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Concrete flat slabs and footings: Design method for punching and detailing for ductility
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Design and Bridges.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Simple but still realistic physical models suitable for structural design of flat concrete plates and column footings with respect to punching are presented.

Punching of a flat plate is assumed to occur when the concrete compression strain at the column edge due to the bending moment in the slab reaches a critical value that is considerably lower than the generally accepted ultimate compression strain 0.0035 for one-way structures loaded in bending. In compact slabs such as column footings the compression strength of the inclined strut from the load to the column is governing instead. Both the strain limit and the inclined stress limit display a size-effect, i.e. the limit values decrease with increasing depth of the compression zone in the slab. Due respect is also paid to increasing concrete brittleness with increasing compression strength.

The influence of the bending moment means that flat plates with rectangular panels display a lower punching capacity than flat plates with square panels – a case that is not recognized by current design codes. As a consequence, punching shall be checked for each of the two reinforcement directions separately if the bending moments differ.

Since the theory can predict the punching load as well as the ultimate deflection of test specimens with good precision, it can also treat the case where a bending moment, so called unbalanced moment, is transferred from the slab to the column. This opens up for a safer design than with the prevailing method. It is proposed that the column rotation in relation to the slab shall be checked instead of the unbalanced moment for both gravity loading and imposed story drift due to lateral loads.

However, the risk for punching failure is a great disadvantage with flat plates. The failure is brittle and occurs without warning in the form of extensive concrete cracking and increased deflection. Punching at one column may even initiate punching at adjacent columns as well, which would cause progressive collapse of the total structure. A novel reinforcement concept is therefore presented that gives flat plates a very ductile behaviour, which eliminates the risk for punching failure. The performance is verified by tests with monotonic as well as cyclic loading.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , xviii, 114 p.
Series
Trita-BKN. Bulletin, ISSN 1103-4270 ; 80
Keyword [en]
Construction engineering, bent-down bars, building codes, cyclic loading, deflection, ductility, earthquake
Keyword [sv]
Byggnadsteknik
National Category
Building Technologies
Identifiers
URN: urn:nbn:se:kth:diva-203OAI: oai:DiVA.org:kth-203DiVA: diva2:7891
Public defence
2005-05-26, Sal D2, Lindstedtsvägen 5, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100929Available from: 2005-05-18 Created: 2005-05-18 Last updated: 2010-09-29Bibliographically approved
List of papers
1. Punching of flat plates: a question of concrete properties in biaxial compression and size effect 
Open this publication in new window or tab >>Punching of flat plates: a question of concrete properties in biaxial compression and size effect 
1990 (English)In: ACI Structural Journal, ISSN 0889-3241, E-ISSN 1944-7361, Vol. 87, no 3, 292-304 p.Article in journal (Refereed) Published
Abstract [en]

The paper presents a design method to predict the punching strength and deflection of flat plates at interior columns. Failure is assumed to occur when the compression zone of the slab in the vicinity of the column is distressed by either a high radial compression stress or by a high tangential compression strain. Size effects and the effect of increasing concrete brittleness with increasing strength are both considered. The method shows excellent agreement with results from punching tests reported in the literature, with conditions ranging from ductile flexural failures to brittle punching failures, from small test specimens to a full-sized structure, from symmetrical to unsymmetrical loadings, and from low to high concrete strengths and reinforcement ratios.

Keyword
Columns - Failure, Concrete Construction - Reinforced Concrete, Concrete Testing, Flat Concrete Plates, Punching Shear, Radial Compression Stress, Tangential Compression Strain
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-24893 (URN)A1990DB85800006 ()
Note
QC 20100929Available from: 2010-09-29 Created: 2010-09-29 Last updated: 2017-12-12Bibliographically approved
2. Shear reinforcement for deflection ductility of flat plates
Open this publication in new window or tab >>Shear reinforcement for deflection ductility of flat plates
1990 (English)In: ACI Structural Journal, ISSN 0889-3241, E-ISSN 1944-7361, Vol. 87, no 6, 696-705 p.Article in journal (Refereed) Published
Abstract [en]

The deflection ductility of flat plates with different types of shear reinforcement is tested. Excellent performance is achieved with a combination of bent bars and stirrups, even at reinforcement ratios exceeding 0.75ρb. Code requirements for flat plates are critically reviewed and design recommendations are given for the bent bar/stirrup reinforcement arrangement.

Keyword
Building Codes, Concrete Construction - Reinforced Concrete, Concrete Products - Slabs, Structural Design, Bent Bars, Deflection Ductility, Ductile Failure Mode, Flat Plates, Shear Reinforcements, Stirrups
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-24894 (URN)A1990EG99400011 ()
Note
QC 20100929Available from: 2010-09-29 Created: 2010-09-29 Last updated: 2017-12-12Bibliographically approved
3. Elimination of flat plate punching failure mode
Open this publication in new window or tab >>Elimination of flat plate punching failure mode
2000 (English)In: ACI Structural Journal, ISSN 0889-3241, E-ISSN 1944-7361, Vol. 97, no 1, 94-101 p.Article in journal (Refereed) Published
Abstract [en]

A design concept that eliminates the punching failure mode of flat plates is verified by testing, and design recommendations are given. Only standard reinforced is used, which is so easy to fabricate and rapid to install that the solution should be suitable as standard practice ordinary flat plates as well as for such extremes applications as air-raid shelters. The system provides excellent safely against progressive collapse of flat plate buildings, a basic requirements that seems to be overlooked in many current concrete codes. The provisions of the ACI Code 318-95 for two-way shear capacity of slabs are found unsafe for critical sections outside shear reinforcement and presumably also around large columns.

Keyword
bent-up bars, building codes, deflection, ductility, flat concrete plates, flexural strength, punching shear, stirrups, structural design, tests
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-24895 (URN)000087123400011 ()
Note
QC 20100929Available from: 2010-09-29 Created: 2010-09-29 Last updated: 2017-12-12Bibliographically approved
4. Ductility Reinforcement for Flat Slabs in Seismic as well as Non-seismic Areas
Open this publication in new window or tab >>Ductility Reinforcement for Flat Slabs in Seismic as well as Non-seismic Areas
2005 (English)In: Magazine of Concrete Research, ISSN 0024-9831, E-ISSN 1751-763XArticle in journal (Other academic) Submitted
Keyword
cyclic loading, design, ductility, earthquake, flat concrete slabs, punching shear, shear reinforcement, story drift, tests
National Category
Building Technologies
Identifiers
urn:nbn:se:kth:diva-24896 (URN)
Note
QS 20120319Available from: 2010-09-29 Created: 2010-09-29 Last updated: 2017-12-12Bibliographically approved

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