Conventional Building Construction

Conventional Building Construction

Conventional Building Construction refers to the traditional method of construction where the construction knowledge is passed from one generation to the other

Associated to the wet construction (in-situ) using reinforced concrete.

Not utilizing new technologies – particularly true..

Labour intensive method

In Malaysia – still the preferable method among the contractors as labour is abundance and the cost is still cheap

Contrary to IBS – New SPP stated that every government projects must use up to 70% of IBS components

In-situ / Cast-in-place concrete

• Concrete structural members have traditionally been built in-place by placing the wet concrete into forms and allowing it to harden. The forms are then removed after the concrete has develop sufficient strength to support its own weight and the weight of any construction loads.

• 3 discrete components : formwork and falsework, reinforcement and concrete

Advantages of in-situ concrete

1. Amenable to almost any shape – does not limit creativity2. Connections are homogenous with the rest of the structure –

by providing adequate anchorage, bond and lapping, hardy require complicated detailing, easy to do renovation

3. Alteration can be made at the last minute – as long as the formwork and rebar are not ready , easy to modify technically

4. Design can proceed as the structure is built – suitable for fast track projects

5. Whole production activities are on site – easy to monitor and coordinate

6. Many players in the industry – competitive 7. Easy to adopt structural resistant concept to overcome

disaster such as earthquake, explosion, collision etc

Disadvantages of in-situ concrete

1. Quality of the finished work – not easy to control, as a result honeycombing, bulging, cracks occur.

2. Labour intensive – not practical for countries (European) where labour cost is high

3. Foreign labours – always associated to social problems 4. Foreign labours – majority only work as to accumulate

the money and send it back to their native land and not spending it locally.

5. Site condition – dirty, dangerous, difficult (3D)6. Site wastages – concrete and formwork & falsework7. Speed of construction – relatively slow

Construction sequence

For a typical construction of a duoble storey building (without lift core), the construction sequence for a in-situ concrete frame would be :-

1. Setting out – building lines and establishing piling point2. Piling work – skin + end bearing OR end bearing3. Pile cut off, lean concrete to pilecap, pilecap formwork + bar , pilecap

concreting, stump concreting4. Ground beam concreting (partly is use suspended slab/full if use non-

suspended slab), starter bar for column is placed5. Ground floor slab concreting, starter bar for staircase is placed 6. Column from ground floor to first floor concreting, starter bar for column

extension is placed 7. 1st floor beam + slab concreting8. Column from first floor to roof level concreting , terminate column bar

(staircase from ground to first floor concreting)9. Roof beam concreting 10. Roof truss construction11. (Architectural work)

Main RC Components • Foundation : piling (deep) or pad/strip/raft (shallow)• Stump : short column between a foundation and ground floor level,

depth to suit engineering requirements such as piping and M&E services.

• Column – many shapes, normally vertical; vertical load bearing member of a structural frame that transmits the beam loads down to the foundation

• Beam – normally rectangular sections; either horizontal, slanting, curves; simply supported, continuous, cantilever; main beam transfer load to column, secondary beam transmit slab load to main beam; shallow beam and deep beam

• Slab – many shapes; one way, two way and others ; mainly supporting selfweight, live load, finishes, machineries

• Staircase – simple, stringer, helical, spiral, jack knife

Staircase

HelicalSpiral

Pre-cast

Free standing/jack knifeSimple

FORMWORK

Definition

• Formwork: Formwork may be described as a mould or box into which wet concrete can be poured and compacted so that it will finally set to the inner profile of the mould or box.

• Falsework: The part of the formwork which support the forms, usually for a large structure such as bridge.

Beam FormworkBasically a 3 sided box supported and propped in the correct position and to the desired level. The sides have to retain the wet concrete in the correct position and be able to withstand the initial hydrostatic pressure of the wet concrete.The formwork soffit, apart from retaining the concrete, has to support the initial load of the wet concrete and finally the set concrete until gaining sufficient strength to be self supporting

Column FormworkConsists of a vertical mould of the desired shape which has to retain the wet concrete and resist initial hydrostatic pressure of the wet concrete.To minimize the formwork material, horizontal clamps are used Head of formwork – 2 types ; used to support incoming beam or cast to the underside of the beam and at later stage a collar or formwork can be clamp around the cast column Column forms are located at the bottom around 75 mm high concrete kicker ; kicker has dual function – to establish the column location and preventing grout loss from the bottom of the column formwork

Necessity of Formwork

• Necessity– Formwork form a very important part of concrete construction. Most

permanent structures simply could not be built without formwork.

• Safety– Failure of formwork could cause injuries or death at construction site.

Materials for Formwork

• Among the materials that can be used for construction of formwork: – Sawn timber – the most widely use – Plywood – when fair face is necessary– Steel form – the use of steel is largely confined to proprietary formwork

system– Plastic form – plastic sheets are normally used as liners to produce an almost

endless variety of patterns.– Aluminium – Aluminium has many of the properties of steel but with

substantial weight savings.– Rubber – Rubber materials are generally used as form liners when complex

shapes are required to be cast into the concrete.– Glass fiber reinforced plastic (GFRP) – GFRP material is strong light weight

material manufactured from polyester or epoxy resin, reinforced with glass fiber.

Concrete Formwork • Forms are a type of temporary structure that provide containment for the

fresh concrete and support it until it can support itself.

• Forms must be designed to support loads of the fresh concrete, equipment, workers, impact of various kinds, or sometimes wind without collapse or excessive deflection.

• The cost of FW is between 30% to 60% of the cost of concrete structure. Design of a good forming system could both expedite a project as well as reduce costs.

Formwork Requirements - 1

• Safety – FW must be:– Strong ( to carry the full load and side pressure

from freshly placed concrete, together with construction traffic and equipment).

– Sound (made of good quality, durable materials).


• Quality – FW must be:– Accurate (within specified tolerances for form dimension)– Rigid (adequately braced and tied to prevent movement,

bulging, or sagging during construction).– Tight Jointed (to prevent cement paste leakage which

disfigures the surface of concrete).– Properly Finished (to provide a concrete surface of good

appearance).


• Economy – FW must be:– Simple (simple to erect and dismantle)– Easily handled (the sizes of units should not be too

heavy to handle)– Standardized (ease of assembly and possibility of

reuse)

Formwork for cast in-situ

• Inspection is very important in all stage of formwork and should consist of checking the various forms, to ensure sufficiently strong and rigid enough to support the dead load of concrete as well as to allow for temporary live load of workmen wheeling barrows and the tamping vibrating of concrete. The checking should include all wedges, braces and bulge, as it is practically impossible to rectify when pouring has commenced.

Striking of formwork

• The period which should elapse before the formwork is struck will vary from job to job and will depend on the concrete used. Formwork must not be removed until the concrete is strong enough to be self-supporting and able to carry imposed load. Time of striking is normally related to the strength of the concrete, and obviously soffit forms to beams and slabs must be left in place longer than is necessary for side form.

• Striking must be carried out with care to avoid damages to arises and projections, and it may be necessary to protect some of the work from damage immediately after removing the form. Before the soffit forms and props are removed, the concrete surface should be exposed carefully to ascertain that the concrete has hardened sufficiently.

• Beam sides – 9 to 12 hours • Beam soffit – 8 to 14 days (props left under)• Beam props – 15 to 21 days• Columns – 9 to 12 hours

Striking formwork

Striking formwork at for Ground Beam (Note the use of suspended slab)

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.

• Normally being placed at location where it would be impractical or difficult to recover (underside of raised slab – TNB trenches)

Potential Advantages of Permanent Formwork

• Reducing the skill level needed on site. • Increasing the potential for standardisation and repetition. • Permitting off-site fabrication in factory conditions followed by

scheduled and appropriate deliveries. • Speeding up erection times, particularly in building works. • Eliminating the need to strike formwork and falsework.• Allowing early access for following or concurrent operations. • Eliminating the programme limitations of reuse of formwork. • Potential reduction in construction and maintenance costs • Potential to shorten construction time• Improve safety by reducing hazards during construction. • Reduces construction waste generation during construction.

Potential Causes of Formwork Failures

• Improper stripping and shore removal• Inadequate bracing• Vibration• Unstable soil • Inadequate control of concrete placement.• Lack of attention to FW details• When FW is not at fault

Formwork collapsecauses injuries, lossof life, propertydamage, andconstruction delays

Formwork failures are the cause of many accidents andfailures that occur during concrete construction whichusually happen when fresh concrete is being placed.

Generally some unexpected event causes one memberto fail, then others become overloaded or misalignedand the entire formwork structure collapses.

Improper stripping and shore removal

New York ColiseumFormwork collapse,where rapiddelivery ofconcreteintroduced lateralforces at the top ofhigh shoring.

Inadequate bracing

Vibration

•Forms sometimes collapse when their supporting shores or jacks are displaced by vibration caused by:• passing traffic• movement of workers and equipment on the formwork• the effect of vibrating concrete to consolidate it.

•Diagonal bracing can help prevent failure due to vibration.

Unstable Soil

•Formwork should be safe if it is adequately braced and constructed so all loads are carried to solid ground through vertical members.

•Shores must be set plumb and the ground must be able to carry the load without settling.

Inadequate Control of Concrete Placement

•The temperature and rate of vertical placement ofconcrete are factors influencing the development oflateral pressures that act on the forms.

•If temperature drops during construction operations,rate of concreting often has to be slowed down toprevent a build up of lateral pressure overloading theforms. If this is not done, formwork failure may result.

•Failure to regulate properly the rate and order of placing concrete on horizontal surfaces or curved roofs may produce unbalanced loadings and consequent failures of formwork.

Lack of Attention to Formwork Details

•Even when the basic formwork design is soundly conceived, small differences in assembly details may cause local weakness or overstress loading to form failure.

•This may be as simple as insufficient nailing, or failure to tighten the locking devices on metal shoring.

•Other details that may cause failure are:• Inadequate provisions to prevent rotation of beam forms where

slabs frame into them on the side.• Inadequate anchorage against uplift for sloping form faces.• Lack of bracing or tying of corners, bulkheads, or other places

where unequal pressure is found.

Formwork Safety• The frequency and serious consequences of formwork failure that

special attention be paid to this aspect of construction safety. Some of the safety precautions that should be observed in constructing formwork :-

1. Provide adequate foundations for all formwork. Place ‘mudsill’ under all shoring that rests on ground. Always check surrounding excavations to ensure that formwork does not fail due to embankment failure

2. Provide adequate bracing of forms being particularly careful of shores and vertical supports. Ensure that all connections are properly secured, especially nailed connection. Concrete vibrators may cause connection to loosen. A skilled carpenter is required to be present at site during any concreting work

3. Control the pouring rate so that design loads are not exceeded. Use drop chutes (also to avoid segregation) when placing concrete into high vertical forms. Free fall distance should be limited to 2m or less

4. Avoid inserting large vibrator deep into previously placed, partially set concrete

Mudsill

Formwork Safety5. Ensure that forms and support are not removed before the

concrete has developed the required strength. ‘Reshoring’ exercise must be done exactly as specified by the designer. Only a limited area should be stripped and reshored at one time

6. No construction load should be allowed on a partially hardened concrete while reshoring is under way

7. Monitor any nearby movement especially cranes (transporting materials)

8. Protruding nails are a major source of injury on concrete construction sites. As forms are stripped, form lumber must be promptly removed to a safe location and nails pulled.

Timber & Steel Props

Planning for Formwork

• The contractor should plan FW at the time of making bid considering the following factors:– Placing schedule and stripping time requirements– Capacity of equipment available to handle form sections and materials– Capacity of mixing and placing equipment– Construction joints– Reuse of forms as affected by stripping time– Relative merits of job-built, shop-built and ready-made forms.– Weather (protection requirements and stripping time)

• Compare alternative methods to determine the most efficient plan.

Construction & Expansion Joints

Key area of Cost Reduction - 1

• Planning for maximum reuse– A form designed for max reuse is stronger and more expensive, but it

can save on the total form cost.

• Economical form construction– Shop-built (greatest efficiency in working conditions and in the

purchase and use of materials and tools)– Shop area on the site (form sections too large or transportation cost

too high)– Job-built (for small jobs, or where forms must be fitted to terrain)

Key areas of Cost Reduction - 2

– Buying prefabricated forms(large number of reuses)– Renting prefab forms(better flexibility in regulating volume of work)

• Setting and stripping– Repetition of the same functions to increase the crew efficiency as the

job progresses– Use of metal clamp or special wedge pin connections that are secure,

yet easy to assemble and dismantle– Add extra features that make handling, erection, and stripping easier

(handles, lifting eyes)

Other costs affected by FW - 1

• Cranes and Hoists– Size of form sections should be limited to the capacity of the largest

crane planned for the job.– Stair towers may be completed early in the schedule to be used for

moving men and materials.– Leave one bay open to permit mobile crane and concrete truck

movement.

• Bar Setting– Form design can permit the rebar to be pre assembled before

installation (more favorable condition)

Other Costs affected by FW - 2

• Concrete Placement– High lifts in wall construction make placing and vibration difficult.– Placing rate is limited by form design.

• Other Trades– The plan should permit other trades to perform their work efficiently

and minimize interruptions in placing.

Among the precautions that can be taken to ensure formwork function as it suppose to be are as follow: - Material used for the construction of formwork must fulfill the

specification. - Formwork is fixed firmly & properly - Construction area must be protected to prevent vandalism of formwork. - Warning sign must be put up at the area where the formwork is fixed to prevent entrance of people that may damage the formwork. - The formwork must be inspected before the concrete is poured.

How to increase the speed of conventional construction?

1. Use slipform techniques where possible especially when lift core is present (in high rise). Cores control the vertical rise speed

2. Arrange for a large area pouring of floors (avoid cold joints), continuous operation

3. Introduction of pre-cast elements where possible (especially in horizontal locations)

4. Simplification of floor details – less varieties in bar size and spacing

5. Prefabrication of slab reinforcement (especially when using mesh reinforcement)

6. Use of straight bar reinforcement7. Repetition as much as possible in formwork and falsework

allows less labour intensive method

Concrete Form Design

1. Wall and column formsFor vertical forms, design load consists of the lateral pressure of the concrete against the form. ACI recommendation:

a. Vertical rate of placement 2.1 m/h or less p = 7.2 + [785R/(T + 18)] wherep = lateral pressure (kPa) R = rate of vertical placementT = temperature (0C) Maximum pressure ;143.6 kPa for columns, 95.8 kPa for walls OR 150h (h = height of form) whichever lesserMinimum pressure;28.7 kPa


b. Vertical rate of placement 2.1 m/h – 3.0 m/h p = 7.2 + [1154/(T + 18)] + [244R/(T + 18)] Maximum pressure ;95.8 kPa OR 150h (h = height of form) whichever lesserMinimum pressure;28.7 kPa

c. Vertical rate of placement more than 3.0m/hp = 23.6h Minimum pressure;28.7 kPa


Conditions ;

When forms are vibrated externally, it is recommended that twice the design load of equation a. & b. to be used

When a retarder, pozzolan or superplasticiser has been added equation c. should be used

When concrete is pumped from the bottom, equation c. should be used together with a minimum additional pressure of 25% to allow for pump surge pressure


2. Lateral load for slab forms

H = 0.02 x dl x ws whereH = lateral force applied along the edge of the slab (kN/m)dl = design dead load (concrete + formwork) (kPa)ws = width of slab perpendicular to the form edge (m)Min pressure = 1.46 kN/m


2. Lateral load for wall

wf = wind force prescribed by local codes but minimum of 0.65 kPa (in Malaysia under MS 1553)

Wall Height, h (m) Design Lateral Force Applied at top of form (kN/m)

Less than 2.4 (h x wf/2)

2.4 m to 6.7 m 1.46 OR (h x wf/2) whichever

greater

6.7 and over 0.358 h OR (h x wf/2)

whichever greater

Conventional Building Construction

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