Gujarat Technological University

Topic:--Special Structures(Tall Structures, Prestressed Concrete & Demolition)

Subject:-ACE(2160601)

Prepared by,

Sujith Velloor S.

Lecturer

Civil Department

Tall Structures

Usually structure or building having height more than 80m is considered as a tall structure.

Generally tall structure may be defined as one that because of its height it is affected by lateral.

Classification: 1. Multi storeyed residential building.

2. Multi storeyed commercial building.

3. Tall chimneys.

4. Transmission Towers

5. Cooling towers

Materials used in construction

1. Concrete

2. Bricks

3. Glass

4. Metals

5. Plastics

6. Natural and Manmade Fibers

Structural System

Moment Resisting Frames

Shear Wall-Frame Systems

Shear Truss-Outrigger Braced Systems

Framed-Tubes

Tube-in-Tube Systems with interior columns

Bundled Tubes

Truss Tubes without interior columns

Modular TubesShear wall-Frame, IBM Tower, Kansas City, MO

Boatmens Tower, St. Louis, MO

Composite System, Houston, TX

Partial Tube System, Southfield, MI

Partial-Tube, Athens, GA

Mercantile Bank Tower, Kansas City, MO Torre Infinito, Caracas Venezuela

Structural System

Methods of construction

Basic Aspects are Planning, Designing & Construction

Mainly tall building construction is divided into four stages

1. Foundation laying

2. Underground structure laying

3. Aboveground structure erection

4. Roofing, finishing works..

World Trade Center New York 1973-2001

First buildings to be tested in a turbulent

boundary-layer flow wind tunnel (mid 1960s)

Prestressed Concrete

Prestress is defined as a method of applying pre-compression to control the stresses resulting due to external loads below the neutral axis of the beam tension developed due to external load which is more than the permissible limits of the plain concrete.

Advantages 1. The use of high strength concrete and steel in prestressed

members results in lighter and slender members than is possible with RC members.

2. In fully Prestressed members the member is free from tensile stresses under working loads, thus whole of the section is effective.

3. In Prestressed members, dead loads may be counter-balanced by eccentric prestressing.

Advantages 4. Prestressed concrete member posses better resistance to shear

forces due to effect of compressive stresses presence or eccentric cable profile.

5. Use of high strength concrete and freedom from cracks, contribute to improve durability under aggressive environmental conditions.

6. Long span structures are possible so that saving in weight is significant & thus it will be economic.

7. Factory products are possible.

8. Prestressed members are tested before use.

9. Prestressed concrete structure deflects appreciably before ultimate failure, thus giving ample warning before collapse.

10. Fatigue strength is better due to small variations in prestressingsteel, recommended to dynamically loaded structures.

Disadvantages

1. The availability of experienced builders is scanty.

2. Initial equipment cost is very high.

3. Availability of experienced engineers is scanty.

4. Prestressed sections are brittle

5. Prestressed concrete sections are less fire resistant.

Terminology

1. Tendon: A stretched element used in a concrete member of structure to impart prestress to the concrete.

2. Anchorage: A device generally used to enable the tendon to impart and maintain prestress in concrete.

3. Pretensioning: A method of prestressing concrete in which the tendons are tensioned before the concrete is placed. In this method, the concrete is introduced by bond between steel & concrete.

4. Post-tensioning: A method of prestressing concrete by tensioning the tendons against hardened concrete. In this method, the prestressis imparted to concrete by bearing.

Tensioning Devices

1. Mechanical devices: The mechanical devices generally used include weights with or without lever transmission, geared transmission in conjunction with pulley blocks, screw jacks with or without gear devices and wire-winding machines. These devices are employed mainly for prestressing structural concrete components produced on a mass scale in factory.

2. Hydraulic devices: These are simplest means for producing large prestressingforce, extensively used as tensioning devices.

3. Electrical devices: The wires are electrically heated and anchored before placing concrete in the mould. This method is often referred to as thermo-prestressing and used for tensioning of steel wires and deformed bars.

4. Chemical devices: Expanding cements are used and the degree of expansion is controlled by varying the curing condition. Since the expansive action of cementwhile setting is restrained, it induces tensile forces in tendons and compressive

Classifications and Types

Prestressed concrete structures can be classified in a number of ways depending upon the feature of designs and constructions.

1. Pre-tensioning: In which the tendons are tensioned before the concrete is placed, tendons are temporarily anchored and tensioned and the prestress is transferred to the concrete after it is hardened.

2. Post-tensioning: In which the tendon is tensioned after concrete has hardened. Tendons

are placed in sheathing at suitable places in the member before casting and later after hardening of concrete.

In post-tensioning the concrete unit are first cast by incorporating ducts or grooves to house the tendons. When the concrete attains sufficient strength, the high-tensile wires are tensioned by means of jack bearing on the end of the face of the member and anchored by wedge or nuts.The forces are transmitted to the concrete by means of end anchorage and, when the cable is curved, through the radial pressure between the cable and the duct. The space between the tendons and the duct is generally grouted after the tensioning operation.

Demolition

Demolition

The action or process of destroying(demolishing) the building or other structures.

In congested area, in particular, the quality of demolition technique becomes an essential element which determines the success of revitalization of city.

In addition to efficiency in demolition, strategies must be adopted to avoid noise, vibration and dust which affect the surrounding environment and there must be efficient disposal of waste products.

Factors affecting evaluation of demolition (a)Access and safety.

(b) As built details of the structure.

(c) Confirmation of presence/absence of prestressing.

(d) Necessity for initial exploratory work or reinforcement cover checks to locate reinforcement.

(e) Equipment and methods to be used.

(f) Method statements required.

(g) Sequence of operations.

(h) Possible temporary support requirements.

(i) Possible protection requirements.

(j) Environmental compliance.

(k) Disposal of materials.

(l) Consultation with the design office.

Classification of Demolition Methods..

1. Simple Demolition by machine.

2. Hammering: (a) Stone Chisel.

(b) Hand hammer

(c) Large size hammer

3. Hitting:

(a) Steel Ball

(b) Balling Machine

4. Jack up by Hydraulic jack

5. Breaking by Hydraulic Breaker

(a) Hydraulic breaker

(b) Pile crusher

6. Abrasion

(a) Hand hammer drill

(b) Large size hammer drill

(c) Diamond boring machine

(d) Diamond disc cutter

(e) Diamond wire saw

(f) Water jet

7. Bursting

(a) Buster with wedge

(b) Chemical expansive demolition agents.

(c) Water gun

(d) Gas cylinder

(e) Mild explosive.

(f) Explosives

8. Melting.

(a) Thermal Lance.

(b) Fire oil flame.

(c) Plasma.

(d) Laser beam

Methods of Demolition

1. Demolition by human operatives

2. Deliberate collapse method

3. Using of applied pull/pushing forces

4. Using Wrecking Ball.

5. Use of powered machines.

6. Using saw cut method

7. By implosive (Explosive)

Safety Measures

1. Environmental protection

2. Safety of workers.

3. Safety of surrounding peoples and buildings.

4. Safety of animals.

5. Safety of electric lines, water lines, cables etc.