GROWTH – DEDICATED CALL – 10/00
TOPIC IV.4
Shear and torsion interaction of hollow core slabs
1. CONFORMITY WITH THE WORK PROGRAMME
This topic falls under the Competitive and Sustainable Growth Programme, generic
activity Measurement and Testing. Specifically, it is related to Objective GROW-20006.2.1 Methodologies to Support Standardisation and Community Policies for which
expressions of interest have been called.
CEN/TC229 - Precast Concrete Products - is preparing under Mandate 100 by the
European Commission, a standard prEN 1168-1 "Precast concrete products - Hollow
core slabs for floors - Part 1:Prestressed slabs". The specifications given in this standard
concerning the calculation and testing of the shear and torsion capacity, are conservative
as far as torsion concerns and do not take account of the interaction effect of both
actions on the actual tensile stresses in the concrete section. In this way, the standard
provisions do not allow to use the full capacity of the product.
The hollow core slab is a precursor in the context of sustainable construction because of
the econoy with materials: about 40% less concrete and 50% less reinforced steel than
classical building methods. In addition, all waste material during production is recycled.
The impact could still be enhanced by the result of the present research project.
The project will also improve the market and export potential of the product itself and
the technical know-how for example in worldwide commercialization of turn-key
production plants.
2. KEYWORDS
Shear, Torsion, Hollow-core slabs, Precast concrete, Prestressing,
3. SUMMARY OF OBJECTIVES AND JUSTIFICATION
The objective of the project is to improve the current knowledge of the interaction effect
of shear and torsion in prestressed hollow core slabs and to develop a calculation model
verified by experimental results. The program includes both the interaction for
individual slabs and for complete floors designed according to the Eurocodes and the
product standard PrEN 1168-1. The present calculation of the interaction effect is based
on a reduction of the shear force due to the applied torsional loading. The formula gives
no possibility to consider the redistribution of the forces and the tensile stresses in the
total floor and in the individual elements. Preliminary studies and tests at the Technical
University of Lulea in Sweden have shown that such distribution exists. However,
models for design and testing are missing.
DC 10/00/Topic IV.4/ Pg 2
4. BACKGROUND
Prestressed hollow core elements are among the most advanced products in the
precasting industry. Today, the annual production in West Europe amounts to more than
25 million m². Different properties make the product so interesting: half the material
used for the same performances than classical plain concrete slabs, industrialized
production process, high durability etc. Prestressed hollow core floors have no other
reinforcement than the longitudinal prestressing wires or strands, anchored by bond.
Research and practical experience have learned that this does not affect the shear and
torsion capacity of the floor on condition that the design principles set forth in the
Eurocodes and the CEN Product Standard are met.
Torsional loading of a single hollow core element causes shear forces in the outermost
webs. They are calculated according to Eurocode provisions for plain concrete cross
sections. This shear forces act upwards in one and downwards in the other of the
outermost webs. The resulting stresses are to be cumulated with those resulting from the
normal shear force, which is equally distributed over all the webs. From a pure
theoretical standpoint, this means that one web in the total cross-section receives much
more stresses than the others. However, practically, there is a certain distribution of
stresses over the whole section.
For a single element, a complete distribution over all the webs would probably result in
high demands on the material ductility. However, in an actual flooring structure the
elements have lateral support from the neighbouring elements through the shear keys in
the longitudinal element joints. These effects will enable to redistribute the shear forces
over the webs of a unit subjected to a combined shear and torsion loading.
The study comprises the elaboration of a theoretical design model and the following test
series:
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-
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Hollow core element loaded in torsion, shear and bending
The aim is to verify the shear capacity for different torsional and bending moments.
Further parameters are the slab thickness, span and loading positions
Transversal load distribution
The aim is to verify the load transfer and the torsional forces between elements, for
a floor composed of 5 elements, with different spans and loading positions
Deformability of floor structure
The aim is to investigate the potential risk of torsional failure of a floor structure
simply supported along 3 edges and subjected to vertical loading. Also here the slab
thickness and the span length are varied.
5. ECONOMIC AND SOCIAL BENEFITS
The economic and social objectives are to develop a design model for the interaction of
shear and torsion in precast prestressed hollow core floor elements. The result of the
study will be directly used in the Eurocode 2 and the CEN Product Standard prEN
1168.
DC 10/00/Topic IV.4/ Pg 3
For the European Precast Concrete industry, the turnover in 1995 approximated 20.600
million Euros and the employment was estimated at about 189.000 people in 5000 plants.
The results of the study will lead to a decrease of the required material quantities for
concrete and steel and decrease the self-weight of the floor. The possible direct savings are
in the magnitude of 5 % of the production costs. In addition, the more favourable
economical position will result in a significant acceleration of the market penetration, both
inside and outside the E.U. market.
The social benefits are related to the safety of the units, by a more correct calculation of the
shear capacity and the effects on the environment: less extraction of raw materials, a
reduction of the energy costs for the manufacture and less total waste.
6. SCIENTIFIC AND TECHNOLOGICAL OBJECTIVES
The technical objectives are to remove the barriers to full exploitation of the hollow
core elements by the existing code prescriptions. The quantitative results of this project
would be important. The simultaneous occurrence of shear and torsion happens in the
corners of floors and around large openings, for example for stairs. However, since the
slab thickness has to be the same over the whole floor, the most unfavourable situation
is governing the whole floor. The possible saving by the results of the proposed project
are in the magnitude of 5% of the cost.
7. TIME SCALE
The maximum duration of the project will be 3 years. The results will be included in a
first revision of the Product Standard EN 1168, in 2004/2005.
8. IMPORTANT ADDITIONAL INFORMATION
References
PrEN 1168-1 "Precast concrete products - Hollow core slabs for floors - Part
1:Prestressed slabs" prepared by CEN/TC229
Gabrielsson, Henrik "Ductility of high performance concrete structures " Doctoral thesis
Lule University of Technology 1999:15
Comparison of the environmental effects of three concrete storey floors, CREM report
n° 95.107 - Consultancy andResearch for Environmental Management - Spuistratt 104d,
1012 VA Amsterdam, August 1996.