Stifferness

SE21 p36-42 Ji

29/10/03

3:42 pm

Page 36

paper: ji

Concepts for designing stiffer structures
Synopsis
The paper demonstrates concepts for designing stiffer structures. They are: (a) the more direct the internal force path, the stiffer the structure; (b) the more uniform the internal force distribution, the stiffer the structure; and (c) the smaller the internal forces, the stiffer the structure. These concepts are applicable to the design of many structures. Two ways of implementing the concepts into practice are provided. Simple examples are given to illustrate the implementation and the efficiency of the concepts. Laboratory tests and the demonstration of two physical models further confirm the findings. Several practical designs are also provided to show the applicability and significance of these concepts. An alternative definition of structural stiffness is given which complements the existing definition and allows for designing stiffer structures. It is interesting to note that using the concepts may lead to not only stiffer but also more economical and elegant designs. K= P D

Tianjian Ji
...(3)

Introduction
Buildings have become taller, floors wider and bridges longer in recent years. It is expected that the trend of increasing heights and spans will continue in the future. How can engineers cope with the ever-increased heights and spans, and design structures with sufficient stiffness? The basic theory of structures provides the conceptual relationships between span (L), deflection (∆), stiffness (K) and natural frequency (f) for a single-span structure carrying distributed loads as follows: D = c1 = c2 L4 K f = c3 K = c 4 L2 ...(1) ...(2)

This definition of stiffness provides a means of calculating or estimating the stiffness of a structure, but does not suggest how to find a stiffer structure. How to design a stiffer structure (the form and pattern of a structure) is a fundamental and practical question and may be more important

References: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. Longman: Dictionary of English Language and Culture, (1992), Longman Group UK Ltd, ISBN 0582 08676 0 Parker, S. P.: Dictionary of Engineering, 5th Edition, (1997), McGraw-Hill, ISBN 0-07052435-1 Ji, T. and Ellis, B. R.: ‘Effective bracing systems for temporary grandstands’, The Structural Engineer, (1997), 75/6, p 95-100 Gere, J. M. and Timoshenko, S. P.: Mechanics of Materials, (1990), PWS-KENT Publishing Company, ISBN 0 534 92174 4 Coates, R., Coutie, M. and Kong, F.: Structural Analysis, (1998), Chapman & Hall Roohi, R.: Analysis, testing and model demonstration of efficiency of different bracing arrangements, Investigative Project Report, (1998), UMIST Ji, T. and Bell, A. J.: ‘Seeing and touching structural concepts in class teaching’, Proc. of the Conf. on Civ. Eng. Education in the 21st Century, Southampton, UK, 26-28 April 2000 Anderson, J.: ‘Teaching health and safety at university’, Proc. of the Inst. of Civ. Eng., Journal of Civil Engineering, (1996), 114/2, p 98-99 Bennett, D.: Skyscrapers – Form & Function, (1995), Simon & Schuster ISBN 0-684-80318-6 Bobrowski, J.: ‘Design philosophy for long spans in buildings and bridges’, Journal of Structural Engineer, (1986), 64A/1, p 5-12 The Structural Engineer, 72/3, 1994 Lian, Q., Xie, Y. and Steven, G.: ‘Optimal topology design of bracing systems for multistorey steel frames’, J. of Structural Engineering, ASCE, 126/7, p 823-829 Zalka, K. A.: Global Structural Analysis of Buildings,(2000), E & FN Spon, London, ISBN 0-41523483-2