.

Footing

• The minimum depth and width of footings are specified, and depth can also be determined using Rankine theory.

Raft Foundation

• The minimum thickness of a raft foundation is set at 300mm.