Shear Capacity of Existing Reinforced Concrete Slab Bridges under Traffic Loads

RESULTS

b

_eff

: Experimental approach

A series of specimens with different widths are tested [1]. For

comparison to the 2,5m wide slabs, specimens with widths of 0,5; 1m; 1,5m and 2m are tested under a concentrated load close to the support. To observe the concept of the effective width, the following behavior is expected: for specimens with small widths (b < b_eff) an increasing width leads to an increasing shear capacity until a

threshold value of the width is reached (the effective width) and no further increase of the shear capacity is observed for the increasing width of the specimen. Experimental results (Fig. 7) indeed confirm this expectation.

The resulting effective width as observed in the experiments on slabs with different widths can be compared to the theoretical effective

width based on a horizontal load spreading method. This comparison shows that the experimental effective width most closely resembles the effective width from the French load spreading method (Fig. 5b), as this method takes into account the size of the loading plate.

Experimentally it is thus proven that the French load spreading method is to be preferred.

Shear Capacity of Existing Reinforced Concrete Slab Bridges under Traffic Loads

Eva O.L. Lantsoght, Cor van der Veen, Joost C. Walraven

Department of Design & Construction: Concrete Structures, Faculty of Civil Engineering and Geosciences, Delft University of Technology, The Netherlands

Introduction

In the Netherlands, 60% of the existing bridges were built before 1975, while the traffic volumes and loads have

increased over time. The results of a first assessment of the existing bridges showed that particularly the shear capacity of reinforced concrete solid slab bridges is often lower than the resulting shear stresses due to dead loads and traffic loads.

In the design or rating of reinforced concrete slab bridges, the one-way shear capacity (Fig. 1) at the support is typically

determined with the same model as used to calculate the shear strength in beams. Due to the two-dimensional

characteristics of a slab, however, the distribution of forces caused by concentrated traffic loads does not correspond to the behavior observed in beams. In a slab, horizontal

redistribution of forces occurs after cracking.

Fig 1: Slab failing in beam shear (S3T4) Fig. 2: Slab failing in punching shear (S11T2)

Research objectives

• Determine the additional capacity of slabs under wheel loads as compared to beams.

• Study the effective width in shear.

• Determine a horizontal load spreading method which can be used to calculate the effective width in shear.

• Formulate recommendations for the assessment of solid slab bridges.

Experiments

To verify the shear capacity of slabs, continuous slabs of 5m x 2,5m x 0,3m are loaded up to failure. Two series of experiments are carried out: slabs under a concentrated load close to the support and slabs under a combination of a line load and a concentrated load close to the support. A total of 160

experiments is carried out.

A sketch of the setup for the first series of slabs is given in Fig. 3. When both a line load and a concentrated load are used, an asymmetric situation is created in which additional moments need to be carried by the test frame. Therefore, a frame with a higher stiffness is constructed (Fig. 4). A description of the

experiments and overview of the results can be found in [1].

Literature

From experiments reported in the literature, a database of 207 shear experiments on wide beams and slabs is compiled [2]. To study the redistribution capacity of slabs under wheel loads close to the support, a subset is made of the slabs under a concentrated load, for which the effective width in shear (b_eff2 in Fig. 5) is smaller than the total width b. When this requirement is fulfilled, the slab can benefit from transverse stress distribution.

Furthermore, to study the influence of direct load transfer, only

experiments with a/d < 2,5 (a = shear span, center-to-center distance between the load and the support) are included in the subset. The

resulting subset contains 22 experiments, most of which are carried out on slabs with a small effective depth d< 150mm. Comparing the results from this subset to beam shear equations from EN

1992-1-1:2005 and ACI 318-08 shows higher shear capacities for slabs under concentrated loads close to the support than expected for beams.

load simple

support continuous _support

300 mm 300 mm load 2500m m 300mm 3600mm 600mm 500mm pre st re ss ing ba rs 1250m m (M ) 438m m (S ) sup 2 CS sup 1 SS sup 1 SS sup 1 SS sup 2 CS sup 2 CS

Fig. 3: Top view sketch of setup for slabs on line supports under a concentrated load.

Fig. 4: Picture of setup for second series of experiments: slabs under a concentrated load and a line load.

b_eff1 load support b_eff2 load support (a) (b)

Fig. 5: Horizontal load spreading methods: (a) traditional method, and (b) method from French practice.

b

_eff

: Statistic approach

Both horizontal load spreading methods from Fig. 5 have been combined with EN 1992-1-1:2005 to verify which load spreading method is to be preferred. Analysis [6] showed that the comparison between the

experimental results and the calculated capacities for both the first series of experiments and the experiments from the database resulted in a

smaller average and standard deviation when the effective width based on the French load spreading method is used. Statistically it is thus

proven that the French load spreading method is to be preferred.

Set of recommendations

For use with EN 1992-1-1:2005 and load model 1 from EN 1991-2:2002, the following recommendations are made:

1. Reduce the contribution of forces close to the support with the factor β = a_v/2d (a_v ≤ 2d; a_v is the clear shear span, the face-to-face distance between the load and the support).

2. Reduce the contribution of the concentrated loads close to the support with a factor 1,25 (the slab factor).

3. For concentrated loads on slabs close to the support,

recommendations 1 and 2 can be combined into β_new = a_v/2,5d for a_v ≤ 2,5d.

4. Determine the effective width for the concentrated loads based on the horizontal load spreading method as used in French practice (Fig. 5b).

5. The minimum effective width in shear for a concentrated load is 4d for loads in the middle of the width and loads close to the edge.

Acknowledgement

The authors wish to express their gratitude and sincere appreciation to the Dutch Ministry of Transport, Public Works and Water Management

(Rijkswaterstaat) for financing this research work.

Quick Scan method

An initial evaluation of the existing slab bridges is based on a “ Quick Scan” method, which compares the design shear force due to the dead load, superimposed loads and traffic loads to the design shear

resistance as determined in EN 1992-1-1:2005.

For the traffic loads, a combination of a lane load and the design

vehicular loads is considered. The most unfavorable position (Fig. 8) of the vehicular loads to determine the maximum shear force at the edge (situation considered for the Quick Scan) is obtained by placing the first design truck such that the distance between the face of the

support and the face of the tire contact area equals 2,5d.This distance is governing since the set of recommendations takes the influence of direct load transfer into account up to 2,5d. In the second and third lane, the design truck is placed such that the effective width (Fig. 5b) associated with the first axle reaches up to the edge of the viaduct.

References

[1] Lantsoght, E., van der Veen, C., and Walraven, J. (2011). "Shear capacity of slabs and slab strips loaded close to the support." SP – Recent Developments in

Reinforced Concrete Slab Analysis, Design and Serviceability, 17pp.

[2] Lantsoght, E. (2012). “Shear in reinforced concrete slabs under concentrated loads close to the support – Literature review”, Delft University of Technology, 289 pp.

[3] ACI Committee 318. (2008),“Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary,” American Concrete Institute, Farmington Hills, MI, 465 pp.

[4] CEN. (2002), “Eurocode 1 – Actions on Structures - Part 2: Traffic loads on bridges, EN 1991-2,” Comité Européen de Normalisation, Brussels, Belgium, 168 pp.

[5] CEN. (2005), “Eurocode 2 – Design of Concrete Structures: Part 1-1 General Rules and Rules for Buildings, EN 1992-1-1,” Comité Européen de Normalisation, Brussels, Belgium, 229 pp.

[6] Lantsoght, E., van der Veen, C., and Walraven, J. (2012). "Shear assessment of solid slab bridges." ICCRRR 2012, 3rd International Conference on Concrete

Repair, Rehabilitation and Retrofitting, Cape Town, South Africa, 6pp. (to appear)

b

400mm

400m

m

1200mm

2000m

m

a

b

+ 3m

b

+ 2*3m

b

l

a

b

_a

lane 1

lane 2

lane 3

Fig. 8: Top view of bridge with at least 3 theoretical lanes, showing the most unfavorable position according to load model 1

from EN 1991-2:2002. b_eff b τ distributed load τ concentrated load

Fig. 6: Principle of the application of the hypothesis of superposition

Fig. 7: Maximum load for specimens of increasing width in different series (size of loading plate, loading at simple (SS) or

continuous (CS) support, distance between load and support)

0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 500 1000 1500 2000 2500 P u ( k N ) b (mm) 300 mm x 300 mm SS a/d= 2,26 300 mm x 300 mm CS a/d= 2,26 200 mm x 200 mm SS a/d= 1,51 200 mm x 200 mm CS a/d= 1,51 200 mm x 200 mm SS a/d= 2,26 200 mm x 200 mm CS a/d= 2,26

Slabs

The first series of experiments confirms the additional capacity of slabs in one-way shear under a concentrated load as compared to beams. While a significant influence of the geometric properties (size of the concentrated load, distance between load and support and type of support) is found, the influence of the concrete compressive strength seemed to be only marginal [1].

In the second series of experiments, slabs are loaded under a combination of a line load and a concentrated load to verify the

hypothesis of superposition of loading. These experiments confirm that it is a conservative approach to combine the stress due to the

concentrated load over its associated effective width to the stress due to all forces which act over the full width of the slab, Fig. 6.