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Design of Concrete Structure I

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Dr. Ali

Tayeh

First Semester 2009

Design of Concrete Structure I

Lecture 12

One Way Slab

Part 1

Thickness of One-Way Slabs

Fire Rating

This is equal to the number of hours for unexposed surface to rise a set amount usually 120 o C

9.0 cm. 1 hour 12.5 cm. 2 hours 16.0 cm. 3 hours

ACI Sec. 7.7.1 (min. cover for corrosion protection)

( 1.) Concrete exposed to earth or weather.

< f16 = 4.0 cm

> f16 = 5.0 cm

( 2.) Concrete not exposed to earth or weather.

< f32 = 2.0 cm

Min. covers for fire ratings should also be considered.

- According to ACI-Code 7.12.2.1 and for steels yielding at , the shrinkage reinforcement is taken not less than 0.0018 of the gross concrete area, or

Solid slab

Two way slab

One-way slab

Ribbed Slab

Two way slab

One-way slab

Reinforcement

Typical Reinforcement in a one-way slab:

Cutoffs

If requirements for use of ACI Moment Coefficients:

Need to confirm thickness is adequate for one-way shear. Difficult to place shear reinforcement in a slab. Minimum area of shear reinforcement required in slabs if

Usual use

More than two slabs

Two slabs

More than two slabs

Two slabs

- Select representative 1-m wide design strip/strips to span in the short direction
- Choose a slab thickness

- Calculate the factored load by magnifying service dead and live loads according to this equation . The dead load includes own weight of the slab in addition to the covering materials it supports. The live load is dependent on the intended use of the slab.
- Draw the shear force and bending moment diagrams for the strip

Using the ACI-Code approximate structural analysis, design for a warehouse, a continuous one-way slab supported on beams 5.0 m apart as shown in Figure . Assume that the beam webs are 30 cm wide. The dead load is 3 kN/m2 in addition to the slab own weight, and the live load is 2 kN/m2. Use

,

.

O.k.

O.k.

- According to ACI-Code 8.11.6.1, topping slab thickness is not to be less than 1/12 the clear distance between ribs, nor less than 5.0 cm.
- According to ACI-Code 8.11.8, shear strength provided by rib concrete Vc may be taken 10 % greater than those for beams. It is permitted to increase shear strength using shear reinforcement or by widening the ends of ribs
- Ribs are designed as rectangular beams in the regions of positive moment between the supports and as T-shaped beams in the regions of negative moments at the supports. Effective flange width beff is taken as half the distance between ribs, center-to-center

Summary of One-way Ribbed Slab Design Procedure

- The direction of ribs is chosen.
- The overall slab thickness h is determined based on deflection control requirement.
- The factored load on each of the ribs is computed.
- The shear force and bending moment diagrams are drawn.
- The strength of web in shear is checked.
- Design positive and negative moment reinforcement.

Design a one-way ribbed slab to cover a 4 m x 10 m panel, shown in Figure below. The covering materials weigh 2 kN/m2, the live load is 2 kN/m2