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Evaluation of the shear capacity of precast-prestressed hollow core slabs: numerical and experimental comparisons

Abstract

Since eighties, 400 and 500 mm thick precast-prestressed concrete hollow core slabs, characterized by increasingly optimized cross-sections with non-circular voids, became very common. However, deeper slabs with long spans, which have to resist high line loads acting close to the supports, are subjected to initial web shear cracking and may fail at loads less than those predicted by traditional codes prescriptions. The shear strength capacity of these members without transverse reinforcement is evaluated through a campaign of detailed nonlinear finite element analyses, matching experimental test data collected from past programs. Constitutive models, based on nonlinear fracture mechanisms, are considered to numerically reproduce the experimental response of single span, simply supported, isolated hollow core units, highlighting web-shear failure mechanism, due to short development length and lack of transverse reinforcement. The adopted diffuse smeared fixed cracking constitutive model allows a reliable prediction of shear stress distributions and crack patterns for these members in their inelastic branch. The presence of a variable inclined strut is clearly evident. Peak shear stress is localized at the bottom side of the cross-section, rather than at the level of the centroid. The experienced brittle web-shear failure mechanism is governed by hollow core shapes with circular or non-circular voids, as evidenced by the evolution of the principal tensile strain distributions. Typically, less inclined, more rounded, diagonal crack, controlled by the smooth web width variation along depth, is opposed to a fairly constant variation of the fracture angle inclination, governed by the abrupt and irregular web width drop.

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Abbreviations

a :

Shear span

a 1 :

Distance between two line loads

α 1 :

Ratio of distance from end of member to transfer length

A low :

Area of lower strands

A up :

Area of upper strands (if present)

b w :

Sum of web widths

D :

Cross-section depth, assumed as H S

D i/D i,max :

Normalized depth

D low :

Diameter of lower strands

D up :

Diameter of upper strands (if present)

D z :

Mid-span vertical displacement

d p,low :

Average distance from lower strand to the soffit

δ C50 :

Standard deviation of f C50,mean

E 1 :

Principal tensile strain

E 3 :

Principal compressive strain

F z :

Shear force at the support

f c :

Concrete compressive strength

f C50,mean :

Mean concrete compressive strength by cylinder tests on 50 mm cores drilled from the tested specimens

f ctd :

Design tensile strength of concrete per EC2

f t :

Concrete tensile strength

ϕ :

Diameter of strand

H :

Nominal slab depth

H H :

Average depth of hollow core

H S :

Average depth of slab cross-section

I :

Moment of inertia of the section about the centroidal axis

LF :

Normalized load level

L s :

Distance between slab end and centre of support

L t :

Transfer length of prestressing force in strands, assumed as 55ϕ (slow release)

N low :

Number of lower strands

N up :

Number of upper strands (if present)

R i :

Ratio between experimentally observed and predicted shear strength capacity

S :

First area moment of the section about the centroidal axis

S low :

Largest slippage of lower strands

S up :

Largest slippage of upper strands (if present)

σ cp :

Fully effective concrete compressive stress at the centroid due to prestressing

σ p :

Prestress in the strands within the transfer region

σ p0 :

Fully effective prestress in the strands outside the transfer region

σ p0,low :

Initial fully effective prestress in lower strands outside the transfer region

σ p0,up :

Initial fully effective prestress in upper strands (if present) outside the transfer region

t b :

Length of bearing

t low :

Average concrete thickness below the hollow core

τ yz :

Shear stress

V obs :

Experimentally observed shear strength capacity

V pre :

Predicted shear strength capacity

x :

Coordinate along the longitudinal axis of the member

x/D :

Ratio between longitudinal distance from the support and cross-section depth

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Correspondence to E. Brunesi.

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Brunesi, E., Bolognini, D. & Nascimbene, R. Evaluation of the shear capacity of precast-prestressed hollow core slabs: numerical and experimental comparisons. Mater Struct 48, 1503–1521 (2015). https://doi.org/10.1617/s11527-014-0250-6

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  • DOI: https://doi.org/10.1617/s11527-014-0250-6

Keywords

  • Hollow core slab
  • Shear strength
  • Prestressed concrete
  • Precast members
  • Non-circular voids