Design of Buildings of Steel and Concrete: Comparative Assessment of Limited Continuity

Approach and Composite Construction

Dr. P. Suryanarayana, Professor, Maulana Azad National Institute of Technology, Bhopal

Public buildings requiring large floor area can be economically constructed adopting steel framework of columns, girders and flooring of reinforced concrete. The recently published steel code (IS:800-2007) includes Load and Resistance Factor methods of design adopted from the latest foreign codes. The outdated clauses for design of composite beams and columns are now removed from IS:800-2007. It is expected that new versions of composite construction, based on European codes will be published soon by the Bureau of Indian Standards.

This paper presents various alternatives for design of Steel-Concrete Framed Buildings and examines the economy of each arrangement. Comparison is made between two types of steel construction (traditional and limited continuity type) as well as economy and constructional advantage achieved by composite construction. The new Indian steel code as well as the latest European codes are used.

A four storey industrial frame of 18mx36m plan area is designed using five alternate approaches. Each of the five alternative designs is done by Allowable Stress Design (ASD) as well as Load and Resistance Factor Design (LRFD) and results compared.

Introduction

Public buildings requiring large floor area can be economically constructed by adopting steel frame work of columns, girders and flooring of reinforced concrete slabs. The traditional design of steel-concrete buildings has the following features. (Figure 1) The floor system consists of a slab supported by a grid work of beams. The beams frame into columns in such a way that the centre lines of beams in longitudinal and transverse directions intersect at the column centre. The beam column joints are assumed to be pinned. Hence the beams are designed as single span beams with hinge supports.

An alternate arrangement wherein the secondary beams are continuous over the primary beams and the primary beams are continuous over column brackets is suggested by British designers. In this arrangement, the centre lines of primary beam, secondary beam and column do not intersect at a common point. [5*]

A comparison of traditional and alternate designs is made by designing a building panel of 18m x 36m for a four storey building by both the approaches. Calculations for each design are done both by Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD). The designs are based on elastic and plastic analysis, lateral stability calculations and deflection checks [1,2,3,4]. The building frame consists of 6.0m x 9.0m slab panels. Columns are 4.0m high per storey.

The five design alternatives are as follows:
  1. Traditional design (Fig.1): Primary beams (9.0m) and secondary beams (6.0m) designed as single span beams hinged at ends. It is assumed that the slab provides full lateral restraint to beams.
  2. Traditional design (Fig.2): Secondary beams at 3.0m spacing. All beams designed as single span beams hinged at ends.
    * References are listed alphabetically at the end of the paper
  3. Limited continuity design (Fig.3): Secondary beams (at 4.5m spacing) designed as 3 span continuous beams, transmitting concentrated loads to primary beams. Primary beams (9.0m) designed as single span beams hinged at ends.
  4. Same as design 3, except that secondary beams are spaced at 2.25m (Fig.4)
  5. Steel-concrete composite construction (Fig.1): Secondary beams are designed as 3 span continuous beams and primary beams as 4 span continuous (Propped Construction with two props at one third points for all spans).
Design of Buildings of Steel and Concrete


In all designs dead load of 4.0 kN/sqm. (slab, floor, finish, partitions) and live load of 4.0 kN/sqm is assumed. Slab 140mm thick, M20 concrete, steel sections of Fe 250 grade adopted.

Codes and Design Methods

Design of steel beams: The beams are designed both by allowable stress design (ASD) and load and resistance factor design (LRFD). ASD Methods developed by Brett, Galambos, Kirby, Nethercot and Trahair [5] were used and the lightest section obtained from these designs are adopted. The designs are repeated by LRFD methods given in [2.4].

Design of steel columns is done by the latest Indian Code. [2]

Design of composite beams and columns: The Indian code for composite beams (3) does not include design of continuous beams. There are no code provisions regarding the design of beams with slender sections. In this paper, the design procedures given in [4] were followed.

The outdated and irrational design procedure given in IS 800-1984 [1] for the design of composite columns is now removed in the 2007 version [2,7]. At present there are no guidelines available in Indian Codes for composite columns. Therefore the procedure given in [4] is adopted.

Design of Buildings of Steel and Concrete

Summary of Designs

The cross sections of beams, girders and columns obtained by LRFD methods are shown in figs. 5 to 10 and tables 1 and 2. A comparison with ASD results is given in tables 3 and 4.

In the steel column (2 channels ISMC 400) (Fig.6) the channels arranged to form a box section 350mm x 400mm so that the column is equally strong for major axis and minor axis buckling. The steel requirement of the composite column is much less (Fig.5) The same column section can be adopted at all locations with appropriate orientation as shown in Fig.1

Span Beams

Span Beams

From Tables 1 and 2 it can be seen that all the beams are designed for optimum performance. In Table 1, (Design 3) the section is slightly over designed. However the section cannot be reduced, if the deflection criterion is to be fulfilled.

It is seen from Tables 3 and 4 that designs obtained by LRFD methods result is lighter sections as compared to ASD methods. Of the five alternative designs considered, it is seen that composite construction is the best option, resulting in great savings of steel.

Comparison of ASD and LFRD/LSD
 
Comparison of ASD and LFRD/LSD

There are many structural and constructional advantages of composite construction vis-a-vis steel frame work. These can be summarised as follows:
  1. Due to the composite action of concrete flange and steel rib, the steel section required in a composite beam is smaller that the section needed for a steel beam, both for single span beams and continuous beams.
  2. Due to increased stiffness of the composite section, beam deflections are reduced.
  3. The concrete slab provides adequate lateral restraint for the steel rib.
  4. By comparing Fig.1 with Figs. 2,3 and 4 it can be seen that a frame comprising composite beams and columns is the most stable among the alternatives.
  5. Speedy construction is possible by proper sequencing of operations. Providing continuity and a rigid frame work is easier in composite construction (as compared to steel work) due to monolithic action.

Conclusion

This paper presents design alternatives for a four storey, 18m x 36m steel-concrete framed building. The frame consists of 20 columns and 16 primary beams of 9.0m span. The number of secondary beams (6.0m) ranges from 15 to 42 in the five alternative arrangements. The beams and columns are designed for strength and checked for serviceability both by ASD and LRFD methods. Design methods from Indian and foreign codes, as well as methods developed by different designers are used. The most economical designs are adopted.

The following conclu- sions are drawn from the study.
  1. LRFD methods are more comprehensive than ASD methods and result in economical design. Savings of order 11% for secondary beams and 16% for primary beams are achieved.
  2. Steel-concrete composite construction requires less number of secondary beams compared to limited continuity designs.
  3. Due to composite action, the size of steel sections can be reduced.Savings of order 22% for secondary beams and 15% for primary beams can be achieved. These savings are partially offset by the cost of shear connectors.
It is concluded that safe and economic designs can be obtained by the LRFD method given in IS:800-2007 for steel frames.

Steel-concrete composite frames are preferable to steel frames due to structural and constructional advantages. A comprehensive code of practice comprising the LRFD methods mentioned in this paper is expected to be available soon.

References

  • IS 800-1984 IS Code of Practice for General Construction in Steel (Second Revision), BIS, New-Delhi.
  • IS 800 : 2007 IS Code of Practice for General Construction in Steel. (Third Revision), BIS, New-Delhi.
  • IS 11384-1985 IS Code of Practice for Composite Construction in Structural Steel and Concrete, BIS, New-Delhi.
  • Narayanan, R. et al: Teaching Resource for Structural Steel Design Ch. 10, 11, 12, 21, 22. Indian Institute of Technology, Chennai, INSDAG, Kolkata July 2001.
  • Richariya, A.K. "Design of Buildings of Steel and Concrete: Comparative Assessment of Limited Continuity Approach and Composite Constru- ction" M.Tech Thesis, Maulana Azad College of Technology, Bhopal 1993.
  • Sangamnerkar, P. "Design Alternatives for Steel-Concrete Framed Buildings". M.Tech Thesis. Maulana Azad National Institute of Technology, Bhopal 2009.
  • Suryanarayana, P. "A Proposed Amendment to IS : 800-1984 (Encased Column Design). Civil Engineering & Construction Review, Vol. 2, No.5, May 1989 pp. 44-46.
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