Reinforced CONCRETE ROOF SYSTEM

GROUP 2:

Reinforced Concrete

• Is concrete in which reinforcing bars or other types of reinforcement have been integrated to improve one or more properties of the concrete.

• Can be molded essentially into any shape or form, is inherently rigid, and is inherently fire-resistant.

• Reinforced concrete can provide viable and cost-effective solutions in a variety of applications.

Mechanics of ReinforcedConcrete

• Concrete is a brittle, composite material that is strong in compression and weak in tension.

• Cracking occurs when the concrete tensile stress in a member reaches the tensile strength due to externally applied loads, temperature changes, or shrinkage.

• Concrete members that do not have any type of reinforcement in them will typically fail very suddenly once the first tension cracks form because there is nothing to prevent the cracks from propagating completely through the member.

Reinforced Concrete in

Roofs• The most common types are classified as:

- One-way systems

- Two-way systems

A) ONE-WAY SYSTEM

- It is primarily subjected to the effects from bending (and the accompanying shear), members in which

commonly referred to as Flexural members.

- Members in a one-way system are usually horizontal but can be provided at a slope if needed. Sloped members are commonly used at the roof level to

accommodate drainage requirements.

- The one-way slab transfers the load to the joists, which transfer the loads to the column-line beams (or, girders). This system utilizes standard forms where the clear spacing between the ribs is 30 in. or less.

B) TWO-WAY SYSTEM

- Two-way roof systems transfer the supported loads in two directions. Flexural reinforcement must be provided in

both directions.

- This popular system, which is frequently used in residential buildings, consists of a slab supported by columns.

- The simplest of all floor and roof systems because the underside of the slab is flat, it is commonly used as the ceiling of the space below; this results in significant cost savings.

Insulated Reinforced Concrete Roof Decks• Because of their weight, concrete roofs typically

must be supported by concrete walls.

• In order to assure that the concrete shell structure will resist the imposed destructive forces, it is essential that the roof structure provides:

- a competent structural diaphragm incorporating a cast-in-place reinforced concrete slab.

- or well-connected precast concrete planks in order to develop the full potential of a concrete shell.

•Reinforced Concrete Roof Slabs

Cast-In-Place Construction

• Pan joist construction o refers to a one-way structural system using a ribbed slab

formed with pans. The system achieves economy through the re-use of standard forming pans.

o Standard pan forms produce dimensions of 20 or 30 inches and depths range from 6 to 20 inches, although other sizes are available. Spans normally range from 15 to 50 feet, but may be extended by posttensioning.

o Joists may have openings in the ribs to accommodate mechanical systems. Slabs between the joists can readily accommodate duct openings or sleeves.

Waffles• Waffle plates are coffered flat plates that result in a

two-way structural system. Forming domes are available in standard sizes or may be custom-made to suit specific job requirement s. Repetition of form use permits overall economy.

• Standard 30-by- 30- inch- square domes have depths of 8, 10, 12, 14, 16 or 20 inches. They have 3-inch flanges from which 6-inch-wide joist ribs at 36-inch centers are formed. Standard domes 19 by 19 inch square have a depth of 6, 8, 10, 12, or 14 inches, and 5-inch-wide joist ribs at 24-inch centers are formed from 21⁄2-inch flanges. Waffle plates provide convenient two-way cantilevering. Recesses formed by the domes provide convenient space for lighting fixtures and other mechanical services. Holes may be provided to accommodate mechanical functions within the depth of the slab.

Flat Plates• A flat plate is a two-way reinforced concrete framing

system utilizing the simplest of structural shapes—a slab of uniform thickness. The flat ceiling is economical to form and may be used for the finished surface without additional treatment. The simple forming and two-way structural action permit economical cantilevers and other architectural projections.

• Slabs generally range from 5 to 14 inches in thickness. Spans are up to 35 feet, but may be extended by post- tensioning. Flat plates provide a continuous solid ceiling with complete flexibility for locating partitions and mechanical equipment. Columns need not be in straight lines to accommodate the building arrangement. Only a minimum structural depth is required, thus providing savings in wall height and total enclosed volume. Electrical conduits and ducts may readily be embedded in the flat slab. A flat-plate system is well suited for roof parking or where other heavy loads are anticipated.

Flat Slabs• A flat slab is a two-way reinforced structural system

that includes either drop panels or column capitals at columns. It is essentially a flat plate roof with additional depth near the columns to resist heavier loads, thus permitting longer spans.

• Flat-plate thicknesses, usually 2.5 or 3 percent of the span, are a minimum of 4 inches with drop panels and 5 inches without drop panels. The size of the drop panel is about 33 percent of the span and 25 to 50 percent of the slab thickness. The diameter of the column cap, if required, can be 8 to 10 times the slab thickness. Spans up to 40 feet are normal. Columns should be about equal distance apart. Flat slabs are well suited for heavy roof loads and may be especially useful for roof-top parking. Electrical raceways and conduits may be embedded in the slab and lighting fixtures may be placed within the depth of the drop panel.

For a strong, ductile and durable construction the reinforcement needs to have the following properties:

• High relative strength• High toleration of tensile strain• Good bond to the concrete, irrespective

of pH, moisture, and similar factors• Thermal compatibility, not causing

unacceptable stresses in response to changing temperatures.• Durability in the concrete environment,

irrespective of corrosion or sustained stress

WATERPROOFING OF REINFORCEDCONCRETE FLAT ROOF

• ROOF MEMBRANE SYSTEMS

• The waterproofing membrane is considered to be most important component of the roofing system as it serves the main function of keeping water out of the building.

• Below are some common roof waterproofing membranes:

o a. Liquid-applied membranesLiquid-applied membranes are applied on site in a liquid form which is allowed to set and form into a water impermeable membrane. Bituminous-based (except those containing coal tar) and polymeric-based membranes can be applied when they are cold while those containing coal tar are usually applied when heated. Liquid-applied membranes are seamless, semi-flexible, easy to apply, detail, maintain and repair. However careful supervision and control during application is needed, particularly in ensuring proper curing of concrete, consistent thickness and uniform application.

• b. Pre-formed sheet membraneso One type of pre-formed membranes is the polymer-modified bitumen

membranes and they are applied by heat or attached with an adhesive. These blended or ‘modified’ asphaltic product are bonded to a high strength fabric of polyester or fiberglass and produced into rolls. They have elongation and recovery properties which make them suitable to protect against stresses created by wind, temperature fluctuation and normal structural expansion and contraction of the building. Some of these products are also modified to increase their resistance to fire, thus increasing their fire-rating. Styrene-butadiene-styrene (SBS) rolls are modified with 'rubbers' and compatible with petroleum products. Atactic Polypropylene Polymer (APP) rolls are modified with 'plasticizers' and not compatible with all petroleum-based products, grease and oils.

o Another type of pre-formed membrane is made of Polyvinyl Chloride (PVC). Although PVC is a hard resin, it is modified with the addition of plasticisers to make it more supple and pliable for use as roofing membranes. PVC membranes are mainly produced by either the calendering or extrusion process. In the calendering process, a reinforcement layer of glass fibre or polyester scrim is normally incorporated into the membrane to provide greater strength and dimensional stability. Like all thermoplastic membranes, they turn soft when subjected to heat. Some proprietary membranes are formulated with heat reflective compounds capable of lowering the surface temperature of the roof membrane by as much as 15%.

Failure of steel reinforced concrete

• Reinforced concrete can fail due to inadequate strength, leading to mechanical failure, or due to a reduction in its durability. Corrosion and freeze/thaw cycles may damage poorly designed or constructed reinforced concrete. When rebar corrodes, the oxidation products (rust) expand and tends to flake, cracking the concrete and unbinding the rebar from the concrete

oMechanical failureoCarbonation (or neutralisation)oChlorides o Alkali silica reactionoConversion of high alumina cemento Sulphates

M A T E R I A L S:• Depends on what type of reinforced concrete

roof system

• Important Materials:a) Reinforced Concrete b) Waterproofing materialc) Insulation

• Green Roofing

ADVANTAGES AND DISADVANTAGESAdvantages- Capable of supporting high dead loads

- Classified as a 'stable deck' simplifying waterproofing detailing.

- Long 'design life'.

Disadvantages

- Construction process is slow, requiring specialist contractors.

- Long 'drying out' period, before

Advantages

- Cost efficiency: The high durability preserves maximum value and guarantees minimal maintenance costs.

- Safety: Compact, fully bonded insulation system preventing large-scale

and renovations in the event of a leak caused by a puncture of the roofing membrane

- Functionality: Flexible and easy installation of a gradient through prefabricated tapered slabs.

E X A M P L E S:• NOTRE-DAME DU HAUT Le Corbusier – 1954

Chapel, International Style brought the organic trends of the 19th century back to geometric structure, the shapes are created with reinforced concrete

Villa SavoieBy: Le Corbusier, in France

Habitat 67Montreal, Canada, By : Architect Moshe Safdie

• Habitat 67 comprises 354 identical, prefabricated concrete forms arranged in various combinations, reaching up to 12 storeys in height. These units create 146 residences of varying sizes and configurations, each formed from one to eight linked concrete units

• Each unit is connected to at least one private terrace, which can range from approximately 225 to 1,000 square feet (20 to 90 m2) in size.

The J. Edgar Hoover Building  office buildingLocated at 935 Pennsylvania Avenue NW in Washington D.C. in the United States

• Headquarters of the Federal Bureau of Investigation (FBI)

• The exterior is buff - colored precast and cast-in-place concrete with repetitive, square, bronze-tinted windows set deep in concrete frames.  A massive, three-story roof deck overhanging the main building on E Street, with glass curtain wall-enclosed walkways

Saarinen TWA Building 1962

• This is an airline terminal, first made for TWA, recently remodelled by the new owners Jet Blue. The building is soaring and birdlike, at once dynamic and symbolic .

• The building is made from reinforced concrete

RIZAL THEATRE, Ayala Avenue, Makati City

• Built in 1960, its 40 meter span reinforced concrete folded plate roof was the first and one of the longest spanned structures of this type ever constructed in the Philippines at that time.