3.5.1 – In a single bearing line, bearings must be of the same type (susceptible of having, in particular, the same settlement), although their translation possibilities do not necessarily have to be the same (figure 3.12 & 3.13).
3.5.2 – Lengthwise, it is inadvisable to juxtapose several bearings that are intended to form a single load transfer point (upper part of figure 3.12). This restriction does not apply to split bearings, where the distance between the axis is generally around 2m or above.
Figure 3.12: examples of authorized and highly inadvisable layouts lengthwise.
N.B: in the above example, this layout makes rotations difficult, which should be taken into account in the design.
Figure 3.13: examples of authorized and highly inadvisable layouts transversally.
Laminated elastomeric bearings –Use on bridges, viaducts and similar structures OV 3.5.3 – Crosswise, several bearings that are intended to form a single point of support can be juxtaposed (figure 3.12 upper part). These bearings must be identical in composition and size. It must be remembered that such layouts should be justified, taking into account in particular rotations resulting from installation defects that are likely to exist crosswise.
Generally speaking, it is inadvisable to place bearings that do not have the same dimensions perpendicular to the same point of support, due to differences in stiffness (figure 3.12). In the case of a skew bridge, with a number of girders, it is generally preferable to lay in the same line identical bearings, the size of which corresponds to that of the most stressed bearing, but paying attention to the minimum stress of the least stressed bearing in order to avoid slipping.
3.5.4 – When bearings exert high compression stress on the supports, special precautions need to be taken.
When the supports are made of reinforced concrete, allow for a minimum clearance of 10 to 15 cms in order to ensure correct stress distribution, the installation of the plates and their anchorages (figure. 3.14). In all cases, the recommendations relating to reinforced concrete constructions should be followed.
3.5.5 – Care should be taken to position, insofar as is possible, the lower face of the bearing above the highest known water level or the hundred-year flood.
3.5.6 – Bearing markings
The position on the structure, the size and direction of any pre-settings, together with the installation direction must be clearly indicated on the bearings.
3.5.7 – Replacing bearings
In the case of a change of bearings on a bridge in service, as with any repair, when a replacement bearing is sized, this sizing will be a compromise between the calculation rules of the present document and the possibilities on the existing structure (available height, plan dimensions, etc.). To assess the adjustments to the present rules, contact the design department of the technical network offices.
Figure 3.14: an example of a construction layout, highlighting the necessity of plating perpendicular to the jacking location.
Laminated elastomeric bearings –Use on bridges, viaducts and similar structures PN
`Ü~éíÉê=Q=Ó= Design principles for a structure with bearings 4.1 – General points – The regulatory context
The design principles for laminated elastomeric bearings have been outlined in the previous chapter. However, a certain number of measurable variables - deformations and longitudinal force in particular – arise from an interaction between the bearings and the structure due to the flexibility of the supports.
The exact dimensioning of a bearing therefore requires the dimensions to be pre-determined prior to introducing the characteristics of flexibility (vertical, horizontal and rotation) throughout the structure (deck and piers) in order to obtain the force and horizontal displacements that allow for verification that the dimensioning correctly respects the limits set out in the previous chapter. If it is not the case, iteration is neccessary.
In NF EN 1337-3, the calculation of bearings is only made at Ultimate Limit State. Combinations to be used are therefore basic combinations in which, aside from permanent action, actions occur that are due to road loads, the effects of temperature (uniform and thermal gradient) and the wind.
These verifications should be completed by accidental combinations if the piers of the structure are likely to suffer from impacts from boats or trucks and combinations under seismic actions if the structure is subject to these. Finally, in some specific cases, other verifications should be carried out, for example, for a beam laid during construction on its definitive bearings.
For the calculations given hereafter, we have used the combinations provided by the following texts:
• NF EN 1991-1-5: this standard specifies the values to be used for uniform temperature actions ΔTN and thermal gradient actions ΔTM. It also specifies the way in which these actions should be combined to account for their simultaneity and obtain the characteristic overall effect Tk
• Appendix A2 of NF EN 1990: this appendix defines the combinations to be used in particular for the calculations of supports and bearings.
Firstly, we can apply the basic combinations given in table 4.1:
N°
+ 1.35 {UDLk +TSk + q fk,comb} + 1.5 min{FW* ; 0.6 FWk} 1 + 1.35 {UDLk + TSk + q fk,comb } + 1.5 {0. 6 Tk} 2
+ 1.35 gr1b 3
+ 1.35 gr2 4(1)
+ 1.35 {gr3 ou gr4} + 1.5 {0.6 Tk} 5
+ 1.35 gr5 6
+ 1.5 FWk 7
+ 1.5 Tk + 1.35 { 0.4 UDLk + 0.75 TSk + 0.4 q fk,comb} 8
Table 4.1: list of basic combinations (1) including braking
1.35 Gk,sup + Gk,inf + P + S + C
PO================Laminated elastomeric bearings – Use on bridges, viaducts and similar structures
In the calculation example, to simplify, we will only consider the UDL, TSk and qfk loads, together with the braking loads, which give table 4.2.
Table 4.2: the combinations chosen for the example
Horizontal force that intervenes in the preceding combinations is to be calculated using the following methods:
• For braking:
NF EN 1991-2 defines the breaking force to be applied to the deck as a fraction of the maximum load that can be put on the busiest lane of the load model 1 (NF EN 1991-2 § 4.4.1). These fractions are 10% for UDL distributed load and 60%
for TS concentrated loads.
If we take a class 2 structure, the main lane of which is 3 metres wide, the total braking force, in characteristic value, for a deck of length L is given by:
L
HK=324+1,89× with L in metres and HK in kN.
The breaking force varies from around 340 to 400 kN for smaller structures 10 to 50 metres long and reaches the maximum value of 900kN for structures 350 metres long between expansion joints. This value is far higher than those normally used in former regulations (300kN for the braking of a Bc truck, for example). As regards structures on laminated elastomer, the breaking force is spread over all the deck bearings, which should not cause problems for the pier reinforcements. However, for large structures with fixed bearings that take nearly all the horizontal force, the sizing of the piers can be complicated with such high breaking values. If the structure has high, flexible piers, it is advisable to have several fixed bearings. Otherwise, the fixed bearing should be put on a short pier, or even on an abutment, which may lead to difficulties in sizing the expansion joints (and the slide plates) on the abutment located at the other end of the structure.
This maximum breaking force will most probably be reduced in the National Annex as NF EN 1991-2 allows for this.
The maximum breaking force could then be brought down to 500 kN, except if the structure carries military loads that comply with the STANAG (Char Mc 120) standardisation agreements.
• For thermal force:
The effects of temperature are defined in section 4 of NF EN 1991-1-5. Temperature differences Te, max and Te, min in characteristic values are to be calculated according to the material from which the deck is made and the region in which the structure is built. These temperatures are to be determined using maps supplied in the National Annex 13 of NF EN 1991-1-5. In the meantime, the following values, found is the National Annex, can be used:
Te, min Te, max
Deck material Concrete Composite Steel Concrete Composite Steel Brittany – Provence
Temperature variations resulting from these maximum and minimum temperatures can be calculated according to a temperature T0 which is taken as equal to 10 °C in the absence of any specification on the project.
To calculate the positioning of bearings or their slide plates, NF EN 1991-1-5 recommends that a supplement be added to this temperature variation range. This supplement is ± 20 °C, or ± 10 °C if the installation temperature is specified. The
13 At the time of writing, the National Appendix is being updated in view of future publication.
Laminated elastomeric bearings –Use on bridges, viaducts and similar structures PP interpretation that we give to this recommendation is that if the bearing is loaded at a temperature close to + 10°C (equilibrium temperature), the supplement will be ± 10°C.
The coefficients of expansion provided for in the Eurocode are 1 x 10-5/°C for concrete decks and 1.2 x 10-5/°C for steel bridges (NF EN 1991-1-5 – Appendix C). For decks of composite structures, NF EN 1994-2 specifies in paragraph 5.4.2.5 (3) that this coefficient must be taken as equal to 1.2 x 10-5/°C for the calculation of expansion, and 1 x 10-5/°C for the calculation of thermal gradients.
We should also specify, even though the Eurocodes do not indicate it explicitly, that the calculation of forces distribution in the various bearings, and therefore the forces in the piers, should be carried out using the instantaneous modulus of concrete.
4.2 - Dimensioning
4.2.1 - Introduction
The best way to understand the procedure for calculating the dimensions of bearings is to use an example (which is not a real case and is only used to illustrate the procedure).
We shall consider the dimensioning of laminated elastomeric bearings of a structure made of prestressed concrete cast-in-place (PSI-DP).
The structure in question has 3 spans and an overall length of 62 m. The width of the slab is 12.30 m for a thickness of 0.90 m.
Figure 4.1: lengthwise section of the structure
Each bearing line has two bearings. The forces and deformations imposed are summarized in table 4.3 (forces for a single bearing to basic ULS for the abutment C0. These forces are the result of a general dimensioning calculation of the structure (a computer calculation completed by manual notes that supposes a uniform distribution of forces on each bearing in a same line).
PQ================Laminated elastomeric bearings – Use on bridges, viaducts and similar structures
V (MN) α (10-3 rad) Vx* (m) Hx (MN) Comb N°
Maxi 4.50 5.7 0.070 - 2 1.35 Gsup + Gmin + S + C + 1.35 LMcara + 1.5 (0.6T)
Mini 0.71 3.3 0.068 - 2 bis
Maxi 3.75 4.9 0.061 0.055 4
1.35 Gsup + Gmin + S + C + 1.35 gr2 + 1.5 (0.0T)
Mini 0.79 1.8 0.059 0.055 4 bis
Maxi 3.82 6.7 0.080 - 8 1.35 Gsup + Gmin + S + C + 1.5 T + 1.35 LMfreq
Mini 0.75 3.3 0.078 - 8 bis
Gmin (at service start date of the bearings) 0.89 9
N.B: S refers to shrinkage, C to creep, P to pre-stressing. Vx* refers to displacement without the effect of breaking force (Hx).
Table 4.3: calculated force and deformations
The following calculations correspond to the recommended procedure for dimensioning a bearing.