Keywords: numerical models, finite element method, strut-and-tie model, reinforced concrete, structural design, structural failures.

Structural failures usually result from the coincidence of several reasons but those associated with erroneous design can have the most severe consequences. Several examples of design errors as well as statistics about their occurrences in concrete structures have been extensively reported in the literature [1,2,3,4,5,6]. Nowadays due to the experience in all type of reinforced concrete structures, the adequate calculation programs and the establishment of various controls during the course of a job, the possibility of a gross design error is increasingly remote. However concrete design demands more than plain numerical checking and, as a result, associated failures continue to appear. A well known example of such kind of design errors with severe consequences were those that caused the SLEIPNER A platform collapse in Norway [6,7].

Highly hyperstatic structures such as reinforced concrete (RC) buildings are even more sensitive to these problems. RC design is based on simplifying assumptions which however are not perfectly correlated with actual conditions in the structure. In this sense, Finite Element-based programs (FE) are a useful tool for the analysis and calculation of bi- and tri- dimensional concrete structures (independently of their degree of complexity) since both non-linear material and second order effects can be considered. The currently available FE programs include powerful tools like the automatic mesh generation that permit fast and efficient data introduction but inexperienced use on the other hand could result to errors in the design of some of the parts of the structure. For example, local discontinuities that appear frequently in concrete structures are often disregarded during automatic mesh-generation and as a consequence specific checks are necessary especially when a design is hastily altered before or during construction.

In order to perform the checks and even solve constructive D structural zones a useful alternative tool is the strut-and-tie method [8,9,10]. This method, based on the lower bound theorem of plasticity, allows such kind of D zones to be solved in a expedite way without the necessity of a computer. Moreover, the behaviour of the area concerned and the possible mechanisms of failure can be graphically checked.

In this paper, the cause of the collapse in 1998 of the slab of a water reservoir is described and analysed. The possibility of errors due to the use of automatic mesh generation for the structural calculation with a FE analysis is also checked. In order to explain the failure mechanism and to calculate the necessary steel that must be placed in the joint area, the strut-and-tie model is used.

As a conclusion, a proposal showing the advantages of using both methods to solve complex structural problems such as the one presented here is described. Combining both methods can result into solving discontinuities in an adequate way and at the same time saving computing cost and time.

1

"Study of failures of Concrete Structures" Bulletin, American Railway Engineering Association, V. 20, no 211, Nov. 1918, pp. 3-28.

2

"Collapse Blamed on Bearing Failures", Engineering New-Records, V. 165, no 24, Dec 1960, p.28.

3

Matousek, Miroslav and Schneider Jorg, "Untersuchungen zur Struktur des Sicherheitsproblems bei Bauwerken", Report no 59, Institute of Structural Engineering, Swiss Federal Institute of Technology, Zürich, Feb. 1977.

4

ACI "Avoiding Failures in Concrete Construction", Seminar Course Manual, SCM-19, American Concrete Institute, USA, 1989.

5

http://www.ima.umn.edu/ arnold/disasters/disasters.html

6

Calavera, J., "Patología de estructuras de Hormigón Armado y pretensado". Tomo I. INTEMAC, ISBN: 84-88764-02-2, 1996. Pages. 147-148.

7

http://www.findarticles.com/cf_0/PI/search.jhtml?key=SLEIPNER+A

8

Collins M.P., Mitchell D. "Shear and torsion design of prestressed and non- prestressed concrete beams". Journal of the Prestressed Concrete Institute, Vol. 25, No 5. Sept.-Oct. 1980. Discussion, Vol. 26, No 6, Nov-Dec. 1981.

9

Schlaich J., Weischede D.: Ein praktisches Verfahren zum methodischen Bemessen und Konstruieren im Stahlbetonbau, CEB, Bulleti d'Information no 150, Paris. 1982

10

http://cee.ce.uiuc.edu/kuchma/strut&tie/

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