One Way Slab and Two Way Slab
Slab is constructed to provide flat surfaces, usually horizontal, in building floors, roofs, bridges and other types of structures. in a civil language one-way slab stated One-way slab is a slab which is supported by beams on the two opposite sides to carry the load along one direction. One way slab is supported by beams in only 2 sides. One way slabs will behave in the same manner as a support beam. One way slab bends or deflects in a direction perpendicular to the supporting edges. one way slab has structural strength in the shortest direction. The main reinforcement is provided in only one direction for one-way slabs.
Compare Difference Between One Way Slab and Two Way Slab
One way and two-way slabs as floor systems are the most used forms in construction works. in particular of structural, reinforced thin plate elements are supported over beams. Beam and slab arrangement can be the best example for one way and two-way slabs. Each single slabs are referred to as panels, if the slab is continuous over supports. A slab or panel is said to be two-way slab if the ratio of longer span to the shorter span is not greater than 2 (two). Hence, one way slabs have ratio between longer to shorter span greater than 2.
One-way slabs span in shorter direction and is designed for one way bending, in other hand two-way slabs span in both directions. Load distrubutions and behavior of two way slab slab supported over masonry wall/beam on both sides is expected to bend in the shorter direction due to its self weight. One way slabs are the easiest to design as the direction is simple and are usually designed as set of beam strips spanning in one direction. one way slabs are designed in per meter strips. System with of only one panel, resting over wall, is analyzed & designed as a simply supported beam of 1m width.
The slabs that are support on its all four edges are basically divided as one way and two way slabs based on this load path. If the loads can be distributed to all four edges of the slab it is two way spanning slab. But even if the slabs are supported on all four edges and a huge fraction of the total load can be transferred mainly in one direction, that slab can be assumed as one way spanning slab. If the is supported on two opposite sides, it is called a one-way spanning slab. In this type of slab, loads are transferred on two opposites.
The slab thickness should be sufficient against shear, although shear is usually not a problem in two-way slabs subjected to uniformly distributed loads.For one way slab, the main reinforcement is calculated by formula (In limit state design) that is determined by comparing compressive force as well as tensile forces.
One Way Slab
One way slab : – If the support is only on the two sides, then that slab is known as One way slab. Load is carried in the perpendicular direction of the supporting beam. If the ratio of longer span panel (L) to a shorter span panel (B) is equal or greater than 2. Thus, L/B >= 2. One way slab is designed in shorter direction so that bending moment & shear force are least. Main reinforcement is provided only in one direction.
Two Way slab
If the slab is supported from from sides then it is known as Two-way slab. Load will be carried in both the directions. The ratio of longer span panel (L) to a shorter span panel (B) is less than 2. Thus, L/B < 2. Main reinforcement is provided in both the directions.
Analysis & Design of One-way Slabs & Two-way Slabs
One way slabs are the easiest to design as the direction is simple and are usually designed as set of beam strips spanning in one direction. For the simplicity in design, one way slabs are designed in per meter strips. System with of only one panel, resting over wall, is analyzed & designed as a simply supported beam of 1m width. Depth of the beam is taken as slab depth. Continuous slabs, continuous over supports in one direction are designed as continuous beam of a frame. Maximum moment of simply supported slab is at mid-span under self weight and uniform pressure loads. Design reinforcement will be Bottom reinforcement and is provided in shorter direction.
One Way Slab Effective Depth:
Effective Depth (d) For deflection control
L/d = 20 X M.F
M.F. Modifiction factor from— IS: 456, p.38.Fig-4
Assume % steel 0.3 to 0.6%
Fs = 0.58 Fy X (Ast requierd / Ast Provied)
Initially assume that Ast reqierd = Ast Provided
Fy = 250 N/ Sq.mm —– Fs = 0.58 X 250 = 145 N/ sq.mm.
Fy = 415 N/ Sq.mm —– Fs = 0.58 X 415 = 240 N/ sq.mm.
Fy = 500 N/ Sq.mm —– Fs = 0.58 X 500 = 290 N/ sq.mm.
Check for Deflection:
Allowable L/d = 20 X M.F.
M.F is Obtained from IS:456-200 P-38 Fig 4
Find actual, L/d
If Actual L/d < allowable L/d ———- Ok
One Way Slab Two Way Slab Load Calculations
Total Load = D.L. + F.L. + L.L.
Dead load of slab = (d x 25)
Floor Finishing load = (as floor finishing near 1 kn/sq.mm)
Live load = ( as per calculation)
Factor Load = 1.5 x Total load
Main reinforcement spacing:
Maximum spacing for main steel reinforcement in one way slab according to ACI 7.6.5 is minimum of:
3hf = 3 × 6 =18″
18″ z Finally use, #3 @ 9″ c/c.
One Way slab, Two Way Slab Reinforcement detailing of a slab is done based on its support conditions.