STEEL CONSTRUCTION Structural steel construction is a specialized task that is usually performed by specialty subcontractors; however, construction managers and inspectors must understand the principles and procedures involved. Three process of steel construction can be broken down into the three major elements of advanced planning, steel fabrication and delivery to the job site, and field operations. Each of these elements involves a number of operations which are described in this chapter.

Types of steel

The types of steel contained in a structural steel member is designated by the letter

A followed by the American society for testing and materials (ASTM) designation number. The principal types of structural steel include:

A36. Carbon structural steel.

A572. High-strength low-alloy structural steel.

A588. Corrosion-resistant high-strength low-alloy structural steel.

Steel strength is designated by the symbol Fy, which indicates the minimum yield point of the steel expressed in thousands of pounds per square inch(ksi), pound per square inch (psi), or mega Pascal (MPa). The high-strength steels (types A572 and A588) are available in yield strengths of 42 ksi(289.6 Mpa) to 65ksi (448.6 mpa).

Weathering steel is a type of steel develops a protective oxide coat on its surface upon exposure to the elements so that painting is not required for protection against most atmospheric corrosion. That natural brown color that develops with exposure blends well with natural settings. However, care must be taken to prevent staining of structural elements composed of other materials which are located in the vicinity of the weathering steel and thus exposed to the runoff or windblown water from the weathering steel.

Standard rolled shapes

There are a number of rolled steel shapes produced for construction which have been standardized by the American society for testing and materials, the following figure illustrates five major section shapes,

A list of standard shapes and their AISC designations is given in in the following table. Note that the usual designation code includes a letter symbol (identifying the section shape) flowed by two numbers (indicating the section depth in inches and the weight per foot). Designations for angles, bars, and tubes are slightly different.

Built-up members

Girders are used when regular rolled shapes are not deep enough or wide enough to provide the required section properties, plate girders normally consist of a web and top and bottom flanges. Stiffeners may be added if needed to prevent buckling of the web. Box girders are constructed using two webs as shown in the following figure

Open-web steel joists and joist girders are other forms of built-up steel members. These are lightweight open trusses that are strong and economical.

STEEL ERECTION

Erection procedure

The usual steel erection procedure employs three crews (a raising crew, a fitting crew, and a fastening crew) which operate in sequence as erection proceeds.

The raising crew lifts the steel member into position and makes temporary bolted connections that will hold the member safely in place until the fitting crew takes over.

The fitting crew brings the member into proper alignment and tightens enough bolts to hold the structure in alignment until final connections are made.

The fastening crew makes the final connections (bolted or welded) to meet specification requirements.

LIFTING EQUIPMENT

1. The mobile crane and tower crane are often used for handling steel and lifting it in to final position.

2. The gin pole is one of the simplest types of powered lifting devices.3. The guy derrick is probably the most widely used lifting device in high-rise

building construction 4. Stiff leg derricks may be mounted on tracks to facilitate movement within a

work area.

FIELD CONNECTIONS

Fastening systems

The three principal systems used for connecting steel members are bolting, riveting, and welding.

Bolted connections

While unfinished (ASTM A307) bolts are still available for low stress applications, high strength bolts are used in most of today's steel construction, to prevent confusion in identification. ASTM has prescribed special markings for high- strength bolts, which are illustrated in the following figure.

Wood construction

INTRODUCTION

Wood is one of humankind's oldest construction materials. Today it is still wildly used to construct residential, commercial and industrial buildings as well as such varied structures as piers, retaining walls, and power transmission towers. In the United States, for example, 90% of all houses are constructed of wood. In this lecture we will consider the properties of wood that influence its use in construction, together with the principles and practices of both frame and timber construction.

WOOD MATERIALS AND PROPERTIES

Types

Wood is divided into two major classes, hardwood and softwood, according to its origin.

Hardwood is produced from deciduous (leaf-shedding) trees. Softwood comes from conifers( trees having needlelike or scale like leaves),which are primarily evergreens, the terms "hardwood" and " softwood " indicate only the wood species and may be misleading, because some softwoods are actually harder than some hardwoods. In the United States, Lumber is grouped into several grading types, which have similar properties. Most of the lumber used in the United States for structural purposes is softwood.

MOISTURE CONTENT

The moisture content of lumber (which identified as the weight of moisture in the wood divided by the oven dry weight and expressed as percentage) has a great influence on its strength properties. At moisture contents above 30%, wood is essentially in its natural state, and no changes in size or strength properties occur. At moisture contents below30%, wood shrinks and its strength properties increase,

WOOD PRESERVATION

Wood is subject to damage by decay and by wood-boring insects, mechanical shields of solid meal or stainless steel mesh may be used to reduce exposure to insect damage. However, wood preservation by chemical treatment is the principle method used today to provide protection against decay and insect damage. Surface treatment of wood has largely been replaced by pressure treatment, which forces the preservatives deep into wood cells, the principal wood preservatives now used include creosote, pentachlorophenol, copper azole (CA), alkaline copper quaternary (ACQ), and sodium borates (SBX).

FRAME CONSTRUCTION

Frame construction utilizes suds [typically spaced 16 or24 in.(04 or0,6m)

On center]. Joists and rafters to form the building frame. Framing members are usually of 2in (5cm) nominal thickness, this frame is then covered with siding and rood sheathing of plywood or lumber. Frame construction is widely used in the United States for single – family residences, as well as for small multiple- family residences, offices, and shops. Building codes frequently specify procedures or minimum dimensions to be used in frame construction,

The procedures described in this section are those widely recommended in the absence of specific code requirements. The two principal forms of frame construction platform frame construction and balloon frame construction are described next.

Platform frame construction

Platform frame construction is illustrated in the next figure. In this type of construction, the subfloor of each story extends to the outside of the building and provides a platform for the construction of the building walls, this method of framing is widely used because:

It provides a good working platform at each level during construct

It permits preassembled wall section to be quickly set in place once the subfloor is completed.

BALLOON FRAME CONSTRUCTION

In balloon frame construction, exterior wall studs extend all the way from the sill to the top of the second floor wall, as shown in the next figure. The outside ends of second floor joists are supported by ribbon strips notched (or let-in) into the studs. Balloon framing is especially well suited for use in tow –story building that have exterior walls covered with masonry veneer, since this method of framing reduces the possibility of movement between the building frame and the exterior veneer.

ROOF CONSTRUCTION

One method of roof construction called joist and rafter framing. As illustrated in next figure. Rafter are notched where they rest on wall plates and are held in place by nailing them to the wall plats or by the use of metal framing anchors, the collar beam shown is used to assist in resisting wind loads on the roof.

Roof trusses are now widely used in wood frame construction in place of rafter framing. The use of roof trusses permits interior walls to be non-bearing because all roof loads are supported by the exterior walls. Additional advantages of prefabricated roof trusses over rafters include:

high strength economy controlled quality less skilled labor required on site an open web design which facilitates installation of plumbing,

electrical, and HVAC systems

Components of common roof trusses are illustrated in next figure

TUNNEL CONSTRUCTION

TUNNEL

A tunnel is an underground passageway, completely enclosed except for openings for egress, commonly at each end.

A tunnel may be used for:

1. Foot or vehicular road traffic, for rail traffic, or for a canal.2. Some tunnels are aqueducts to supply water for consumption or

for hydroelectric stations or are sewers.3. Other uses include routing power or telecommunication cables.4. To permit wildlife such as European badgers to cross highways.

In the United Kingdom, a pedestrian tunnel or other underpass beneath a road is called a subway. In the United States that term now means an underground rapid transit system.

GEOTECHNICHAL INVESTIGATION

1. A tunnel project must start a comprehensive investigation of ground conditions by collecting samples from boreholes and by other geophysical techniques.

2. An informed choice can then be made of machinery and methods excavation and ground support, which will reduce the risk of encountering unforeseen ground conditions.

3. In planning the route the horizontal and vertical alignments will make use of the best ground and water conditions

4. In some cases conventional desk and site studies yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large diameter tunnels

5. To give more information a pilot tunnel, or drift, may be driven ahead of the main drive, this smaller diameter tunnel will be easier to support should unexpected conditions be met, and will be incorporated in the final tunnel.

TUNNEL CONSTRUCTION

Tunnels are dug in types of materials varying from soft clay to hard rock.

The method of tunnel construction depends on such factors as the ground conditions. The ground water conditions, the length and diameter of the tunnel drive the depth of the tunnel, the logistics of supporting tunnel excavation, the final use and shape of the tunnel and appropriate risk management.

There are three basic types of tunnel construction in common use:

1. Cut and cover tunnels . Constructed in a shallow trench and then covered over.

2. Bored tunnels , constructed in situ, without removing the ground above. They are usually of circular or horseshoe cross-section.

3. Immersed tube tunnels , sunk into a body of water and sit on, or are buried just under its bed.

Cut and cover

Is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel.

Two basic forms of cut-and-cover tunneling are available.

Bottom-up method

1. In the cut-and-cover bottom-up or caisson wall method, a drilling rig is used to install caisson walls down to the existing bedrock.

2. Once the caisson walls are in place, soil between the walls is excavated to a depth below the tunnel floor.

3. The tunnel floor, a slab, is poured followed by the sidewalls of the tunnel from the bottom-up.

4. After the walls of the tunnel are completed, the roof is constructed and the roadway or ground on top the tunnel restored.

5. Material used to provide the structure and support in the construction of the tunnel may include concrete, pre-cast concrete arches, or corrugated steel arches.

TOP DOWN METHOD

In the cut-and –cover top- down or diaphragm wall method, the opposite process takes place in constructing the tunnel.

A trencher or trench cutter is typically used to dig a trench out of the ground first before concrete wall are built.

These processes consist of using a slurry mixture to build a slurry wall. The slurry wall provides temporary support to the sides of the trench before concrete is poured for a permanent wall structure.

Once the concrete walls of the tunnel are completed, the roof of the tunnel is constructed and the surface roadway restored.

Excavation of the tunnel is then carried out through openings in the tunnel roof top-down to the tunnel floor.

BORING MACHINES

1. Tunnel boring machines (TBMs) and associated back-up systems are used to highly automate the entire tunneling process, reducing tunneling costs.

2. Tunnel boring in certain predominantly urban applications, is viewed as quick and cost effective to laying surface rails and roads.

3. Expensive compulsory purchase of buildings and land with potentially lengthy planning inquiries is eliminated.

4. There are a variety of TBMs that can operate in a variety of conditions, from hard rock to soft water-bearing ground.

BRIDGEA bridge is a structure built to span physical obstacle such as a body of water, valley or road, for the purpose of providing passage over the obstacle.

Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed, the material used to make it and the funds available to build it.

TYPES OF BRIDGES

There are six types of bridges:

1. Beam bridges.2. Cantilever bridges.3. Arch bridges.4. Suspension bridges.5. Cable-stayed bridges 6. Truss bridges.

Beam bridges

Beam bridges are horizontal beams supported at each end by abutments, hence their structural name of simply supported when there is more than one span the intermediate supports are known as piers.

Cantilever bridges

Cantilever bridges are built using cantilevers-horizontal beams that are supported on only one end.

Most cantilever bridges use a pair of continuous extending from opposite sides of the supporting pries, meeting at the center of the obstacle to be crossed.

Cantilever bridges are constructed using much the same materials and techniques as beam bridges.

The difference comes in the action of the forces through the bridge. The largest cantilever bridge is the 549-meter Quebec Bridge in Quebec, Canada.

ARCH BRIDGES

Arch bridges have abutments at each end. The earliest known arch bridges were built by Greeks and include the Arkadiko Bridge.

The weight of the bridge is thrust into the abutments at either side.

Dubai in the United Arab Emirates is currently building the sheikh Rashid bin Saeed crossing which is scheduled for completion in 2012 when completed, it will be the largest arch bridge in the world.

SUSPENSION BRIDGES

Suspension bridges are suspended form cables. The earliest suspension bridges were made of ropes or vines covered with pieces of bamboo.

In modern bridges, the cables hang from towers that are attached to caissons or cofferdams, the caissons or cofferdams are implanted deep into the floor of a lake or river.

The longest suspension bridge in the world is the 12.826feet (3,909m) Akashi Haikyo Bridge in Japan.

FACTORS CONSIDERED IN DECIDING BRIDGE TYPE

In general all the factors are related to economy, safety and aesthetics.

1. Geometric conditions of the site.2. Subsurface conditions of the soil3. Functional requirements4. Aesthetics 5. Economics and ease of maintenance 6. Construction and erection considerations.7. Legal considerations.

Geometric conditions of the site

The type of bridge selected will always depend on the horizontal and vertical alignment of the highway rout and on the clearances above and below the roadway.

Relatively high bridges with larger spans over navigable waterways will require a different bridge type than one with medium spans crossing a flood plain.

Subsurface conditions of the soil

The foundation soils at a site will determine whether abutments and piers can be founded on spread footings, driven piles, or drilled shafts

If the subsurface investigation indicates that creep settlement is going to be problem, the bridge type selected must be one that can accommodate differential settlement over time.

Drainage conditions on the surface and below ground must be pressures, movement of embankments, and stability of cuts or fills.

Functional requirements

Bridge must function to carry present and future volumes of traffic.

Decisions must be made on the number of lanes of traffic, inclusion of sidewalks and/or bike paths, whether width of the bridge deck should include medians, drainage of the surface waters, snow and future wearing surface.

Aesthetics

1. It should be the goal of every bridge designer to obtain a positive aesthetic response to the bridge type selected.

2. There are no equations, no computer programs or design specifications that can make our bridge beautiful.

3. It is more an awareness of beauty on our part so that we can sense when we are in the presence of something good.

4. Aesthetics must be a part of the bridge design program from the beginning. It can

5. T be added on at the end to make the bridge look nice.at that time it is too late. From the beginning, the engineer must consider in the selection of spans, depths of girders , piers, abutments, and the relationship

Economic and ease of maintenance

1. The initial cost and maintenance cost over the life of the bridge govern when comparing the economics of different bridge types.

2. Generally, concrete structures require less maintenance than steel structure. The cost and hazard of maintenance painting of steel structures should be considered in type selection studies.

3. One effective way to reduce the overall project cost is to allow contactors to propose and alternative design or designs.

Construction and erection considerations.

1. The length of the time required to construct a bridge is important and will vary with the bridge type.

2. Generally, larger the prefabricated or pre-cast members shorter the construction time. However, the larger the members, the more difficult they are to transport and lift into place.

3. The availability of skilled labor and specified materials will also influence the choice of a particular bridge type.

TEMORARY WORKS: FORMWORK AND SCAFFOLDING

General

When undertaking works such as concreting, brickwork, plastering, erection of prefabricated members, it is necessary to install some works to carry loads temporarily or to give access for workmen to the works. These are called temporary works.

Centering, Formwork, scaffolding and shoring are the principle types of temporary works.

Centering

Temporary work used for construction of arches is called centering

Formwork or shuttering

Temporary works used as a mold in which fresh concrete is poured for it to harden is called formwork or shuttering

Scaffolding

Temporary works erected for construction of masonry works, plastering, painting etc. is called scaffolding.

CONCRETE FORMWORK

Definition:Form works for in-situ concrete work

A mould or box into which wet concrete can be poured and compacted so that it will flow and finally set to the inner profile of the box or mould.

Function Mold forms the concrete to desired size and shape and control its position

and alignment Formworks also act as a temporary structure that support:

a)Its own weight

b)The freshly placed concrete

c) Construction live loads (material, human, logistics)

Form work is a classic temporary structure in a sense that:

a) It can be erected quicklyb) Highly loaded for a few hours during the concrete placement c) With a few days it is disassembled for future use

A good formwork would have the following characteristics that are:

a) Safeb) Cost effective or economicalc) High qualityd) Finished concrete surface is of acceptable qualitye) In the correct locationf) Able to produce the required shape and surface

FORMWORK DESIGN

Loads include in design process are as follows

a) Fresh concrete b) Rebar c) Formwork material d) Wind and lateral loads e) Live loads due to

Formwork construction Reinforcing installation Concrete placement

MATERIAL FOR RORMWORK CONSTRUCTION

Among the material that can be used for construction of formwork:

a) Timberb) Steel c) Glass reinforced plastic (GRP)

TIMBER FORWORK

Timber Formwork:

After Concrete Was Poured

Timber Formwork: For The Slab

ADVANTAGES OF TIMBER FORMWORK

The advantages of timber formwork are as follows:

1. Easy handling because it's light weight 2. Easy to disassemble 3. Damaged parts can be replaced with new ones 4. Very flexible

DISADVANTAGES OF TIMBER FORMWORK

The disadvantages of timber formwork are as follows:

1. Can't be used for long. Have limited re-use .it can be re-used 5 or 6 times only.2. If the timber is dry. It will absorb moisture from wet concrete which could

weaken the resultant concrete member.3. Timber with high moisture content (more than 20% moisture content), wet

concrete will shrink and cup leading to open joints and leakage or grout.

Timber formwork used for the construction of 2nd and the 3ed floor.

ADVANTAGES OF STEEL FORMWORK

1. Very strong and able to carry heavy load2. Easy to be fixed 3. Uniform size and surface 4. Can be used for a very long time

DISADVANTAGES OF STEEL RORMWORK

1. Limited size or shape

2. A very smooth surface will be produced which give problems for finishing process.

The first floor circular columns were constructed using steel column forms. The steel column form should be oiled before concreting

After concreting the first floor columns the steel column forms were dismantled.

CASUSES OF FORMWORK FAILURE

1. Improper stripping and shore removal 2. Inadequate bracing 3. Vibration4. Unstable soil under mudsills5. Inadequate control of concrete placement6. Lack of attention to formwork details

Inadequate bracing

More frequent causes of formwork failure Inadequate cross bracing and horizontal bracing of shores is one

of the factors most frequently involved in formwork accidents.

Vibration

Form sometimes collapse when their shores / jacks are displaced be the vibration caused

1. Passing traffic 2. Movement of workers and equipment on the formwork3. The effect of vibrating concrete to consolidate it.

SAFETY PRECAUTION

Among the precautions that can be taken to ensure formwork function as it supposed to be are as follows:

1. Material used for the construction of formwork must fulfill the specification.2. Formwork is fixed firmly and properly 3. Construction area must be protected to prevent vandalism of formwork. 4. Warning sign must be put up at the area where the formwork is fixed to

prevent entrance of people that may damage the formwork.5. The formwork must be inspected before the concrete is poured.

PERMANENT FORMWORK It's a part of the permanent structure of the building Permanent formwork is a structural element that is used to contain the

placed concrete, mould it to the required dimensions and remain in place for the life of the structure.

Permanent formwork capable of supporting various slab thicknesses

Use of permanent formwork

1. Reduce construction and maintenance costs 2. Shorten construction time 3. Improve safety by reducing hazards during construction 4. Reduces construction waste generation during construction.