In reference to the conventional construction method of two-way flat slab, such need for longer spans and heavier loads necessitate the increased slab thickness for the deflection limit. The increased slab thickness then turns the dead load into the main design load, wherein structural solution would be required to the rhythmic arrangement of voids for the elimination of neutral axis and reduction in dead weight. Not only does the waffle slab system add up to the economical consideration, it also yields to the structural weight and efficiency of materials such as steel and concrete. This attractive structural system brings about speed and versatility in application with its higher stiffness and smaller deflections. Usually a combination of flat flange plate, the system contains an array of equally spaced parallel ribs or grillage in an orthogonal assembly with large square voids or recesses between the ribs. Pods are the square, lightweight, cellular EPS blocks evenly spaced with plastic spacers and constructed edge boards, then reinforced with steel mesh and covered with concrete.
One-way spanning system is called ribbed slab while two-way ribbed system is known as waffle slab. In comparison with the normal reinforced solid flat plate, the reinforced waffle slab system proves to be much more efficient with carrying design loads, hence the various applications on office buildings, auditoriums, hotels, theater halls, vestibules and show rooms where column-void spaces are the main requirement. The attractive soffit appearance from the underside exposure of the slab speaks to the users structurally and aesthetically. Also, consideration of the reactive soil proves to be a significant factor in the construction of waffle slab.
Construction Techniques
Traditional construction technique of waffle slab system includes the storage of materials, establishment of supports and formwork assembly in situ for worker convenience. Consequently, reinforcement is cut and tied followed by the insertion of opening steel cylinder into the reinforcement. Grouting, curing and removal of formwork is steered for the completion of waffle slab system. However, this traditional technique provides an excessive waste of resources such as manpower, machinery and materials wherein the precast technique can alleviate some of the site problems. Initially, material planning is conducted wherein components are modulated for the production at precast plant. After which, the production of precast components speeds up the process into the delivery onsite after completion. Hoisting machinery is required to assemble the waffle slabs onto the construction floor. Accuracy before concrete pouring connection is critically considered as altitude positioning is conducted after the hoisting. Materials, labor and time are factors greatly considered for the economic and structural integrity of the waffle slab system.
Methods of Analysis
Three types of structural analysis can be applied for the waffle slab system. Firstly, Rankine-Grashoff Method is an approximate method used for small span grids with rib spacing not exceeding 1.50 m. This method treats the slab as simply supported on edges, computing moments and shear force per unit width of slab strip. Second, the Plate Analogy Method of Timoshenko requires a rigorous method in consideration of plane stress and orthotropic plate analysis but overall has the same method with Rankine-Grashoff regarding the forces per unit width of slab strip. Finally, the Stiffness Method analyzes the rigid supports and provides closed form solution based on the matrix formulation of structure's stiffness. By use of application software, the simple supports then are studied at a closer distance referencing the similarity with the two methods aforementioned.
What modern and innovative techniques have you incorporated with the construction of waffle slab system? Share your thoughts below!
I have created a parametric Waffle Slab object for ArchiCAD: https://youtu.be/xIsLcNxvyoo
(Click on the gear icon and choose English subtitles.)