This document summarizes a study on the influence of load patterns on the shear strength of hollow core slabs in uncracked zones. Numerical models were developed to analyze three slab thicknesses under concentrated and distributed loads. The results showed that distributed loads produced 22.9-41.1% higher shear strengths than concentrated loads. Additionally, the brittle shear failure of individual webs limits load redistribution. The study suggests using test methods that replicate real loading or reduced safety factors, and optimizing slab cross-sections for simultaneous web shear failure.

1. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 1/16
Department of Civil and Environmental Engineering
University of Perugia - Italy
Influence of load pattern on the shear strength
of hollow core slabs in uncracked sections
Marco Breccolotti1, Marco Pecetti2, Costanza Vittoria Fiorini3
1 Department of Civil and Environmental Engineering, University of Perugia
2 Generale Prefabbricati S.p.A., Perugia, Italy,
3 Department of Astronautical, Electrical and Energy Engineering, “Sapienza” University of Rome
8th International Conference on Structural
Engineering and Concrete Technology
29 – 31 March 2023, Lisbon

2. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 2/16
Introduction
Shear strength represents a crucial issue in the design
of hollow core (HC) slabs due to the absence of shear
reinforcement and to the brittle collapse mode.
There are still today significant inconsistencies between
experimental data of shear strengths of these elements
and design strengths provided for by the specific
structural standards, especially for slabs of greater
thickness.
HC slab shear strength is verified experimentally by
applying concentrated loads at specific locations.
Nevertheless, real loading conditions are generally
represented by distributed loadings.
In the present paper numerical investigations are
carried out to evaluate the influence of load type on the
HC slab shear strength in the uncracked zone.

3. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 3/16
Introduction
Full scale tests (Annex J, EN 1168)

4. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 4/16
Introduction
Shear strength represents a crucial issue in the design
of hollow core (HC) slabs due to the absence of shear
reinforcement and to the brittle collapse mode.
There are still today significant inconsistencies between
experimental data of shear strengths of these elements
and design strengths provided for by the specific
structural standards, especially for slabs of greater
thickness.
HC slab shear strength is verified experimentally by
applying concentrated loads at specific locations.
Nevertheless, real loading conditions are generally
represented by distributed loadings.
In the present paper numerical investigations are
carried out to evaluate the influence of load type on the
HC slab shear strength in the uncracked zone.

5. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 5/16
Programme of the investigation
HC slabs
• 3 slabs with thickness 260, 320 and 400 mm belonging to
the family of extruded HC slabs have been investigated.
• ½” (99 mm2) prestressing strands in every web
• Short spans have been assumed in order to achieve
shear failures rather than bending failures.
• Numerical models have been developed with the
capabilities of the FE code Abaqus.
• Concentrated (distributed on narrow surfaces) and
distributed loads have been considered.
• A total number of 15 load cases have been considered for
each slab

6. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 6/16
Programme of the investigation
simulation n. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
load pos. (mm) 0-L 250-350 350-450 450-550 550-650 650-750 750-850 850-950
side --- in in+fin in in+fin in in+fin in in+fin in in+fin in in+fin in in+fin
load type dist conc conc conc conc conc conc conc conc conc conc conc conc conc conc
One side concentrated load

7. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 7/16
Programme of the investigation
simulation n. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
load pos. (mm) 0-L 250-350 350-450 450-550 550-650 650-750 750-850 850-950
side --- in in+fin in in+fin in in+fin in in+fin in in+fin in in+fin in in+fin
load type dist conc conc conc conc conc conc conc conc conc conc conc conc conc conc
Two sides (symmetrical) concentrated load

8. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 8/16
Programme of the investigation
simulation n. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
load pos. (mm) 0-L 250-350 350-450 450-550 550-650 650-750 750-850 850-950
side --- in in+fin in in+fin in in+fin in in+fin in in+fin in in+fin in in+fin
load type dist conc conc conc conc conc conc conc conc conc conc conc conc conc conc
Distributed load

9. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 9/16
FE models
• Rubber pads have been introduced in the numerical
models in order to reproduce the effective conditions,
thus reducing stress concentrations.
• Concrete parts and rubber pads have been modelled
with 8-node linear bricks (C3D8R) while 2-node linear 3-
D trusses (T3D2) have been used to model prestressing
strands.
• Prestressing strands have been embedded in the
concrete without relative slips between the two
materials.
• Concrete damaged plasticity (CDP) model has been
used to represent the inelastic behavior of concrete
Numerical investigations

10. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 10/16
Concrete properties
Stress-strain law (short-term loading, fib Model Code 2010)
 
2
,lim
1 2
c cm c c
k
f for
k
 
  

 
 
  
 
 
  
 
Numerical investigations
Property u.m. value
fck Characteristic compressive strength MPa 40
fcm Mean compressive strength MPa 48
c1 Strain at maximum compressive stress - 0.0024
c,lim Ultimate strain - 0.0035
Ec0 Reference elastic modulus GPa 21.5
1 3
0 1
1 1 1
10
c ci cm cm
ci c c
c c c
E f f
k E E E
E


 
 
    
 
 
 2 3
0.3
ctm ck
f f
 

11. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 11/16
Prestressing steel properties
Stress-strain law (fib Model Code 2010)
Prestressing
Temperature fields assigned to the prestressing strands to
generate prestressing.
Temperature with linear variation in order to reproduce the
prestress transferring lengths.
Numerical investigations
Property u.m. value
f0.1k 0.1% Proof strength MPa 1670
fptk Characteristic tensile strength MPa 1860
pu Total elongation at maximum force - 0.035
Es Elastic modulus GPa 200

12. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 12/16
Results
Numerical investigations
HC slab 320
HC slab 260 HC slab 400

13. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 13/16
Comments
Numerical investigations
HC slab 260
Distributed
Conc. 850-950 mm
Conc. 550-650 mm
Conc. 250-350 mm

14. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 14/16
Comments
Numerical investigations
HC slab 320
Distributed
Conc. 850-950 mm
Conc. 550-650 mm
Conc. 250-350 mm

15. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 15/16
Comments
Numerical investigations
HC slab 400
Distributed
Conc. 850-950 mm
Conc. 550-650 mm
Conc. 250-350 mm

16. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 16/16
Concluding remarks
In this study are reported the first results of numerical investigations carried out on the shear
performance of HC slabs in the zones uncracked by bending moments. They can be resumed as
follows:
1) the shear strength in the uncracked zones is strongly influenced by the type of applied load. In
particular, it was observed that the shear strengths of hollow core slabs for distributed loads are
always greater (between 22.9% and 41.1% for the investigated slabs) than those observed for
concentrated loads;
2) the fragility of the collapse mechanism due shear of the individual webs does not allow the
implementation of an effective transversal redistribution of the applied loads.
Based on these observations, it is suggested the use of test methods that reproduce the loading
conditions of real structures or the adoption of reduced partial safety factors for shear collapse,
and the development of optimized HC slabs cross section so as to make the web shear collapse
of all webs simultaneous.

17. Marco Breccolotti Seismic Influence of load pattern on the shear strength
Lisbon, 29 – 31 March 2023 of hollow core slabs in uncracked sections 17/16
Thank you for the kind attention
Department of Civil and Environmental Engineering
University of Perugia - Italy
8th International Conference on Structural
Engineering and Concrete Technology
29 – 31 March 2023, Lisbon