ARE EXAM STUDY MATERIALS (PPD/PDD)


1 way vs 2 way slabs





explanation 1

The columns could be in a lot of locations. Putting them at the corners just maximizes the amount of usable space below the slab. 2 way slabs work by transferring the load in 2 directions. The center of the slab experiences "+" bending (tension on the bottom) and therefore needs reinforcing on the bottom in 2 directions since concrete has essentially no tension capacity. At the columns, the slab experiences "-" bending (tension on the top) and therfore needs reinforcing on the top in 2 directions.


2 way slabs can be cantilevered past columns but the distance is important. Columns can be placed too far towards the center making the cantilever too big or making the distance between columns too small. As a pretty strong guideline, columns need to go in corners.


1 way slabs would primarily need reinforcing along the bottom. Mostly in 1 direction.



explanation 2

this requires an image of the empire state plaza in albany, ny


A one-way slab has its primary reinforcement in the direction of the load. The slab is simply supported between beams or walls. The floors of the tower and four agency buildings are supported by the walls of the core on one side and tied into the columns at the perimeter of the building. One-way slabs do have reinforcement in the other direction or transverse direction to control shrinkage cracking.


Two-way slabs have primary reinforcement in two directions to resist loads in both directions. These are typically supported by columns in each corner. Two-way slabs are usually used to cover large areas and minimize obstructions in spaces like parking garages and factories. Most two-way slabs will be continuous slabs over many rows of columns. The plaza, concourse and underground parking levels (P1 to P3) are all supported by two way slabs They are enhanced by the use of waffle pattern which gets the reinforcement deeper in each slab. There are two to three rebar in the base of each web of the waffle (both directions). The span between columns is 30’-0” with 6 waffles per direction.


The longer the span between columns will require a thicker slab to resist the load and more importantly the deflection.


The questions may not get into this but If a slab is 8” or thicker, the upper part of the slab will need reinforcement to resist shrinkage cracks. The slab will also need primary reinforcement in the top of the slab, over the columns to resist the moment in that area. The slab may also get thicker as it gets close to the column to resist the moment.



slab references

Reinforcement Detailing of Reinforced Concrete Slabs

One way Slabs & Two way Slabs | Spanning of Slabs | One way & Two way Action

Recommended Details for Reinforced Concrete Construction

Notes on One Way Slab Design -Dr. E. R. Latifee

SLAB DESIGN

Difference between one way slab and two way slab

Reinforced concrete design

Slabs

Engineer Boy (youtube video)