Structures: Slabs

Classification of Slabs

• One-way slabs

• Two-way slabs

One-way slabs

• load carried by shorter side

• supporting beams on longer side

• bending done in parallel to shorter side

less steel reinforcement and more slab thickness in comparison to a two-way slab

• uses wood + steel structures

One-Way Slab Bay

Bay determined by

• Width: Limited by how far the floor or roof slabs (the one-way elements) can stretch.

• Length: Limited by the strength of the beams or walls that hold up those slabs.

Two-way slabs

• load carried in x and y direction

• therefore, supports on all side

• bending done in two directions

• more steel reinforcement and less slab thickness (less

concrete) in comparison to a one-way slab.

Ratio for One-way slabs

The ratio of the longer side to the shorter side is ≥ 1.5 – 2.0.

Ratio for Two-way slabs

The ratio of the longer side to the shorter side is ≤ 1.5 – 2.0.

Types of One-Way Slab

• One-way solid slab with (i.e., supported by):

  Load bearing walls.

  Beams.

  Beams and girders.

• One-way solid slab with slab bands.

• One-way concrete joist system (rib slab).

One-way solid slab with walls

One-way solid slab with beams

One-way solid slab with beams & girders

• The slab transfers the load to a beam, which then transfers it to a girder and then to a column.

• It enables longer spans between columns.

One-way solid slab with slab bands

• economical alternative to deeper beams

• The depth of the slab itself may be reduced as well, since with the broader beams, the span of the slab between the beams is lessened

• as reduced beam depths = Savings in floor-to-floor heights

One-way concrete joist system (rib slab)

• Suitable for spans between roughly 5m and 10m

• Slab thickness, including the ribs, is roughly between 25 cm and 60 cm, depending on the span.

Types of Two-Way Slab

• Two-way solid slab.

• Two-way flat slab (plate) (a slab with no dropping elements).

• Two-way flat slab with hollow block (Hourdy).

• Two-way flat slab with drop panels and/or mushroom capitals.

• Two-way concrete joist system (waffle slab).

Two-way solid slab

• advantage = rigid frame action that is made possible by the column-beam interaction for resisting lateral loads.

• disadvantages = are the increased cost of formwork and greater construction depth.

• Span up to roughly 12 m

Two-way flat slab (plate)

• uniform thickness supported directly by columns

without beams or girders.

• due to the simplicity of form, lower floor-to-floor heights,

• flexibility in column placement make flat plates practical for apartment and hotel construction.

• high shear forces around the top of the column require either added conventional reinforcing or a Studrails system

Two-way flat slab with hollow block (Hourdy)

• Very commonly used in the region for residential buildings.

• Masonry replaces part of the expensive concrete and forming ribs.

• Slab thickness between 20cm and 30cm.

• Can be used to roughly 9m span.

Two-way Flat Slab with Drop Panels and/or Mushroom Capitals

• Flat slabs are flat plates thickened at their column supports to increase their shear strength and moment-resisting capacity

Two-way Concrete Joist System (waffle slab)

• Suitable for longer-span, heavily loaded applications, its underside presents rich architectural opportunities as an exposed ceiling.

• The complexity of waffle slab formwork also makes it the most expensive and least used of site-cast framing systems.

One-Way Slab thickness+span

Slab span (shorter direction) is up to 6m

Slab thickness roughly equals span /28

• Minimum thickness is 10 cm