Good Construction Practices-Joining Walls to Roof

Good Construction Practices-Joining Walls to Roof

Connecting walls to roof structure. Join walls and roof to strengthen each other.

·       Column reinforcement has to be protruded from the top of columns and be bent around roof trusses for structural strength, or roof trusses should be strapped with metal ties to the wall structure.

·       Exposed metal must be painted with rust proof paint to avoid corrosion.

An example of binding roof trusses to building structure. Roof trusses should be constructed over columns, and for a concrete column the reinforcement should protrude at the top and be bent over roof trusses to join the roof structure with the wall structure. Metal straps or plates can be used for this role, particularly where the building structure is made of wood.

A good roofing example of column steel reinforcement bent over roof truss to tie house structure to the roof structure. This is an essential detail in areas subject to earthquake and to winds, to keep the roof on the house when a natural hazard strikes the structure. Such exposed steel should be painted with rust-proof paint to minimize corrosion.

 Example of good roofing practice. Steel roof trusses are connected to columns through welding of truss members to column reinforcing steel and purlins are welded with ties to roof trusses. This will hold the roof firmly on the house in storms or other hazards from nature.

 Example of incorrect roofing practice. The roof structure is good, but no connection is made between roof trusses and the building structure. The roof is resting on the top of the house walls and is subject to moving with high winds or other forces of nature.

The Deepest Foundation on the Planet!!!![MUST WATCH]

THE DEEPEST FOUNDATION ON THE PLANET

This record belongs to the famous “Petronas Towers”, also known as the Petronas Twin Towers located in Kaulalampur, Malaysia.These are twin skyscrapers & they held a record for the tallest building in the world from 1998 to 2004.This record was surpassed by Taipei 101 in 2004.

It was designed by Argentine architect Cesar Pelli. The Petronas Towers has a tube in tube structural system, patented by Fazlur Rahman Khan.

During geological investigation, when test boreholes were made it was found that the original construction site laid on a cliff edge.

One half of the site was soft rock while the other half was decayed limestone.Hence the entire site was relocated by 61 meters so that the structure completely rests on soft rock.

Since the depth of bedrock was huge, the buildings had to be built on “world’s deepest foundation”. . The concrete raft foundation, comprising 13,200 cubic meters of concrete was continuously poured through a period of 54 hours for each tower.104 concrete piles, ranging from 60m to 114m, were bored into the ground

The raft is 4.6 meters thick, weighs 35,800 tons and held the world record for the largest concrete pour until 2007. The foundations were completed within 12 months.

Due to the high cost of importing steel, the towers were constructed using super high-strength reinforced concrete.High-strength concrete is twice as effective as steel in sway reduction. But, it makes the building twice as heavy on its foundation as a comparable steel building.

Towers are supported by 23m x 23m concrete cores and an outer ring of widely spaced super columns, the towers use a sophisticated structural system which provides 560,000 square meters of column-free space.

Properties of a Good Brick

Properties of a Good Brick

A brick is said to be good quality if it satisfies following conditions

• Colour: A good brick must have uniform & bright colour.

• Shape: Bricks should have sharp and true right angled corners. They should have plane faces.

• Texture: They should not possess fissures, cavities, unburnt lime & loose grit. They should possess fine, dense and uniform texture.

• Size: Bricks should be of standard sizes as prescribed by codes.

The size of the brick is of 90 mm × 90 mm × 90 mm and 190 mm × 90 mm × 40 mm.

With mortar joints, the size of these bricks are taken as 200 mm × 100 mm × 100 mm and

200 mm × 100 mm × 50 mm.

• Strength: Crushing strength of brick should never be less than 3.5 MPa. A field test for strength is that when a brick is dropped from a height of 0.9 m to 1.0 m on to the hard ground, it should not break into pieces.

•  Soundness: When struck by the hammer or by another brick, it should generate a metallic or ringing sound.

• Hardness: The brick should not show any impression when scratched on its surface using fingers.

• Water Absorption: After immersing the brick in water for 16 hours, water absorption should not be more than

·       20 % by weight for first class bricks

·       22.5 % for second class bricks

·       25% for third class bricks

• Efflorescence: Bricks when soaked in water for 24 hours, it should not show deposits of white salts when allowed to dry in shade. White patches are due to the presence of sulphate of calcium, magnesium and potassium. They keep the masonry permanently in damp and wet conditions.

• Thermal Conductivity: Bricks should have low thermal conductivity, so that buildings built with them keep it warm in winter & cool in summer.

• Fire Resistance: Fire resistance of bricks is usually good. Hence bricks are sometimes used to encase steel columns in order to protect them from fire.

• Sound Insulation: Light weight and hollow bricks are good sound insulators. While heavier bricks are poor insulators of sound.

Determining Size of Elastomeric Bearings.

Determining Size of Elastomeric Bearings

For elastomeric bearing, the vertical load is resisted by its compression where as shear

resistance of the bearing controls the horizontal movements. 

The design of elastomeric bearings are based on finding a balance between the provision of sufficient stiffness to resist high compressive force and the flexibility to allow for translation and rotation movement.

The cross sectional area is usually determined by the allowable pressure on the bearing support. Sometimes, the plan area of bearings is controlled by the maximum allowable compressive stress arising from the consideration of delamination of elastomer from steel plates. In addition, the size of elastomeric bearings is also influenced by considering the separation between the structure and the edge of bearing which may occur in rotation because tensile stresses deriving from separation may cause delamination. The thickness of bearings is designed based on the limitation of its horizontal stiffness and is controlled by movement requirements. The shear strain should be less than a certain limit to avoid the occurrence of rolling over and fatigue damage. The vertical stiffness of bearings is obtained by inserting sufficient number of steel plates.

INCREMENTAL LAUNCHING METHOD OF BRIDGE CONSTRUCTION

INCREMENTAL LAUNCHING METHOD OF BRIDGE CONSTRUCTION

Pre-Requisites for incremental launching:

• Used only for straight bridges and curved bridges with fixed radius over entire length.
• Slenderness ratio i.e. span to depth ratio of beam sections must not be more than 17.
• Single cell box and double T are most suitable cross section.

Important measures taken during incremental launching of bridge segments:

• During the process of launching the segments, the leading edge of the superstructure will be subjected to a high hogging moment. In this kind of situation, steel launching nose having a length of about 0.6 to 0.65 times the span length is provided at the leading edge in order to reduce the cantilever moment. However sometimes, a tower and stay system are designed instead of using launching nose which also serves the same purpose.

• During the launching, superstructure will be subjected to alternative positive and negative moments. Usually, a central prestress is provided in which the compressive stress at all points of bridge cross sections are equal. In this way, it counters the possibility of occurrence of tensile stresses at upper and lower parts when it undergoes positive and negative moments respectively. After the process is complete i.e., when the whole superstructure is completely launched, depending on the bending moments in completed bridge condition, continuity prestressing is performed by providing the proper  location and design of continuity tendons. These continuity tendons supplement the central prestress

• Sometimes if the span of the bridge is too long, temporary piers may be provided at suitable locations to reduce the cantilever moment.

• Construction is done without using form work to avoid problem of passing over obstacles.

Components of a an Expansion joint

Components of a an Expansion joint

An Expansion joint has the following components:

·         Joint filler

·         Joint sealant

·         Expansion Cap/PVC dowel sleeve

·         Joint sealant

·         Bond breaker tape

·         Cradle bar

1. Joint filler: 

This material is compressible so that the joint can freely expand without any constraint. Some people are of opinion that even in absence of filler, the joint can still expand freely. But its presence is needed because it serves the purpose of space occupation such that there is no space for dirt and rubbish to accommodate.

2. Joint sealant:

 Main function of a sealant is to cover up the joint width and prevents seepage of water and dirt into the joint. Sealants also prevent corrosion of dowel bars and unexpected joint stress resulting from restrained movement.

3. Expansion Cap/ PVC dowel sleeve: 

It provides to space for the movements of the dowel bar. On one side of the joint, the dowel bar is encased in concrete & on the other side, the PVC dowel Sleeve is bonded directly to concrete so as to allow movement of dowel bar. As seen in the detailing of normal expansion joints in Highways Standard Drawing, a part of PVC dowel sleeve is also extended to the other part of the joint where the dowel bar is directly bonded to concrete. The reason behind this is to avoid water from getting into contact with dowel bar in case the joint sealant fails. As PVC is a flexible material, it only minutely hinders the movement of joint only under this design.

4. Dowel bar: 

This is a one of the major component of the expansion joint. It guides the direction of movement of concrete expansion. If dowel bars are incorrectly placed, it will induce stresses in the joint during thermal expansion. On the other hand, it connects the two adjacent structures by transferring loads across the joints.

5. Bond breaker tape: 

The main function of the bond breaker tape is to prevent the flowing of  liquid sealant at the time of construction since the majority of joint sealant is applied in liquid form.

6. Cradle bar: 

These bars are supporting bars used to hold the dowel bar in position at the time of construction.

Plastering & Pointing-Definition & Difference

• POINTING

Special mortar finishing work is applied on to the exposed joints Instead of plastering entire surface of the masonry.This process is known as "pointing". Pointing involves raking of joints to a depth of 10 mm to 20 mm and then filling it with rich mortar. For cement mortar pointing, mix used is 1 : 3 and for lime mortar pointing mix used is 1 : 2. 

Pointing is most suitable for stone masonry since stones possess attractive colors and has good resistance to penetration of water. Pointing gives protection to weaker part of masonry like joints and it improves the aesthetics of the masonry.

• PLASTERING

Plastering refers to process of applying mortar coats on the surfaces of walls, columns, roofs etc. in order to get a smooth finished surface. Mortar used for plastering may be lime mortar, cement mortar or lime-cement mortar. 

Cement mortar of 1 : 4 or 1 : 6 mix is very commonly used for plastering, richer mix being used for outer walls.  Lime mortar used shall have fat lime to sand ratio of 1 : 3 or 1 : 4. If hydraulic lime is used mix proportion (lime: sand) is 1 : 2.To combine the cost effectiveness of lime mortar and good quality of cement mortar many use lime-cement mortar of proportion i.e cement : lime : sand of 1 : 1 : 6 or 1 : 1 : 8 or 1 : 2 : 8.

Difference between Plastering & Pointing

No.	PLASTERING	POINTING
i	It is applied on to entire surface.	It is applied only at exposed joints.
ii	It provides smooth surface.	It does not provide smooth surface.
iii	It conceals defective workmanship in the masonry construction	It is used to expose beauty of well built masonry work.
iv	It provides a base for applying white/ colour washing	White washing or colour washing are ruled out.

When should an engineer use jacking at one end only and from both ends in prestressing work? [Practical Civil Engineering Question]

When should an engineer use jacking at one end only and from both

ends in prestressing work?

During the pre-stressing process, if prestressing is carried out at one end only, frictional losses will occur and the prestressing force will be minimized along the tendon length till the other end. This frictional loss includes the friction caused due to change in curvature of tendons & loss due to deviation of tendon alignment from centre line i.e. wobbles effect. Because of this the prestressing force in mid span and far end span is highly minimized due to high frictional loss.

In order to avoid such a situation, to provide even distribution of prestress & symmetry of force along the length prestressing is carried out at both the ends.

In fact, pre stressing at one end only has the potential advantage of lower cost compared to stressing from both ends. For multiple spans having unequal span length,

Jacking will be carried out at the end of the longer span so as to provide a higher prestress force at the location of maximum positive moment. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the prestress force. However, if the total span length is sufficiently long,   jacking from both ends should be considered.

Lintels-Definition & Types

Lintels-Definition & Types

A Lintel is a horizontal flexural member that spans over the openings of the walls for windows, doors, Ventilators, etc. The end bearings for the lintel should not be lesser than 20 cm. The width of lintel is usually the same as that of wall.

 The load coming from the masonry above the opening is transferred to wall by flexure

Such that frames of doors, windows etc are not over loaded.

Lintels of various materials are used.

They are:

o   Stone

o   Wood

o   Brick 

o   R.C.C.

o   Steel.

•  Stone Lintels: Stone beams are used as lintels where stones are available. Since stone is weak in tension the use is limited to short spans only. Stones are cut to the width of wall and are dressed prior to using as lintels. Their depth is kept about 0.1 times the span

•  Wood Lintel: These lintels may be a single piece or combination of 2 or 3 pieces. Sometimes wooden lintels are strengthened at top and bottom by using steel plates. Such kinds of lintels are called as flitched beams.

•  Brick Lintels: Good quality & well burnt, lintels are laid on ends or edges to act as lintels. It requires temporary form work during the construction. These lintels should be cured for 7–14 days before stripping of form work. Such lintels are useful to span small openings.

•  R.C.C. Lintels: R.C.C lintels give us flexibility to choose any span for an opening. They can be either isolated or continuous over the openings. They must provided with suitable reinforcement i.e. main reinforcements on lower side i.e. tension zone. Nowadays these lintels are used commonly in buildings.

• Steel Lintels: Rolled Steel I-sections or steel angles are used as lintels. Tube separators should be provided to maintain the spacing between the sections. Frequent painting and maintenance is required if the sections are opened to atmospheric action. Generally they are encased in concrete to avoid maintenance problem. These lintels can be used for large spans