slab (by engg.mohammad asim nosherwani buet khuzdar)

PROJECT TITLE : SLAB

GROUP MEMBERS SAJJAD ASLAM

SABIR BALOCH

MOHAMMAD ASIM

WAHEED AHMED

In reinforced concrete construction, slabs are used to provide flat, useful surfaces.

A reinforced concrete slab is a broad, flat plate, usually horizontal, with top and bottom surfaces parallel or nearly so.

It may be supported by reinforced concrete beams by masonry or reinforced concrete walls, by structural steel members, directly by columns, or continuously by the ground.

INTODUCTION

A concrete slab is common structural element of modern buildings. Horizontal slabs of steel reinforced concrete, typically between 4 and 20 inches (100 and 500 millimeters) thick,  are most often used to construct floors and ceilings, while thinner slabs are also used for exterior paving.

TYPES OF SLABS SLAB

ONE WAY

TWO WAY

1. One way slab is supported by beams in only 2 side.2. The ratio of longer span panel (L) to shorter span panel (B)

is equal or greater than 2. Thus, L/B >= 2.3. Main reinforcement is provided in only one direction for

one way slabs.4. One way slabs will behave in the same manner as a

support beam.5. Main steel is only provided parallel to span

ONE WAY SLAB

One Way Slab Bending

• Examples of one way slab…Shades in the roofing system(cantilever). Slab of stairs. Footings.

Check whether the slab is one-way or two-way. Calculate h(min).

◦ Not less than 110 mm for rooms◦ Not less than 75 mm for sunshades

Calculate dead load acting on the slab◦ Calculate dead load acting on the slab

Calculate live load acting on the slab. Calculate total factored load per unit strip (KN/m)

Design procedure for One Way Slab

Calculate the moments either directly (simply supported) or by using coefficient for continuous slabs

Calculate effective depth.d  = h – (20 + (½)db)db = 10, 13, 15 generally used

Minimum Slab Thickness: For solid nonprestressed slabs overall slab thickness h, shall

not be less than the limits in the table.

One Way Slab (Design Limits)

Support Condition Minimum h

Simply Supported l/20

One end continuous l/24

Both end continuous l/28

Cantilever l/10

 Two way slab is supported by beams in all four sides. The ratio of longer span panel (L) to shorter span panel (B) is

less than 2     Thus, L/B < 2. Main reinforcement is provided in both the directions for two

way slabs. Two way slabs will reduce bending and shifting.

TWO WAY SLAB

3D MODEL OF TWO WAY SLAB

TYPES OF TWO WAY SLAB

ADVANTAGES OF TWO WAY SLAB

Flexibility in room layout

Reinforcement placement is easier

Ease of Framework installation

Building height can be reduced

Less construction time

DISADVANTAGES OF TWO WAY SLAB Span length is medium.

Not suitable for supporting brittle (masonry) partitions

Higher slab thickness

Critical middle strip deflection

For each applicable factored load combination, design

strength at all sections shall satisfy:1. ∅Sn≥U2. ∅Mn≥Mu3. ∅Vn≥Vu

DESIGN STRENGTH

The Direct Design method consists of a set of rules for distributing moments to slabs and beam sections to satisfy requirements and most serviceability requirements simultaneously. These fundamental steps are involved as follows:

Determination of the total factored static moment. Determination of the total factored static moment to negative

and positive sections

Distribution of the negative and positive factored moments to the column and middle strips and to the beams, if any.

DESIGN OF TWO WAY SLAB BY DIRECT DESIGN METHOD

LIMITATIONS There shall be a minimum of three continuous spans in

each direction.

Panels shall be rectangular, with the ratio of longer to shorter span not greater than two.

All loads shall be due to gravity only and uniformly distributed over an entire panel. The unfactored live load shall not exceed two times the unfactored dead loads.