Design of steel piles for integral abutment bridges using Eurocodes

Hans Pétursson

ABSTRACT

Integral Abutment Bridges are becoming more popular in Europe, but the traditions differ from country to country. This leads to different technical solutions for the same problem in each country.

In this paper some of the different solutions that are used in Europe are presented. The Eurocodes are set of technical rules for the structural design of construction works. Since 2010 the

Eurocodes are mandatory for specification of public works and are intended to be the standard used in all the EU zone for the design of construction works. There are no special rules for bridges with integral abutments in the Eurocodes for integral abutment bridges. It is up to the designer to come up with a design that complies with the rules in the Eurocodes that are common with other type of bridges. In this paper some of the aspects of designing piles according to Eurocodes are described.

_______________ Hans Pétursson, Steel Bridges Specialist, Swedish Transport Administration, Röda vägen 1,SE-781 89 Borlänge, Sweden

180

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Precast (PC) and prestressed (PS) concrete piles are not commonly used in Europe, with the exception of Sweden. In Sweden, PC/PS concrete piles are common in all bridge types, including short FIAB, due to their low cost and ready availability. In the USA, these pile types are typically only used for shorter span FIAB.

Steel Core Piles Although not common, steel core pipe piles have been used for FIAB in Europe. The pile

system consists of a cover pipe, injected concrete and a core steel pile. Drilling is performed down to the bedrock. The cover pipe is drilled 300-500 mm into the rock. When the cover pipe is placed in the hole the interior is rinsed and then injected with concrete. The steel core is then installed. After inspection and driving pile to refusal the pile is cut to the right length. A pressure distribution plate is then fit to the top of the pile. In this way the very slender steel pile can be used and the concrete stabilizes the pile from buckling. A few FIAB have been built in Sweden with steel core piles.

Figure 6. Sketch of steel core pile.

The Eurocodes

The Eurocodes are set of technical rules for the structural design of construction works. Since 2010 the Eurocodes are mandatory for specification of public works and are intended to be the standard used in all the EU zone for the design of construction works. The Eurocodes are published separate European Standards each section having a number e.g. EN 1990 Basis of structural design, EN 1991 Actions on structures, 1992 Design of Concrete Structures and EN 1993 Design of Steel Structures. There are 10 sections published and each section consists of several parts.

There are no specific parts of the Eurocodes that deals with integral abutments nor are there any specific rules in the different parts. It is up to the designer to come up with a design that complies with the rules in the Eurocodes that are common with other type of bridges. The parts of the Eurocodes that the designer needs to look into when designing piles for integral abutment bridges are: EN 1990 Basis of structural design EN 1991-2Actions on structures - Part 2: Traffic loads on bridges

184

EN 1991-5 Actions on structures - Part 1-5: General actions - Thermal actions EN 1993-2 Steel bridges EN 1993-5 Design of steel structures - Part 5: Piling EN 1997-1 Geotechnical design - Part 1: General rules

Design of piles

Philosophies

Integral abutment bridges may be generally designed based on two different concepts. 1. Low flexural stiffness of piles / low degree of restraint

If the integral abutment is supported by one row of flexible piles the superstructure can beanalyzed as a beam with simple hinged supports. 2. High flexural stiffness of piles / high degree of restraint

If the integral abutment is supported on stiffer piles or spread footing the rotation of thesuperstructure and to some extent the displacement will be restraint and a large support moment will be distributed to the superstructure. This means that the connection between the abutment and the superstructure needs to be stronger and more robust but the field moment of the superstructure is somewhat shifted to the support and a more slender superstructure can be constructed. If the piles are relatively short and stiff enough the structure act like a frame and the soil-bridge interaction is not as vital as in bridges with more slender piles. This type is not discussed further in this paper.

Loading

Bridge loading is the same for integral abutment bridges as other bridges. The difference is that some loads cause forced displacement and rotations at the top of the piles and this gives unwanted strains in the piles. The stiffness behind the abutment reliefs these strains to some extent depending on confinement, water content and friction.

Thermal loads

Two types of temperature loads needs to be considered. First, a uniform temperature change which means that the abutment is displaced horizontally. The magnitude of the bridge movement depends on the mean temperature of the structure when the superstructure is locked to the abutments. Also uneven temperature distribution in the superstructure cause movements of the piles as the beam end rotates. The mean bridge temperature is dependent on the ambient air temperature, wind effect and solar radiation. Temperature gradients through the depth of the bridge beams generate secondary bending moments due to the fact that the centroid of the temperature distribution curve and the centroid of the cross-section of the bridge beams may not coincide. The maximum temperature differentials (with positive gradient) occurs when the concrete deck slab is exposed to sun radiation during the summer and winter, resulting in a concrete deck slab that is warmer than the steel beams. The minimum temperature differential (with negative gradient) occurs when the concrete deck slab is suddenly drenched with cold rain or snow, thus cooling the concrete deck slab at a faster rate than the steel beams. The rotation will also cause a horizontal displacement of the pile top if there is a vertical distance between the centre of gravity of the composite cross-section and lower edge of the abutment wall where the pile is clamped.

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Soil with cohesion (clay)

If the soil surrounding the soil is considered to be an elastic continuum with Young´s modulus E and Poisson´s ratio of 0.5 the lateral soil modulus, ku, can be derived e.g. according to Baguelin (1997). If the Young´s modulus is supposed to be E=50cu, where cu is the cohesion the lateral soil modulus can be written:

ku =k0cu/b, 157≤k0≤242 (2)

For long term loading the creep effect must be considered. This done approximately by reducing the lateral soil stiffness:

ku =50cu/b (3)

The pressure against the pile is described by the expression:

p=kuu (4)

Where u is the lateral displacement of the pile. The pressure p is limited to a value py that for drained conditions can be calculated by:

py=Nccu (5)

Where Nc is a constant that varies between 8-12 for deep soil layers (<3b) and decreases to 2 for layers closer to the surface. For short time loading Nc=9can be used and for long time loading the creep is considered by a lower value of Nc=6.

Cohesion-less soil (friction soil)

For cohesion-less soils there are no unambiguous expression that describes the relation between the lateral soil modulus and the strength parameter �´. The relationship between lateral displacement and soil pressure against the pile is therefor based on suggested empirical “p-y” curves based on experimental data. There are numerous suggestions on different “p-y” curves among them are Reese (1994). The “p-y” curves are un-linear and a soil model with linear curve based the initial slope of the “p-y” curve suggested by Reese et. al.(1994) is used here. The lateral soil modulus is half of the initial modulus according to Reese et. al.(1994) and is expressed as follows:

Ku=1/2ksz (6)

where ksis given in Table Ⅲ.

TABLE Ⅲ. VALUES FORTHE CONSTANT KsFOR LATERAL SOIL MODULUS ACCORDING TO REESE ET. AL. (2007) Water[MN/m3] Sand above water table [MN/m3]

Loose sand 5 7 Medium dense sand 16 24 Dense sand 34 61

188

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189

Initial imperfections and second order moments

When designing piles second order effects are taken into consideration. This is done by enlarging moment along the pile and is depending on the initial imperfections of the pile. The initial imperfection of the pile can be assumed according to Table 5.1 in EN 1993-1-1 and is:1/200

e0/lc=1/200 (8)

for steel pipe piles that are cold formed. For piles with H-profile the value is 1/300 for strong axis bending and 1/250 for weak axis bending.

Concluding remarks

This paper describes some of issues that have to be dealt with when designing steel piles for integral abutment bridges, but is far from complete. For a more comprehensive material about designing integral abutments according to Eurocodes the reader can look into the design guide from the INTAB project Feldman et. al 2010.

REFERENCES

Baugelin F, Frank R, Said Y. H., 1977, Theoretical study of lateral reaction mechanics of piles, Geotechnique, Vol. 3, pp. 405-434

Collin P., Veljkovic M., Petursson H., 2006, “International Workshop on Bridges with Integral Abutments”, Lulea University of Technology, ISSN: 1402-1536, ISRN: LTU-TR06/14-SE

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EN 1991-1-5 // Eurocode 1: Actions on structures - Part 1-5: General actions – Thermalactions. - Brussels : European Committee for Standardization (CEN), 2003.

EN 1991-2 // Eurocode 1: Actions on structures - Part 2: Traffic loads on bridges. -Brussels : European Committee for Standardization (CEN), 2003.

EN 1992-1-1 // Eurocode 2: Design of concrete structures - Part 1-1: General rules and rulesfor buildings. - Brussels : European Committee for Standardization (CEN), 2004.

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EN 1993-2 // Eurocode 3 - Design of steel structures - Part 2: Steel Bridges. - Brussels :European Committee for Standardization (CEN), 2006.

EN 1993-5 // Eurocode 3 - Design of steel structures - Part 5: Piling. - Brussels : EuropeanCommittee for Standardization (CEN), 2007.

EN 1994-2 // Eurocode 4 - Design of composite steen and concrete structures - Part 2:General rules and rules for bridges. - Brussels : European Committee for Standardization(CEN), 2005.

EN 1997-1 // Eurocode 7: Geotechnical design - Part 1: General rules. - Brussels : EuropeanCommittee for Standardization (CEN), 2005.

Feldmann, M., Naumes, J., Pak, D., Veljkovic, M., Eriksen, J., Hechler, O., et al.,2010, Design Guide. Aachen: RWTH AachenUniversity, Institut für Stahlbau, http://www.bridgedesign.de

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Emerson, M. (1977). Temperature differences in bridges: basis ofdesign requirements, TRRL Laboratory Report 765, Transport and Road Research Laboratory, Crowthorne, Berkshire, pp. 39.

Iles D., 2005, “Integral Steel Bridges: A Summary of Current Practice in Design and Construction”, SCI PublicationP340, p. 8

Kerokoski O., 2006, Soil Structure Interaction of Long Jointless Bridges with Integral Abutments, Tampere University of Technology, p.16

Maruri R., Petro S., 2005, Integral Abutments and Jointless Bridges (IAJB) 2004 Survey Summary, Federal Highway Administration (FHWA)/Constructed Facilities Center (CFC) at West Virginia University

Pétursson H., Möller M., Collin P., 2013, Structural Engineering International, Vol. 23 , Number 3 ,pp. 278-284 Reese L. C., Cox W. R., Koop F. D., 1974, Analysis of Laterally Loaded Piles in Sand, Offshore Technology

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191