Report on Flat Slabs

Introduction Reinforced concrete slabs are used in floors, roofs and walls of buildings and as the decks of bridges. The floor system of a structure can take many forms such as in situ solid slab, ribbed slab or pre-cast units. Slabs may span in one direction or in two directions and they may be supported on monolithic concrete beam, steel beams, walls or directly by the structure’s columns. In conventional reinforced concrete, the high tensile strength of steel is combined with concrete's great compressive strength to form a structural material that is strong in both compression and tension. Another method of overcoming concrete’s natural weakness in tension is by a method called pre-stressing. The principle behind pre-stressed concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service. Pre-stressing removes a number of design limitations conventional concrete places on span and load and permits the building of roofs, floors, bridges, and walls with longer unsupported spans. This allows architects and engineers to design and build lighter and shallower concrete structures without sacrificing strength. There are various types of slabs that result in designs and pre-

stressing have corresponding applications in construction and are chosen based on several factors. Economy of construction, lead time, design loads, required spans, serviceability requirements, among other construction requirements are all important. One type of specialized reinforced concrete slab is the flat slab which is most commonly used in concrete-framed buildings. Other types include flat plate, ribbed floor slab, waffle slab, lift slab, spanstress floor system, and slipform method, all of which have their own design characteristics and applications in construction.

Figure 1. Flat slab construction What is a Flat Slab? A flat slab is a reinforced concrete slab supported directly by concrete columns without the use of intermediary beams (Figure __). The slab may be of constant thickness throughout or in the area of the column it may be thickened as a drop panel. The column may also be of

constant section or it may be flared to form a column head or capital. This type of slab features highly versatile elements and are appropriate for most floor situations, including irregular column layouts, curved floor shapes, ramps etc. The benefits of choosing flat slabs include a minimum depth solution, speed of construction, flexibility in the plan layout (both in terms of the shape and column layout), a flat soffit (clean finishes and freedom of layout of services) and scope and space for the use of flying forms. The flexibility of flat slab construction can lead to high economy and yet allow the architect great freedom of form.

Types of Flat Slabs There are four types of flat slabs (Figure _): (1) a simple flat slab that is directly supported by columns; (2) a flat slab with column head, which is supported by a wider base at the areas of the column; (3) a flat slab with drop panels, which had an even wider support at the column so as to increase primarily the perimeter of the critical section, for shear and increasing the capacity of slab for resisting two-way shear; and (4) flat slab with both drop panels and column heads.

Figure 2. Types of Flat Slabs The early reinforced concrete flat slabs all had drops, and columns with capitals, and were considered to be the structure of choice for warehouse construction and heavy loads. Because of the columns capitals and drops, shear was not really a problem. The use of column heads increases the shear strength of the slab and reduces the moment in the slab by reducing the clear or effective span. Drop panels also increase the shear strength of the slab while increasing the negative moment capacity of the slab thereby causing a “stiffening” effect and reducing deflection.

Benefits of Using Flat Slabs Having no beams necessary to support the floor slab has a number of benefits and frees the architect and engineer from a number

of limitations inherent in conventional methods of reinforced concrete construction. Foremost, it allows an architect to introduce partition walls anywhere required. Such flexibility in room layout also allows the owner to change to size or configuration of the room layout. Also, there is no need for false ceilings and finish soffit of slabs, rendering savings

Figure __. Floor-to-floor height reduction

on both labor and materials, as well as opening the volume of the room. Without beams, the height of each level is reduced (Figure _) and may offer significant savings in the overall building height. This would generally result in 10% savings for vertical members, which means there can be an additional floor for every 10 floors as compared to using two-way conventional slabs. The savings in height lead to other economies for a given number of floors, since mechanical features such

as elevator shafts and piping are shorter. There is less outside wall area, so wind loadings may be less severe and the building weighs less, which may bring cost reductions in foundations and other structural components. Construction reduction

is

time another

advantage not only because of

the

reduced

building

height and weight but also because

the

design

will

facilitate the use of simplified table formwork (Figure ) to increase productivity. Also, mechanical

and

Figure __. Simplified formworks for flat slab

electrical

systems are easier to install, further adding to the reduction in construction time as well as installation costs. M&E systems can be mounted directly on the underside of the slab instead of bending them to avoid beams, or hacking through beams Figure __. Holes through beams

(Figure _) where it may be necessary in conventional reinforced concrete slabs.

Pre-fabricated welded mesh may also be used where the sizes are standardized, allowing for better quality control and minimized installation time.

Figure __. Prefabricated welded mesh

The above mentioned benefits allows for standardized structural members and prefabricated sections to be integrated into the design for ease of construction. As such, the structure will have a higher buildable design score since the number of site workers is reduced and the productivity at the site is increased.

When to Avoid Using Flat Slabs Flat slabs (and other beamless slabs) will be at a disadvantage if they are used in structures that must resist large horizontal loads by frame action rather than by shear walls or other lateral bracing. The transfer of moments between columns and a slab sets up high local

moments, shears, and twisting moments that may be hard to reinforce for. In this situation, the two-way slab is the more capable structure because of the relative ease with which its beams may be reinforced for these forces. In addition, it will provide greater lateral stiffness because of both the presence of the beams and the greater efficiency of the beam-column connections. Flat slabs are best applied only in spans of 5 to 9m. While it is possible to use flat slabs for spans over 9m by subjecting it to posttensioning, it is advisable to apply conventional two-way slabs with such longer spans.

Design Considerations

Figure __. Wall and Column Positioning

Walls and columns must be positioned to maximize the structural stiffness for lateral loads. It is should be done to counter later forces in

the absence of beams. Such positioning must facilitate the rigidity of the building towards the center. In order to improve lateral stability, the elevator shaft positioning must work in tandem with the rest of the structural members. It is also possible to add multiple-function perimeter beams to add rigidity as well as to reduce slab deflection. The sizes of vertical and structural members can be optimized to keep the volume of concrete for the entire superstructure (inclusive of walls and elevator core) to be in the region of 0.4 to 0.5 m3 per square meter of the building. This is considered to be the optimum design which is more economical than conventional two-way slab systems. A deflection check must be undertaken to ensure that unsightly occurrence of cracks on non-structural walls and floor finishes do not appear. The test much include all load cases both for short- and longterm basis, and the acceptable deflections should be less than L/250 or 40mm, whichever is smaller. Openings through the floor slab should not encroach upon a column head or drop. Sufficient reinforcement must be provided to take care of stress concentration. In order to fast-track projects where removal of forms at early strength is required, it is possible to use high-strength concrete. Alternatively, 2 sets of forms may be used.

Construction: In Situ vs. Precast For some sites, a flat slab is poured in situ. In this case, the site is prepared, forms for the concrete are set up, and the reinforcing rebar or other materials are laid down. Then, the concrete is mixed, poured,

Figure __. Preparation for in situ flat slab

and allowed to cure before moving on to the next stage of construction. The time required can vary considerably, with size being a major factor; the bigger the slab, the more complex reinforcement needs can get, which in turn adds to the amount of time required for set up. Once poured, the slab also has to be examined and tested to confirm that the pour was good, without air pockets or other problems which could contribute to a decline in quality. In other cases, a flat slab may be prefabricated off site (precast) and transported to a site when it is needed (Figure __). This may be done when conditions at the site do not Figure __. Pre-cast slab

facilitate an easy pour, or when

the conditions for the slab's construction need to be carefully controlled. Transportation of the slab can be a challenge if it is especially large. Barges, cranes, and flatbed trucks may be required to successfully move it from the fabrication site to the site of the installation. Some precast flat slabs are subjected to pre-stressing to render it stronger in both tensile and compressive capacities. Among the advantages include shallower depth (for the same deflection), quicker stripping of shuttering, and greater shear strengths than plain reinforced slabs of the same depth. Pre-stressing is also applied to waffle type slabs to achieve even greater spans.