COMPUTER MODEL FOR COST ESTIMATION OF STRUCTURES IN HIGH RISE COMMERCIAL BUILDINGS

INTRODUCTION

At the initial stages of architectural design for any project, the architect is to consider various conflicting requirements while making the choice for a suitable structural system/scheme. Thus the structural system/scheme finally selected may not be the one with a minimum cost. While considering different aspects of the problem he thus needs to consider the comparative cost of different systems/ schemes for cost optimization.

Cost estimates for structural works are often made by assuming quantities of concrete, reinforcement and formwork. In the absence of realistic information relating to variations in quantities of material with changes in design parameters, the quantities assumed tend to be very approximate and the percentage error could be large. Where more realistic estimates are required engineer works out alternative structural schemes for computing the quantities and costs for the various schemes. It is extremely unsystematic and wasteful if structural schemes are to be worked out and cost estimated every time a new building project comes up. With this in view an effort was made by the author to develop interactive computer models for cost estimation of structural systems in high rise buildings, up to 50 storeys high, using different structural systems. The structural systems considered were reinforced concrete beam and slab system, reinforced concrete flat slab and waffle slab systems and prestressed concrete beam and R. C. slab system.

The applications of the model developed can be made for comparative cost estimation to assess the effect of various design parameters, for approxima­te structural cost estimation of an overall project given its design features, for checking the estimates for structural works, for calculation of quantity index for structural works and for various other building economics studies.

This paper outlines the computer based cost models developed and illustrates their various applications.

STRUCTURAL SCHEMES, ANALYSIS AND DESIGN

The structural schemes considered for reinforced concrete beam and slab construction and prestressed beam and R.C. slab construction are shown in Figures 1 and 2 respectively. The sizes of the square column grids ranged from 6-10 metres for the first type of construction while for the second the range was from 10-14 metres. The analysis and design were in accordance with the limit state design proposed in the British Code of Practice CP110: Part 1: 1972, the service loads being taken from the British Code of Practice CP3: Chapter V: Parts I and II: 1972. Frame-shear wall interaction was considered in the analysis for lateral loads, sub-frame, grid beam and continuous beam analysis being carried out for gravity load analysis depending on the appropriateness.

For the analysis of structural schemes using flat and waffle slabs empirical method as codified in the British Code of Practice CP110: 1972 has been used. Square grids with sides of 6, 8 and 10 metres for structural schemes with flat slabs and with sides of 6.4, 8 and 10.4 metres for those with waffle slabs were considered. The grid size in the latter case were varied so as to accommodate the standard moulds available locally.

COMPUTER BASED COST MODELS

Computer based cost models (programs) were developed using the results of charts and statistical relationships [1,2,3,4]. The model elements for which quantities and costs can be computed by these programs are slab, beams (if applicable), columns, shear walls and total structure. The programs were perfected to run on the IBM 3081 mainframe computer and a microcomputer version of the same were also made. Flow charts developed to write the programs are illustrated elsewhere [5].

APPLICATIONS

For clarity, in each case, the design problem has been defined first and based on the data given, a solution using the computer has been obtained.

Comparative Cost Estimation

The programs developed are capable of supplying the comparative cost information for the effects of different design parameters on structural cost which is needed at the architectural design stage for cost optimization. Such an application is illustrated in the Problem 1 below.

Problem 1- Structural Schemes A design team is involved in the planning of 40 storey commercial building and the work is at the very initial stages. It is required to present the comparative unit costs for the following structural schemes using the given data:

Prevailing rates of materials may be assumed in the calculations.

A sample solution obtained using the computer model is shown in the Table 1 and an overall in Table 2 which can be considered along with other conflicting factors to select an appropriate scheme for cost optimization.

Approximate Structural Cost Estimation of an Overall Project Given the salient features of a commercial building project, the models developed are capable of computing the structural cost of an overall project. This is illustrated in Problem 2.

Problem 2- Total Structural Quantities/Cost Dete­rmine the total structural quantities for a 25 storey commercial building project (Figure 3) using prestressed beams and R.C. slab construction (Scheme B2, Figure 2) given the following design features:

their numbers

Concrete grade 30 N/sq mm is to be used in all components. Assume prevailing market rates for different constituents for the cost estimation. Solution obtained using the computer model is shown in Table 3.

Checking of Estimates for Structural Works Another important area for application of the computer models developed is in making an overall check of the structural estimates prepared based on the details supplied by the Structural Engineer. The object here is to find out if substantial differences exist between the initial estimates and those computed with the help of the model. If such differences exist, there is a need to look into the original workings and reason out differences. In so doing the errors made, whether by the Engineer or the Estimator, can be located and necessary corrections made.

Calculation of Quantity Index for Structural Works Past historical information of constituent quantities for structural works in completed projects is of immense value for computation of approximate cost estimates for present/future projects. However, invariably, there are always variations between the design features of different projects. To adjust constituent quantities due to these variations, quantity index for structural works can be a useful device.

The computer models developed can be utilized in computing the constituent quantities, for a desired set of design features, which in turn can be utilized for establishing the quantity index. The quantity index so established can in turn be utilized in adjusting the quantities of past projects so as to arrive at the quantities for the new projects. This is illustrated in the solution of the problem which follows.

Problem 3- Quantity Estimates It is required to estimate the constituent quantities for structural works needed for a proposed 30 storey commercial building having the design features similar to the

one recently completed within the organization with the following differences:

Solution obtained using the computer model is shown in Table 4.

Establishing Cost Index for Structural Works

The cost index for structural works in a building can be established using the quantities obtained with models and the index so established can be used for estimating the cost of structural works in new buildings based on similar projects executed in the past.

Budgeting of Materials

Budgets of materials are always needed for arranging in time the supply of materials for construction projects. In cases, where details of projects are not available in time for placement of supply orders, computations of total requirements for structural works can be worked out using the models developed.

Building Economics Studies

The structural costs of buildings are influenced by a variety of factors, some of which are interrelated. It is essential that building economist should be fully aware of the cost consequences resulting from changes in shape, size, structural system, structural scheme, number of storeys, grid size and grid location. The models developed are capable of computing quantities/cost for each of these variables.

Problem 4- Cost Variation Considering a grid size of 10m * 10m and the structural scheme B2 (Figure 1), generate cost per sq m of floor area for an interior grid in various storeys of construction ranging from 5 to 50. The rates stated below for 5 storey construction can be considered for costing. Further, it can be assumed that, on an average, the construction cost rises at one percent per floor.

Solution obtained using the computer model is shown in Table 5.

CONCLUSIONS

Appropriateness of a structural system for a building from the point of view of cost depends on the relative prices of materials of construction which vary disproportionately from time to time. The models developed form a convenient tool in estimating structural costs for different systems/ schemes for cost optimization at the initial stages of design.

The computer based cost models developed are also of great utility for approximate structural cost estimation for an overall project given its design features, for checking the design and cost estimates for structural works, for calculation of quantity index for structural works and for various other building economics studies in the building industry.

REFERENCES