trm 009 comparison of concrete and steel frame buildings .doc

7/28/2019 TRM 009 Comparison of concrete and steel frame buildings .doc

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TECHNICAL REFERENCE 1 of 8MANUAL Rev 1

COMPARISON OF CONCRETE AND Date: 8/94STEEL FRAMED BUILDINGS

INTRODUCTION

This note examines the advantages and disadvantages of concrete and steel framed structures.Particular types of structural frames are not considered, but the following basic design criteria havebeen assumed for the purposes of this note:

Structural grid: 7.5m x 7.5mLive loading: 4kN/m2 (imposed load) + 1kN/m2 (partitions)

The following assessment criteria are used for the purposes of comparison:

CostProgrammeProcurementFire resistanceServices distributionServices penetrationsFlexibilityDurability/MaintenanceSelf-weightFoundationsFrame deflections and vibrationsFrame movementsAcoustic insulation

Thermal insulationAdaptabilityAesthetics

Assessment Criteria

Cost

Two reports have recently been produced to examine the comparative costs of concrete and steelframed buildings:

'A report on the comparative costs of concrete and steel framed office buildings', C.H. Goodchild,

1993 (published by the British Cement Association on behalf of the Reinforced Concrete Council).

Comparative Structure Cost of Modern Commercial Buildings', R.M. Lawson, 1993 (published by theSteel Construction Institute).

WSP Consulting Engineers


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The reports seek to establish the comparative total costs of 'real' buildings designed using a varietyof structural frames. The buildings considered had relatively simple, repetitive structural frames, andtheir findings are summarised below:

Concreteframe(flat

slab)(/m2 gross)

Steel frame(composite)(/m2 gross)

+/-(%)

Building M4 3-storey 770.97 791.11 +2.61

7.5m x 7.5m grid

Budget cost 75/ft2Live load 4+1 kN/m2

7-storey 711.33 729.48 +2.55

Building M62 3-storey 587.13 617.57 +5.18

7.5m x 7.5m gridBudget cost 55/ft2

Live load 4+1 kN/m2

7-storey 545.32 575.32 +5.49

Overall costs per

Building A

2600 m2 floor area7.5m x 6.0m gridLive load 3.5+1 kN/m2

Building B

18000 m2 floor area7.5m x 7.5m gridLive load 3.5+1 kN/m2

Steel frameoptions

Composite beamand slab

554 843

Slim floor (pcunits)

560 857

Composite trusses(long span)

- 863

Concreteframe options

RC flat slab 564 856

RC waffle flat slab 580 872

Precast double-teeunits (long span)

572 892

As expected, both reports establish that the material which they represent is the cheaper form ofconstruction. The main conclusion to be drawn from these reports is that there is little to choose interms of cost between the two materials for a typical building with a 7.5m x 7.5m grid.

Programme

The reports described above also considered the time taken to erect the frames for the two forms ofconstruction. The findings are summarised below.



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Concrete frame(flat slab)(weeks)

Steel frame(composite)

(weeks)

Building M4 3-storey 14 8

7.5m x 7.5m gridBudget cost 75/ft2

Live load 4+1 kN/m2

7-storey 19 15

Building M62 3-storey 13 9

7.5m x 7.5m grid

Budget cost 55/ft2

Live load 4+1 kN/m2

7-storey 20 15

Frame construction

Building A2600 m2 floor area7.5m x 6.0m gridLive load 3.5+1 kN/m2

Building B18000 m2 floor area7.5m x 7.5m gridLive load 3.5+1 kN/m2

Steel frameoptions

Composite beamand slab

5 9

Slim floor (pcunits)

5 10

Composite trusses(long span)

- 8

Concreteframeoptions

RC flat slab 9 16

RC waffle flat slab 12 16

Precast double-teeunits (long span)

9 16

Steelwork frames can start earlier and are erected more quickly than their concrete equivalents. Thisallows the external cladding and roof finishes to start earlier.

However, the need to fireproof the steelwork and the additional amounts of external cladding offsetthis advantage to some degree.

The British Cement Association report concludes that the overall construction programmes for thetwo forms of construction are broadly similar (with concrete being up to two weeks faster in somecases). The Steel Construction Institute Report concludes that the steel framed buildings retain theadvantage, with an overall construction programme some 6 to 12 weeks shorter. Recent experiencehas suggested that concrete framed buildings can be built as quickly as steel framed buildings,particularly when procurement times are taken into account.

Procurement



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The long lead-in times for steel frames (to allow the production of drawings and fabrication to takeplace) can have a significant effect on the overall programme.

In general, 12 weeks should be allowed from the award of the sub-contract to the erection of the firststeel elements on site. Recent experience has suggested that lead-in times are increasing again asthe steel fabricators begin to obtain more work.

The lead-in time for steel will be absorbed if a concrete sub-structure has to be built before the super-structure. The lead-in time can also be absorbed to some extent by pre-tendering the steelworkcontract.

Fire resistance

Reinforced concrete has an inherent fire resistance and can easily achieve up to four hours fireresistance, providing there is sufficient concrete cover to the reinforcement bars.

Most steel-framed buildings will require fire protection. There are a variety of protection systemsavailable depending on the degree of protection required - spray systems, boarded systems,intumescent coatings and concrete encasement. Fire protection may also provide a certain amountof corrosion protection at the same time.

Services distribution

A concrete flat slab (or waffle flat slab) structure provides a uniform structural soffit level to allow

more flexibility for the distribution of services. In contrast, a steel structure gives a larger overallceiling void but there will be 'pinch points' underneath the steel beams.

For a 7.5m x 7.5m grid, typical overall structural depths are given below:

Solid flat slab: 325mmWaffle flat slab 400mmComposite steel frame 585mm (maximum)Slim floor'/precast slab 350mm

The composite steel beam/composite slab solution will require a deeper overall ceiling void (andhence a greater floor to floor dimension).



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Services penetrations

Steel and concrete frame structures will both have an insitu concrete floor slab, which will, in general,allow flexibility in the positioning of services openings. However, certain types of concrete slab (flatslab, in particular) cannot incorporate openings near column positions, or very large openings. Theinherent 'redundancy' in a reinforced concrete slab can often allow openings to be post-drilled.Similarly, post-drilled fixings can be used to the slab soffits for supporting cable trays and pipes.

Precast floor slabs, used in conjunction with a steel frame, are far less flexible, and servicesopenings must be determined at the time the fabrication drawings are produced. Post-drilled holesare not usually possible and there are limits on the location of fixings to the soffits for services.

As discussed above, a level soffit can be achieved with concrete flat slab structures, avoiding theneed for holes in downstand beams. Holes can be formed in steel beams, but their location must bedetermined at the time of the production of the fabrication drawings. If there are a large number ofopenings, a castellated beam or fabricated truss can be used instead.

Flexibility

The lead-in time for the production of fabrication drawings means that steel framed structures are notalways tolerant to late design changes. In contrast, it is possible to make design changes to aconcrete frame up until the moment the formwork is erected on site.

The flexibility provided by steel and concrete frame structures in the location of services openings is

discussed above.

Durability/Maintenance

Concrete has an inherent durability and maintenance will be low, provided that good quality concreteis achieved at the time of construction.

Steel will corrode in the presence of oxygen and water. However, inside a dry, heated building, baresteel will only rust superficially and corrosion rates will be low because of the low amount of waterpresent. Corrosion protection will only be required for internal steel elements if they are likely to beexposed to moisture (such as condensation).

Self-weight

In general, a concrete framed building will have a greater dead load than a steel framed building.This has implications for the design of columns and foundations.

For a 7.5m x 7.5m column grid, typical floor dead loads are as follows:

Solid flat slab .8 kN/mWaffle flat slab 5.4 kN/mComposite steel beams 2.7 kN/m



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Foundations

As discussed above, the foundations for a concrete framed building will be larger than those for asteel framed building.

For a 5 storey building with a 7.5m x 7.5m grid, typical foundation sizes (unreinforced concretefooting) for an internal column may be:

Solid flat slab 4.75 x 4.75 x 2.00 dpWaffle flat slab 4.25 x 4.25 x 1.90 dpComposite steel beams 3.70 x 3.70 x 1.65 dp

Frame deflections and vibrations

The sag in a structural element will become noticeable if the deflection exceeds span/250 and this isused as a design criterion for both steel and concrete structures.eg for a 7.5m span, the sag will become noticeable when the deflection exceeds 30mm.

The deflection of a concrete element will be influenced by elastic, creep, shrinkage and thermalstrains. However, for a composite steel beam, bending deflection is the governing criterion. A largedeflection will have implications for internal partitions, brittle finishes and the depth of the raisedfloor. Deflections of steel beams can be partially reduced by using an initial pre-camber.

Lightweight structures (such as a composite steel beam/composite slab frame) can be more

susceptible to vibration than heavier structures. Composite steel structures are usually designed toensure that the natural frequency of the beams is greater than 5Hz. However, in some cases, quiteperceptible vibration can be induced by a person walking across a floor, even if the floor frequency isabove 5Hz. Experience has shown that a low level of vibration, just above perception levels, is notdisturbing to most people when engaged in normal occupations (e.g. office work). In addition, mostcomputers and other items of normal electronic equipment are not particularly sensitive to vibration.Therefore, although it may not be possible to eliminate perceptible vibration altogether, compositeconstruction provides a serviceable solution.

Frame movements

The horizontal movements of elements within a building may arise from both structural and material

factors. These effects may be summarised as follows:- temperature movement (The coefficients of thermal expansion of steel and concrete are

very similar (about 12 x 10-6). This corresponds to an extensionof 1.2mm over a 10m length for a 10oC rise in temperature).

- long term shrinkage of slabs- early age thermal contraction of slabs- horizontal forces (eg wind loads)

The horizontal movements are restrained by shear walls, cross-bracing or frame action. Movementjoints are provided in a building to control the horizontal movements (particularly those resulting fromtemperature or shrinkage). It is generally considered that overall movement joints should beprovided in concrete frames at plan lengths in the range of 60m to 70m (ref: CIRIA Technical Note107: Design for movement in buildings). Steel framed buildings should be provided with movement

joints at a spacing of 75m to 90m.

Acoustic insulation



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The acoustic separation provided by the floor slab is related to its thickness. In simple terms, thethicker the slab, the greater the acoustic separation. For a 7.5m x 7.5m column grid, likely floor slabthicknesses are shown below:

Solid flat slab 325mmWaffle flat slab 100mmComposite steel frame 100mm

Thermal insulation/absorption

The typical thermal conductivity of lightweight concrete (at 3% moisture) is 0.81 - 0.88 W/m 0K.

The typical thermal conductivity of normal density concrete (at 3%) moisture) is 1.73 W/m 0K.The normal density concrete slab is, therefore, a better absorber of heat than a lightweight concretecomposite slab. Conversely, the lightweight concrete slab is a better insulator than a normal densityconcrete slab.

Adaptability

The need to provide a building frame which can be adapted to suit the needs of a future tenant isbecoming increasingly important. It is usually straightforward to adapt a steel framed building e.g. toadd beams to frame a new opening. It can be considerably more difficult to adapt a concrete framedbuilding, particularly a flat slab structure, which places limitations on the locations of openings.

Aesthetics

The need to provide fire protection to steelwork beams means that it generally needs to be hiddenbehind a ceiling. A good quality finish can be achieved in insitu concrete when the structure isproperly detailed and constructed. Alternative quality finishes can be achieved using needle-gunningor acid etching. Precast concrete offers the best opportunities for achieving a good quality finish.

Summary Table

CRITERIA CONCRETE FRAME STEEL FRAME

Cost

Programme 5 _

Procurement 4 5

Fire resistance 4 5

Services distribution 4 5

Services penetrations

Flexibility 4 5

Durability/Maintenance 4 5

Loading 5 4



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Foundations 5 4

Frame deflections/vibrations 4 5

Frame movements 5 4

Acoustic insulation

Thermal insulation/absorption 4 5

Aesthetics 4 5

4 indicates advantage over other material5 indicates disadvantages when compared to other material

indicates broadly similar to other material

Conclusion

It is concluded that most modern structural systems in steel, composite and concrete construction allhave broadly equal economic merit. However, it is necessary to consider the choice of the structuralsystem in relation to its influence on other non-structural, and often more expensive aspects, of thebuilding construction, in order to make the final decision on the type of building frame.

Evelyn MurrayWSP Consulting Engineers


trm 009 comparison of concrete and steel frame buildings .doc

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