FLAT PLATE SYSTEM

Introduction

A flat plate is a one- or two-way system usually supported directly on columns or

loadbearing walls. It is one of the most common forms of construction of floors in buildings. The

principal feature of the flat plate floor is a uniform or near-uniform thickness with a flat soffit

which requires only simple formwork and is easy to construct. The floor allows great flexibility

for locating horizontal services above a suspended ceiling or in a bulkhead. The economical span

of a flat plate for low to medium loads is usually limited by the need to control long-term

deflection and may need to be sensibly.

Flat plate floor system pre-cambered (not overdone) or prestressed.

An economical span for a reinforced flat plate is of the order of 6 to 8 m and for

prestressed flat plates is in the range of 8 to 12 m. The span ‘L’ of a reinforced concrete flat-plate

is approximately D x 28 for simply supported, D x 30 for an end span of a continuous system, to

D x 32 for internal continuous spans. The economical span of a flat plate can be extended by

prestressing to approximately D x 30, D x 37 and D x 40 respectively, where D is the depth of slab.

Advantages of Flat Plate System:

1. Simple formwork and suitable for direct fix or sprayed ceiling

2. No beams—simplifying under-floor services 3. Minimum structural depth and reduced floor-to floor height.

Burj Dubai Flat Plate System – Copyright Imre Solt 2007

Disadvantages of Flat Plate System:

1. Medium spans

2. Limited lateral load capacity as part of a moment frame 3. May need shear heads or shear reinforcement at the columns or larger columns for shear 4. Long-term deflection may be controlling factor 5. May not be suitable for supporting brittle (masonry) partitions 6. May not be suitable for heavy loads.

Flat plate construction

With increasing demand for flexibility in interior layout, the use of flat plate for landed

houses is gaining much popularity amongst architects. The main and unique feature of this

system is that it provides a way for the architect to achieve the concept of high and completely

flat ceiling with no beam protrusion.

Some projects have reported an improvement in the construction speed and cost savings

from using this system which requires only simple formwork. The use of flat plate appeals to

designers particularly because design flexibility is possible through shifting of walls without the

need for columns to be properly aligned. The services can be installed within or below the slab

and there are flexibilities in relocating vertical small penetrations. The soffit is often flat and high

ceiling height can be achieved.

The columns used in this system are either cast in-situ concrete columns or circular steel

hollow sections. When the columns used are steel hollow sections with concrete in-fill, the

desired finish with exposed steel can be easily achieved.

CONNECTION AND DETAILING

The main consideration for steel column connection to flat plate is to ensure that the base

plate for the steel columns are cast into the concrete flat plate. Hence the positioning and

alignment of the base plates are of utmost importance.

If concrete in-fill and column bars are required within the steel hollow section, the starter

bars for the columns have to be placed and fixed in position prior to casting of concrete flat plate.

In the concrete column with flat plate design, the connection is more simplified without

the need for base plate connection. In this case, reinforcement bars should be properly detailed

between the columns and slabs. Punching shear checks are critical and vertical shear

reinforcement should be detailed accordingly.

PUNCHING SHEAR

Flat plate slab system is widely adopted by engineers as it provides many advantages. The system

can reduce the height of the building, provide more flexible spatial planning due to no beams

present, and further reduce the material cost. However, the main problem in practice is the

brittle failure of flat plate slab under punching shear. In this paper, the punching shear behavior

has been studied and an experimental work using carbon fiber reinforced polymer (CFRP) rods

as shear reinforcement has been conducted in flat plate slab system.

This exploratory research is to study the behavior of the flat plate slab with CFRP-rods reinforced

in punching shear zone under constant gravity load and lateral displacements in a reversed cyclic

manner. Three specimens of interior column-slab connection specimens were tested including

one standard specimen without any shear reinforcement, the second one reinforced with CFRP-

rods and the third one reinforced with stud rails as the reference to the second one. The slabs

were 3000 mm long × 2800 mm wide × 150 mm deep, and were simply supported at four corners.

Punching shear failure occurred for the standard specimens at a lateral drift-ratio, lateral drift

divided by the length of vertical member, of approximately 5%. The specimen reinforced by CFRP-

rods had significant flexural yielding and sustained deformations up to a drift ratio of

approximately 9% without significant losses of strength, and punching shear was not observed in

this specimen. The displacements increased up to 1.79 times larger than that of the standard

specimen. And this specimen showed 42% superior ductile performance than the standard

specimen and even the same capability with the stud-rail reinforced specimen. The results of the

experiment indicate that CFRP-rods using in the flat slab has a better foreground.