N. John Gardner, P.Eng., Ph.D.

       Dr. Gardner's research has been directed to the relationship between the structural behaviour of reinforced concrete structures and how the structural behaviour is affected/altered by construction practices.

        Much of the research has evolved from research into the consequences of the construction method into the safety and serviceability of flat slab structures.  Flat slab structures are usually constructed by casting a new slab onto formwork supported by previously cast, immature slabs.  The popular shore/reshore method of construction imposes large (relative to design service loads) early age loads on the supporting slabs.  If the applied load is greater than the available strength failure can occur - usually with loss of life (Baileys Crossing, Fairfax County, Virginia, Coco Beach, Florida and Sampoong Department Store, Seoul).  In addition, the large construction loads cause large, early age, elastic deflections and consequent creep deflections which give rise to serviceability problems.  Fundamental to understanding these problems is knowledge of the early age mechanical properties of concrete as a material and the relationships between developed concrete strength and punching shear, bond strength, modulus of elasticity, creep and shrinkage.

Construction Loads due to form/reshore method of construction
Initial research was directed to determining the loads imposed upon flat slabs due to the shore/reshore method of construction.  Secondly the early age development of the mechanical properties of concrete were investigated and the relationships between the structural behaviour and the material behaviour.

Gardner's work on shoring and reshoring is referenced in ACI 347 Guide to Formwork for Concrete and is the basis for all the examples in ACI Special Publication SP5 Formwork for Concrete.

Gardner N.J.,  "Review of the form-reshore problem and needed research", ACI Concrete International, Vol.12, No.1, November 1990 pp32-38.

Pressure of concrete against vertical form surfaces
The applicant's work on formwork pressures is also referenced in ACI 347 Guide to Formwork for Concrete and is the recommended method in CSA S269-M92 Concrete Formwork.

Gardner, N.J.  "Pressure of concrete on formwork - A Review".  ACI Journal, September/October l985, No. 5, Vol. 82, pp. 744-753.

Punching Shear
Experimental research has indicated serious deficiencies in the equations suggested to predict punching shear capacity in the N.American design codes ACI 318-99 and CSA A23.3-94.  A number of papers have been published, and further research is in progress, on punching shear

Gardner N. J. , Chung Lan, Huh Jungsuk*,  "What can we learn from the Sampoong department store collapse".  Proceedings, International Workshop on Punching Shear Capacity of RC Slabs.  June 8-9, 2000 KTH (Kungl Tekniska Hogskolan - Royal Technical University), Stockholm, Sweden.  pp.225-234.
 

Gardner N. J. and Sharifi Haki*,  "Punching strength of edge column connections of post-tensioned concrete flat plates".  Proceedings, International Workshop on Punching Shear Capacity of RC Slabs.  June 8-9, 2000 KTH , Stockholm, Sweden. pp.431-440.

Gardner N.J. and Rezai-Kallage Mohammed*, "Punching shear of continuous prestressed concrete flat plates", ACI Materials Journal,  May-June 1998  pp.272-284.

Gardner N. J., "Punching shear provisions for reinforced and prestressed concrete flat slabs".  Canadian Journal of Civil Engineering, Vol 23, No. 2, April 1996,  pp.502-510.

Gardner N. J. and Shao Xiao-yun*  "Punching Shear of Reinforced Concrete Continuous Flat Slabs".  ACI Structural Journal, Vol 93, No.2, March-April 1996  pp.219-229.

Shrinkage and Creep
The N.American design codes ACI 318-99 and CSA A23.3-94 have no provisions for creep and shrinkage and ACI Committee 209-82 report does not include creep for loads applied earlier than 7 days. Code type equations have been developed to describe the time dependent behaviour of concrete. Gardner is currently trying to persuade ACI Committee 209 that expressions developed at the University of Ottawa on shrinkage and creep should be the basis for the next ACI 209 Guide for Recommended Practice for Calculating Long Term Deformations of Concrete Structures.

Gardner N.J., "Design provisions for shrinkage and creep of concrete".  ACI Special Publication SP194 honouring Adam Neville, published August 2000 pp.101-134.

Gardner N.J. and Lockman M.J., "Compliance, Relaxation and Creep Recovery of Normal Strength Concrete". Accepted Second International Conference on Engineering Materials,August 16-19, 2001 San Jose. Organised by Canadian Society for Civil Engineering and the Japan Society for Civil Engineers.

Serviceability - "Deemed to comply" span/thickness limits
A proposal to change the "deemed to comply" span thickness limitations of Clause 9.5 of ACI 318 has been submitted to ACI Committee 435 and to the CSA committee responsible for CSA A23.3.

N. J. Gardner and Zhang Jiehong*,  "Controlling deflection serviceability by span/depth limits and long-term deflection multipliers for reinforced concrete beams".  ACI Special Publication SP 161 honouring the work of professor Dan Branson pp.165-197, 1996.

Early age construction loads on immature concrete slabs can cause significant long-term deflections due to early-age flexural cracking, creep and shrinkage.  In general deflections are not checked during the design process; control against deflection is assured by making the slab thickness larger than some specified fraction of the span.  A layered finite element program has been used to determine flat slab thickness span ratios for which deflections do not need to be calculated including the effects of construction load and rate of construction.  These revised requirements would replace CSA A23.3 clause 13.3.3.
 

Ofosu-Asamoah K.* and Gardner N. J., "Flat Slab Thickness to Satisfy Serviceability including Early Age Construction Loads".   ACI Structural Journal, Nov-Dec 1997.

Hyperbolic Cooling Towers
Alexandridis A. and Gardner N.J.,  Tolerance limits for geometric imperfections in hyperbolic cooling towers, Vol. 118, No.8, August 1992.  ASCE Journal of Structural Engineering,  pp2082-2100.

 SELECTED ABSTRACTS

Punching shear strength of continuous post-tensioned concrete flat plates,  N. J. Gardner and Mahmoud Rezai Kallage

Abstract   Describes a test to failure of a two bay by two bay continuous, unbonded post-tensioned flat plate designed in accordance with the provisions of ACI 318-95 except that no supplementary bonded reinforcing steel was provided.  The prestressing tendons were uniformly distributed in one direction and banded in span/3 column strips in the other direction.  In both directions the average prestress on the concrete was 3.5 MPa.  The dimensions of each bay were 2.7 m by 2.7 m, the slab thickness was 89 mm, and the mean concrete cylinder strength was 44 MPa.  The load was applied monotonically until punching shear occurred at an edge column on the side parallel to the banded tendons.  The failed column was shored and the slab reloaded until punching shear occurred at the interior column.  Finally both the edge column and the interior column were shored and the slab loaded until a corner column failed.  All the punching shear failures were violent and without warning.

The literature was reviewed to locate experimental results of punching shear tests for isolated prestressed concrete flat plates, continuous prestressed flat plate systems and tests of flat plate column connections under shear and moment transfer.  The measured punching strength capacities were compared to those calculated using the provisions of ACI 318-95, BS 8110-85 and a proposed method.

KEYWORDS:    Punching shear, prestressed, continuous flat slabs, structural design.
 

Flat slab thickness required to satisfy serviceability including early age construction loads,  K. Ofosu-Asamoah and N. J. Gardner

Abstract   The use of higher strength concretes and steel yield strengths allow designers to design thinner slab systems which satisfy all the requirements of strength.   However the use of thinner slabs leads to increased deflections and the possibility of violating deflection serviceability criteria.  The shore-reshore method of construction applies significant construction loads to the immature supporting slabs.  The span/thickness provisions of ACI 318-89 do not consider the sensitivity of slab deflections to early-age construction loads or rate of construction.  A parametric study showed the current code requirements for minimum slab thickness to be over conservative in some cases and insufficiently conservative in others.  A span-thickness criterion, which includes the effect of early-age construction loads, concrete strength and panel aspect ratio, was developed from the results of the parametric study.

KEYWORDS:  Flat Slabs, Serviceability, Construction Loads, long term deflections, design aids.
 

Design provisions for shrinkage and creep of concrete.  N. J. Gardner

Abstract   The method is complete in that it includes strength development with age,  relationship between modulus of elasticity and strength, and equations for predicting shrinkage and creep.  The only arbitrary information is determining the factors appropriate to the cementitious materials, which can be improved from measured strength age data.  At the most basic level the proposed method only requires the information available to the design engineer.  The prediction values can be improved by simply measuring concrete strength development with time and modulus of elasticity.  Aggregate stiffness can be taken into account by back calculating a concrete pseudo strength from the measured modulus of elasticity.  Measured short term shrinkage and creep values can be extrapolated to obtain long duration predictions for similar sized elements.

KEYWORDS: concrete, shrinkage, creep, strength development, modulus of elasticity
 

Controlling Deflection Serviceability by Span/Depth Ratio Limits and
Long-term Deflection Multipliers for Reinforced Concrete Beams.,  N. J. Gardner and Jiehong Zhang.

Abstract     This project studied the immediate and long-term deflections of reinforced concrete beams using a layered nonlinear finite element model.  Long term deflections were calculated by a hybrid technique using an effective reduced modulus for concrete creep and a conventional finite element, time dependent load vector for shrinkage and tensile cracking.  The modelling technique was verified using the extensive experimental data of Christianson.
The solution technique used was to analyze successively longer span beams for a constant fraction of the design ultimate moment until the deflection criterion was just satisfied for a given beam cross section and steel ratio.  The span to depth ratios of all the beams which just satisfied the limiting criterion were then summarised.  A total of 290 analyses were carried out for various combinations of beam details.

Span/depth ratios, which include the effects of concrete strength, tensile and compression steel are proposed which satisfy specified deflection criteria.  Long-term deflection multipliers, for beams that satisfy a deflection criterion of span/500, are also proposed.

KEYWORDS: Serviceability, deflections, long term deflections, design aids.
 

Punching shear provisions for reinforced shear provisions for reinforced and prestressed concrete flat slabs,  N. J. Gardner

Abstract     The validity of the CSA A23.3-94 code provisions for punching shear were compared with the punching shear results of 142 reinforced concrete flat slabs, 16 prestressed concrete flat slabs with unbonded tendons and 17 flat slabs with unbonded prestressed and supplementary bonded reinforcement.  The code prediction equations are not capable of direct verification against experimental results without using a correction factor.  Using a justifiable correction factor the CSA A23.3-94 provisions are appropriately conservative for reinforced concrete slabs but the scatter is large.  However it was concluded that the CSA A23.3-94 provisions are not conservative for prestressed concrete flat slabs.  An equation is proposed to calculate the punching shear capacity of reinforced concrete and prestressed concrete slabs, that has a smaller coefficient of variation than the punching shear provisions of CSA A23.3-94, for symmetrically loaded interior columns.  The critical section of the proposed method is the perimeter of the column;  which is easier to justify than an arbitrary critical perimeter 0.5d from the column.

KEYWORDS: reinforced concrete, prestressed concrete, flat slabs, punching shear.
 

Tolerance limits for geometric imperfections in hyperbolic cooling towers,  A. Alexandridis and N.J. Gardner

Abstract     The imperfection forces due to a meridional imperfection in the form of a cosine curve were calculated using a series solution to the classic elastic thin shell theory for hyperboloid cooling tower shells. The closed form solution was extended to include the effects of reduced hoop stiffness due to vertical cracking and yield in the circumferential steel.

Subsequently tolerance limits were derived to predict the maximum permissible radial deviation at any level in a cooling tower shell assuming orthotropic behaviour due to vertical cracking and extended to include yielding in the circumferential direction assuming the shell can safely strain in the circumferential direction to twice the yield strain of steel without progressive failure.  The tolerance limits derived permit larger radial deviations in the more critically stressed zones near the base of a cooling tower than the published limits of Croll and Al-Dabbagh and can accommodate imperfections of very short length characteristic of a kink in the perfect meridian.

Keywords: Cooling Towers, Safety, Design, Construction, Construction imperfections.
 

Punching Shear of Reinforced Concrete Continuous Flat Slabs,  N.J.Gardner and Xiao-yun Shao

Abstract     The paper presents the experimental results for punching shear of a two bay by two bay reinforced concrete flat slab.  The results show that the interior column slab connection is more critical in punching shear than the edge and corner column connections in a properly designed multi-bay flat slab. Large flexural cracks, indicative of steel yield, were evident at all connections where punching shear failures occurred.  Supplementary supports can increase punching shear capacity by about 30% at edge column connections.

An empirical method, using a shear perimeter around the loaded area, is developed.  The code provisions of ACI 318-89, BS 8110-85, and the CEB-FIP 1990 Model Code were recalculated in terms of mean concrete strength, using published experimental results, to remove their conservatism.  The comparison shows that isolated punching tests can represent the punching shear behaviour of interior column slab connections in continuous slab systems.  All four methods were adequate to predict the capacity of edge and corner column connections under gravity loads.

Finally the code format equations and the proposed equation were recalculated to obtain the 95% characteristic values.

KEYWORDS:    Punching shear, reinforced concrete, continuous flat slabs, concrete construction, structural design.
 

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