Costs Associated With Concrete And Steel Framed Buildings Construction Essay

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The cost difference model study compares the costs of constructing the three and seven storey buildings

using in situ reinforced Concrete and flat slabs and composite steel frames in two different locations and design were commissioned for typical commercial office buildings located in the UAE ABU DHABI industrial area- B-C1 , B-S1 and

B-C2, B-S2 .the building were based up on 7.5 x 7.5 m structural grids, pad foundation and specification suited to local

Market conditions performance and cost Criteria dictated square air conditioned building with curtain walling on the

B-C1 and B-S1rectangular naturally ventilated building with insulation block in work cladding on the B-C2 and B-S2.

The designs were

Taken up to normal system stage, the only different being directly attributable to the structural frame material

Budget costs were assigned to all elements of construction from substructure, superstructure and external walls

Through to external works, and preliminaries.

Findings in terms of overall construction cost, the seven storey steel- framed building was found to be 5.5% more expensive than the Concrete equivalent on the B2 site and, 2.6% more expensive on the B1 site. The three-storey steel-framed building was Found to cost 5.2% more for the rectangular B2 building and 2.6% more for the square B1 building. The steel buildings had between 0.30 and 1.48%less net gettable area. The differences in cost are greater if net floor area is considered rather than gross. With regard to overall speed of construction, the study found little to choose between the two materials.

Analysis of construction costs shows that the most significant savings occur with the building frame. For example, the combined Cost of structure and internal walls of the three-storey building in Reading (B1) are 32% more using a steel frame rather than concrete.

Rectangular steel buildings had 10% more external cladding. The results indicate that concrete construction gives more liability space and leads to lower Finance costs. Apart from the frame material, the sizes of cost differentials are mainly related to plan shape, number of storeys and the type of ventilation.

B1 Roof Plan:

B-2 Roof Plan:

Main conclusions

The main conclusions are that, for a wide range to buildings, choosing concrete rather than steel can give up to:

* 24% savings in frame costs

* 5.5% savings in overall construction costs

* 1.5% more gettable area with:

o equal overall construction times

o And lower finance costs.

2-Introduction

The Cost Study was undertaken to provide both a comparison and an understanding of the construction costs Associated with concrete- and steel-framed buildings .There has been much debate over the relative merits of the two forms of construction, particularly in terms of cost and speed, but very little reliable published data Exists. Constantly changing market conditions, estimating practices, contract conditions and prices have confused the issues involved. It quickly became apparent that a detailed study, covering a range of buildings, was required in order to gain comprehensive picture. The objective of the study was to provide an impartial comparison between Concrete and steel frames for buildings of three- and seven-storeys in the UAE ABU DHABI industrial area; identical specifications were required with the only permissible variations being directly attributable to the material used in the Structural frame.

The cost models were developed using best current practice for both concrete and steel and are reported upon in

This publication to an extent the choice of buildings has influenced the answers obtained and questions raised

.nonetheless these, and the factual Aspects of the study, raise many interesting issues for those commissioning, designing

And constructing buildings in the 1999there are many useful conclusions to be drawn. The publication of this

Work is intended to fulfill several function .First, to illustrate the real difference between

Building in concrete and building in steel .Second, to put an in situ concrete sponsored cost model study against

Those prepared by the steel and precast concrete industries in the past. Third, to give the Designer & contractors more information to design and constructing for lower cost building to be economize.

3-Method of study

Brief

The brief given to the design team asked for the scheme designs of multi-storey B-1and B-2. The designs,

In concrete and steel, were to reflect contemporary commercial practice and the design team s best Judgment.

They would be used for preparing Budget costs, making comparisons and for investigating other issues.

Concepts and initial studies:

The buildings were to be located in the B1and B2 motorway corridors and were attributed to cost department & estimating respectively local market conditions prescribed construction cost budgets, for the purposes of the study, of 75AED/ft2 & 55 AED/ft2 .these budgets indicated that different specifications and designs were required for each location .the building configurations therefore reflect the two different budgets, being principally associated with the use or not of air conditioning.

The shape and form of the buildings were determined to suit local requirements in terms of performance and cost and not to favour concrete.

The budget B1 corridor buildings permitted the use of an all-air VAN or fan coil unit air-conditioning system. The use of air

Conditioning called for a square envelope, in order to minimize the wall to floor ratio, arranged around a central services core. Performance, cost and architectural criteria allowed for curtain-wall cladding. The budget for the B2 buildings only allowed for natural ventilation with convection heating. The need for cross ventilation meant that these buildings were configured into two wings of 15m depth, organized around a central core.

Initial layouts involving in board columns and cantilevers were not pursued as they may have unduly favored concrete. Originally six-storey buildings had been envisaged .they were not pursued as the steel schemes would have needed firemen's lifts whilst The concrete schemes would not investigations to determine the optimum structural grid for the proposed buildings were carried out .grids of 7.5 x 7.5m, 9.0x6.0m and 9.0x9.0m were considered.

Based upon overall project costs and floor area calculations, the 7.5x7.5 m and 9.0x6.0m grids appeared to be optimum for the steel schemes and the 7.5x7.5m grid appeared optimum for the concrete schemes. Thus, the 7.5x7.5m grid was established as optimum for both materials and locations, and was adopted for all buildings in the study.

Architectural floor plans were adjusted to suit.

These initial studies therefore determined the basis for the study. Concrete and steel framed options were to be developed using two floor configurations each at three and seven storeys. Thus, eight scheme designs were to be undertaken to give four pairs of buildings for comparison. And need all documents to give Integrated Study.

DOCUMENTS REQUIREMENT TO STUDY THE MODELS:

1. Architectural drawing each project

2. Structural drawing each project

3. All Technical specifications for each project

i) Technical specifications Mechanical & electrical parts

ii) Technical specifications Architectural & civil

4. Contract Agreement &conditions of contract

5. Vendor, Materials and Subcontractors List,

6. BOQ.

7. PROJECT HAND BOOK

NOS OF PROJECTS TO BE STUDY AND TYPE OF THE MATERAIL

S.N

4-DESIGN STANDARDS

Architectural

Vertical dimensions (mm)

Other Services

5-OUTLINE SPECIFICATIONS

B1 corridor buildingsless steel.

B1 corridor buildings

B2 corridor buildings

The outline specifications for the ((B.2)) buildings are identical to

The ((B.1)) buildings except as noted below

Architectural 00mm mineral fiber tiles with exposed slim line grid.

Structural

6-DRAWING S

B1 -3 STOREY CONCRETE FRAMED BUILDING & B1- 7 STORY

B1 C3

Typical cross section and detail: architectural

Notes

1. 500 x 500mm column

2. 200mm wide column zone

3. 500mm deep d/s beam

4. Assumed design imposed loads

Roof: 1.5kN/m2

Plant room: 7.5kN/m2

Offices: 5.0kN/m2

5. Floor zone (mm)

Slab: 300

Tolerances: 20

Deflection: 20

Total: 340

6. Key to pad foundation sizes (m) :

A 2.25 x 2.25 x 0.40 deep

B 2.80 x 2.110 x 0.53 deep

C. 4. 15 x 4. 15 x 0. 8 5 deep

D 24.0 x 2.60 x 0.60 deep

E 24.0 x 2.90 x 0.60 deep

B1 S3 3storey steel-framed building &

B1C7

Typical floor plan: architectural

7-QUANTITIES AND BUDGET COSTS "you can see this item in the excel file"

B1-C3 = Concrete 3-Storey B1-S3 = Steel 3-Storey

B1-C7 = Concrete 7-Storey B1-S7 = Steel 7-Storey

Base date : Dec1999 (Valid Sep 2001)

Budget cost summaries

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8 PROGRAMMES

8-1 PRE-CONTRACT PROGRAMME

ACTIVITY

CONTRACT WK.

8-2 CONSTRUCTION PROGRRAMMES

B1C3 & B1S3

B1 C3

Activity

B1 S3

Activity

B1C7 & B1S7

B1 C7

Activity

B1 S7

Activity

CONSTRUCTION PROGRRAMMES

B2C3 & B2S3

B2 C3

Activity

B2 S3

Activity

B2C7 & B2S7

B2 C7

Activity

B2 S7

Activity

9-STUDY FINDINGS

Overall Costs

In terms of overall construction cost, the Steel-framed buildings were found to be between 2.5 and 5.5% more expensive Than their concrete equivalents, as illustrated in the figure and table below, the seven-storey steel-framed building on the B-2 was found to be 5.49% more expensive than its concrete Equivalent and on the B-1 site 2.55% more expensive. The three-storey steel-framed buildings were found to be more expensive than their concrete equivalents, costing 5.18% more for the Rectangular B-2 building and 2.61% more for the square B-1 building. These figures are based on cost per m2 of gross External area. The differences in cost would have been greater if net Floor areas had been considered (see costs per net getable area) &see table no 10-1.

Cost Savings

Analysis of construction costs showed that the most significant savings occurred with the superstructure, with notable

Contributions from the external cladding and internal planning. For example, the combined cost of the superstructure and stud partitioning (internal planning excluding doors) for the three-storey steel-framed B-1building is 32% greater than the concrete-framed building. Rectangular steel buildings had 10% more area of external cladding than their concrete-framed equivalents.

SUBSTRUCTUES

Costs for the ground floor slab and associated ground works appear to be identical. Foundations for the steel-framed buildings cost between 2 1% and 47% less than the concrete equivalents. This is the consequence of smaller pad foundations for the lighter steel frame. The foundations in the models must be regarded as being very simple.

Table 10-1 Over cost

General note * cost AED (Dec1999) per gross external m2

**Percentage differences, steel, concrete

Superstructure

The structure of the steel-framed buildings cost between 24% and 32% more than the concrete equivalents. This is mainly due to the additional expense office protection for the steel frame. .

External Cladding

The external cladding (main elevations and roof plant cladding and screens) of steel framed buildings cost between 1% and 9% more than for the concrete framed buildings. Costs of roof plant cladding and screens are the same. Cladding to the main elevations costs between 1.5% and 9.6%, more on the steel framed buildings. The cost is related to the area of cladding required. This is due to the necessary increases in storey heights, from 3990mm to 4ll0mm on the B-1 schemes, and from 3300mm to3600mm on the B2 schemes, to accommodate the increased allowance for the structural zones of 340mm and 660mm.

Table 10-3 Structure costs

* Glass curtain wall @ DH 275/m2

# Brick cavity wall @ DH190 /m2

Internal Planning

The internal planning (stud partitions and internal doors) of steel-framed buildings costs more than for concrete framed buildings. Stud Partitioning is between 26% and 63% more expensive in the steel-framed buildings than in the concrete equivalents. This is due to the dual use of the concrete wall columns and core walls for both structure and internal walls in the concrete schemes.

ROOF FINISHES AND INTERNALM FINISHES , FIXTURES AND FITTINGS

There was little or no discernable difference between the concrete and steel frames with regard to the cost of roof finishes and fixtures and fittings .The slight variations in internal finishes reflect the different gettable and non gettable areas and the dissimilar storey heights..

MECHANICAL AND ELECTRICAL SERVICES

There was no difference between concrete and steel frames with regard to the cost of the lifts, mechanical services, electrical installation or builders work in connection with services. This is due to the assumption that the frame Material had no influence on services Provisions.

Preliminaries

The budget for preliminaries for each pair of building was based on a lump sum to allow for one off items such as Establishing site, crane bases etc., and a sum per week to allow for supervision and hire charges. Preliminaries Costs therefore vary according to the difference in the Programmers . Detailed consideration of preliminaries, e.g. Sizes of cranes, hire durations, etc, was beyond the scope of this study.

External works

There was no difference between concrete and steel With regard to external works,

Table 10-5 Preliminaries costs

Table 10-7 NET/GROSS RATIOS

Table 10-9 CONSTRUCTION PROGRAME DURATIONS

Table10-11 ADDITIONAL COST OF AIR CONDITIONING

Areas

The pairs of building were designed to have the same gross external and gross internal floor areas. The steel building had between 0.30 % and 1.48 % less net gettable floor area than the concrete building .This is mainly due to the additional areas required fit toilets and cores around the steel structure. The floor Area lost to accommodate steel Bracing is noteworthy. The B1 steel schemes have Ground floor reception lobbies some 20m2 larger than their concrete Equivalents, due to architectural Considerations. Nonetheless, had Identical reception lobbies been used, the steel buildings would still have had between 0.5% and 1.24% less net gettable floor area.

Net gettable Areas

The difference I areas are also reflected in the net to gross internal floor area ratios.

Costs per net gettable areas

In terms of the cost per gettable area, the steel framed buildings were found to be between 3.7% and 6% more expensive than their concrete framed equivalents. The differentials were found to be Greater for the B-2 building compared to the B1 building and greater at 7 storeys rather than 3 storeys. Analysis of the costs per net gettable Area reveals that most savings emanate from the superstructure. Significant Savings also arise from the external Cladding, internal planning and mechanical and Electrical services .the saving apparent from M&E services is entirely due to the difference in the net gettable areas.

Construction programmes

The construction programmers for the concrete schemes are up to two weeks faster than their steel equivalents. This is due to the need to fire proof steelwork, the effects of fire proofing on the Programmers, and, on the B2 models, Additional amounts of external Cladding. Steelwork frames can start earlier and are erected quicker than their concrete equivalents. This allows the external Cladding and roof finishes starting and, to some extent, finishing sooner. However, the Fire proofing of steelwork completely eliminates this early advantage. This activity requires each floor to be wearing her tight before perimeter details can be completed. The assumed external cladding and roof finish details must be essentially complete, before fire proofing can be completed. The net effect is to delay the effective starts of fire casing, 1st fix and subsequent critical Activities. The additional quantities of external block work cladding on the B-2 steel. Framed buildings called for larger gang sizes than the concrete buildings. One or two weeks would have been added to this critical activity had the same gang sizes been assumed. The durations of 1st fix trades, 2nd fix trades and fixtures and fittings are essentially the same. In the steel options, the start of 1st fix trades is governed by fire proofing. Slight differences in quantities appear to make little difference to the programmers. Generally, the steel Frames allow an earlier start and additional float on then on-critical mechanical and electrical elements. However, the float for lifts on the B1-S7building is less than for the B-1C7 building. External works are non-critical. The B-2 buildings take longer than their B1 equivalents. Whilst the B2 buildings can be treated as two halves and starts made earlier, completions are controlled by the two halves coming to gather over the core areas. They also have additional quantities of cladding

Overall procurement times

Traditional forms of contract under Traditional forms of contract the pre-contract period was assessed as being 20 weeks for all buildings, allowing two weeks for contractor mobilization. Differences contract programmers (and therefore overall procurement times under traditional lump sum forms of contract) hinge on the allowances required for steel s long lead-in time .This lead-in time causes potential delay which can only be absorbed by pre-letting (as assumed). Had tenders, mobilization, foundations and / or substructures taken longer, steel s lead-in time would have become less or non-critical as critical activities could have run in parallel. Alternative methods of procurement there would be overall time advantages, using construction management and design and build forms of contract for the concrete options. These alternative forms of contract appear to be less attractive for the steel options as lead-in times delay the effective starts on site. Lead-in times also make late finalized design and variations more difficult to accommodate. As before, had detailed design, mobilization, foundations and/or substructures taken longer, steel s lead-in time would have become less or non-critical and these alternative methods of procurement would have become more attractive. General Overall procurement times are very Much influenced by the method of Procurement, type of contract used and lead-in times. The more generally held view is that there is no significant difference in overall procurement time between steel- and concrete-framed Buildings.

Other observations

Number of storeys

The cost per gross external m2 was found be between 6.8% and 7.8% cheaper for he seven-storey buildings compared to he three-storey buildings. The savings are due to relative reductions in preliminaries, external cladding, finishes and foundations. 'They more than off set extra costs for Services and superstructure.

Cost per gross m*, storeys/3storeys I

B1 Concrete -7.74%

Steel 7.79%

B2 Concrete -7.12%

Steel - 6.85%

Air conditioning

The air-conditioned buildings appear to be approximately 30% more expensive to construct than their naturally ventilated equivalents. Detailed analysis and comparison is complicated by further differences in specification, and different rates used for the two locations. However the premium for air-conditioned buildings over naturally ventilated buildings are in the above.

Construction costs

Main conclusion

The main conclusion to be drawn from the study is that , for a range of buildings, construction using concrete frames is more economic than using steel frames.

Choosing concrete rather than steel gave to up to 32% savings in frame costs and up to 5%savings in overall construction costs . The structure accounts for a relatively small proportion of total construction cost, yet the choice of frame material has noticeable effects on other cost elements.

Difference in cost

Foundations for the concrete options cost more but account for relatively small proportion of overall cost and so are more than compensated for by savings in other cost elements .the main source of savings using concrete lies in the superstructure. The costs of formwork, reinforcement and concrete for the concrete options must be compared with the costs of steelwork ,decking and concreting floors ,and fire casing for steel options .fire casing accounts for the majority of the difference. Apart from the frame , the other main sources of savings using concrete are cladding of the main elevations and internal partitioning . there are minimal differences in the costs of finishes and ,assuming the same contract periods, there is little apparent difference in the costs of preliminaries . there are no differences in the costs of mechanical services and electrical installation due to the design assumptions made.

Area and costs of net gettable area

The concrete options give greater ratios of net to gross floor areas, with up to 1.5% more net gettable area. This appears to be due to the flexibility of concrete walls as partitions, and the allowance for steel bracing zones . together with the fact that the concrete options are more economic to construct on gross external area basis, the costs per net gettable area are significantly less .

Finance costs

Analyses of each flows during construction show that the steelwork options require greater expenditure earlier than their concrete equivalents .the steel work building require, therefore more financing during the construction phase and are consequently subject to greater finance costs .all other things being equal, concrete construction s pay as you pour principle saves on finance costs this point can be illustrated by using the seven story B1 models as bases.

10- Results

* Overall, concrete is more economic than steel.

* Concrete construction gives more net gettable space (therefore the cost per net gettable area is significantly less)

* Substructure and superstructure account for a relatively small percentage of overall costs.

* The higher costs of concrete frames foundation are more than offset by savings in other cost elements.

* The choice of superstructure has a noticeable effect on other elements of costs.

* Whether using in situ reinforced concrete composite steel frames, overall procurement times and contract durations are the same for these types of buildings.

* Shorter lead-in times favour concrete as a frame material, especially for nontraditional forms of contract.

* All other things being equal , concrete construction leads to lower finance costs.

* Overall, the seven-storey buildings are 7% to 8% cheaper to build per gross floor area than the three storey buildings.

* The air-conditioned buildings are approximately 30% more expensive to construct than naturally ventilated buildings.

9-Appendices

11-A / planning & programming

* Overall procurement time

Method of procurement:

The effects of the method of procurement on pre-contract and overall project programmes were briefly considered . three types of contract were reviewed:

_Traditional lump sum where the works are designed and subsequently measured by the client's consultant, for pricing by tenders .

_Design and build where the main contractor takes on the additional responsibility of design and where the main contractor takes on the additional responsibility of design and prepares his own subcontract tender documentation.

_Construction management where a series of trade contracts are let by the construction manager on behalf of the employer. management contracting where trade subcontracts are let through the management .

The steel-framed options did not appear to be as suited to construction management or design and build forms of contract as the concrete as the concrete options. This is due to the effects of lead-in times delaying effective start construction .

* Pre-contract programme

The pre-contract programme was prepared on the basis of a traditional lump sum form of contract. It was assumed that design times are similar for the two materials.

It was assumed that the start of concrete frame construction on site was controlled by the release of initial bending schedules & procurement of reinforcement "approximately 1-3 weeks for normal delivery ", and that commencement of steel frame construction was controlled by release of its design "& its procurement "currently 10-12weeks". No allowance was made for delayed approvals .

* Construction programmes

Basis

As neither timescale nor time penalties were imposed, the programmes were drawn up to reflect those that might be negotiated between a client and contractor to achieve optimum overall construction cost.

11-B / Cost Breakdowns " you can see this item in the excel file "

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Cost Engineering ID 45449 Page 1

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