undefined | unit 3 design of flat slab

SDD3

UNIT 3Question bank Q1) Define Flat Slabs?A1) The beams reduce the available net clear ceiling height. Hence in warehouses, offices, and public halls sometimes beams are avoided, and slabs are directly supported by columns. This type of construction is aesthetically appealing also. These slabs which are directly supported by columns are called flat slabs. Q2) Draw the example of Flat Slabs and show middle strip and column strip?A2)

Q3) Define drops and column heads?A3)Drops:-The drops when provided shall be rectangular in plan, and have a length in each direction not less than one-third of the panel in that direction. For exterior panels, the width of drops at right angles to the non-continuous the edge, and measured from the centerline of the column shall be equal to one half of the width of drop for interior panels.Column heads:-Where column heads are provided that portion of the column head which lies within the largest right circular cone for pyramid entirely within the outlines of the column, and the column head, shall be considered for design purpose as shown in the figure. Q4) How to calculate thickness of flat slab?A4)The thickness of flat slab:-From the consideration of deflection control IS 456-2000 specifies minimum The thickness in terms of span to effective depth ratio. For this purpose, the largest span is to be considered. If you drop as specified in A is provided, then the maximum value of the ratio of the largest span to The thickness shall be=40 if mild steel is used=32, if Fe-415 or Fe-500 steel is usedIf drops are not provided or the size of drops does not satisfy the specification A then the ratio shall not exceed 0.9 times I.e. =40×0.9=36, if mild steel is used=32×0.9=28.80, if HYSD bars are usedIt is also specified that in no case, The thickness of the flat slab shall be less than 125 mm. Q5) How to calculate bending moment and shear force at flat slab?A5)For this IS 456-2000 permits use of any one of the following two methoda) The direct design methodb) The equivalent frame method The direct design method:-This method has the limitation that it can be used only if the following conditions are fulfilled.

There shall be a minimum of three continuous spans in each direction.

The panels shall be rectangular, and the ratio of the longer span to the shorter span within a panel shall not be greater than 2.

The successive span length in each direction shall not differ by more than one-third of the longer span.

The design live load shall not exceed three times the design dead load.

The end span must be shorter but not greater than the interior span.

It shall be permissible to offset columns a maximum of 10% of the span in the direction of the offsetting net withstanding the provision in II.

Total design moment:- [ IS 456-2000, clause no 31.4.2 pp-55]The absolute sum of the positive, and negative movement in each direction is given by

Where,

= total moment.

W=design load on the area

=clear span extending from face to face of columns, capitals, brackets or walls but not less than 0.65

=length of span in the direction of , and

=length of span transverse to

In taking the values of

the following clauses are to be carefully noted.a) Circular supports shall be treated as squares supports having the same area i.e. squares of size 0.886Db) When the transverse span of the panel on either side of the centerline of support varies,

Shall be taken as the average of the transverse spans. In the figure, it is given by

c) When the span adjacent, and parallel to an the edge is being considered, the distance from the edge to the centerline of the panel shall be substituted for

Q6) How to calculate Distribution of bending moment into negative, and positive moments?A6)Distribution of bending moment into negative, and positive moments [IS 456-2000, clause no 31.4.3 pp-55]a) In an interior span

Negative design moment=0.65

Positive design moment=0.35

b) In an, and span

1) Interior negative design moment=

2) Positive design moment=

3) Exterior negative design moment=

Where is the ratio of flexural stiffness at the exterior columns to the flexural stiffness of the slab at a joint taken in the direction movements are being determined, and is given by,

Where,

= sum of the flexural stiffness of the columns meeting at the joint, and

= flexural stiffness of the slab expressed as moment per unit rotation.

Q7) How to calculate Distribution of bending moment into negative, and positive moments?A7) Distribution of bending moments across the panel width [IS 456-2000, clause no 31.4.4 pp-55]The positive and negative moments found are to be distributed across the column strip in a panel as shown in the table below. The movement in the middle stripe shall be the difference between the panel, and the column strip moments.Table:- Distribution of moments across the panel width in a column strip

SR.NO.	Distributed moment	Percent of the total moment
1	Negative BM at the exterior support	100
2	Negative BM at the interior support	75
3	Positive bending moment	60

Q8) How to calculate slab reinforcement?A8)Slab Reinforcement [IS 456-2000, clause no.31.7 pp-59]a) Spacing:-the spacing of bars in a flat slab shall not exceed two times the slab thickness.b) Area of reinforcement:-when the drop panels are used, The thickness of drop panel for determining the area of reinforcement shall be the lesser of the followingI) The thickness of drop, andII) The thickness of the slab plus one-quarter the distance between the edge of the drop, and the edge of the capital.The minimum percentage of the reinforcement is the same as that in solid slabi.e. 0.12% HYSD 0.15% MILD STEEL Q9) How to calculate Minimum length of reinforcement in slab?A9)Minimum length of reinforcement:-At least 50% of the bottom bar should be from support to support. The rest may be bent up the minimum length of different reinforcement in flat slab should be as shown [ IS 456-2000 fig.16]. If adjacent spans are not equal, the extension of the negative reinforcement beyond each face shall be based on the longer span. All slab reinforcement should be anchored properly at discontinuous edges.

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