waffle slab

Waffle slab

Introduction

           Waffle slab or two way joist slap  is a concrete slap made of reinforced concrete that according to Harris,Dictonary of Architecture and construction is “Characterized by ribs in two directions giving it the appearance of a Waffle or Grid”.

            This slap is preferred for longer spans,over systems like flat slab,flat slab with drop panel, two way slabs, one way slabs, and one way joist slabs that are recommended for span 30 feet to 40feet, being much stronger than the systems just mentioned.

 At Construction

          A waffle slab can be made in different ways but generic forms are needed to give the waffle shape to the slab. The forms needed are: Waffle Pods, Horizontal and vertical supports, Cube Junctions, Hole Plates, Clits and Steel Bars. First the supports are built, then the pods are arranged in place, and finally the concrete is poured. The following are the three ways these slabs are manufactured:

• In-Situ: After the framework has been constructed, concrete is poured at the site and later assembled.
• Precast: The slabs are made somewhere else and then brought to the site and assembled.
• Pre-fabricated: The slabs get reinforcements integrated while being manufactured, without needing to reinforce the assembly at the site. This is the most expensive option.

Waffle slab design          

        Different guides have been made for architect's and engineer's to design and select the thicknesses and rib dimensions for the slabs. In the next four diagrams the rules of thumb are explained further.

          Slab depth is typically 3” inches to 5” inches thick. The rule of thumb is: Span/24.

          The width of the ribs are typically 5 inches to 6 inches and have steel rod reinforcements.

             The distance between ribs is typically 3 feet.

          The height of rib and beam should be = .04(span between columns)

          The width of the solid area around the column =1/8(Span between columns) Its height has to be the same as the ribs.

Advantages:

• Better for buildings that require less vibrations, this is managed by the two way joist reinforcements that forms the grid.
• Bigger spans can be achieved with less material, being more economical materially and cost effective.
• It can be, depending on the person, aesthetically pleasing.
• System with greater load capacity.
• Can be implemented with wood, concrete or steel.
• If holes are provided between the ribs, the mechanical systems can be implemented through them. This system is called Holedeck.

Thanks and regards 

Er.Karthiga.S.B.E.M.E.,