3 Loading 3.1 Design (1964) Dead Loading 3.2 Design (1964) Traffic and Pedestrian Loading 3.3 Design (1964) Environmental Loading 3.4 Traffic Loading for the Study 3.5 Footway/Cycleway Loading 3.6 Design Traffic and Footway Loading 3.7 Railway Live Loading 3.8 Dead Loading Associated with the Railway
3 Loading
3.1 Design (1964) Dead Loading
Current details of the bridge, and any subsequent modifications, are not available at this time. The loading applied in the original design is assumed to be reasonably indicative of the present condition. The following design dead loads are given in Table 2.5 (p.380) of the ICE report:
Item |
Main Span |
Side Span |
|---|---|---|
Cables, hangers and associated details |
38.8 |
39.9 |
Trusses, laterals and cross girders |
48.2 |
45.0 |
Road way deck steel |
31.1 |
11.0 |
Road way deck concrete |
0 |
76.9 |
Road way deck mastic |
14.6 |
15.6 |
Cycle and footway deck steel |
11.4 |
11.7 |
Cycle and footway deck rubber/bitumen |
1.0 |
1.0 |
Parapets, barriers, grillages |
4.8 |
4.8 |
Drainage gullies and downpipes |
0.4 |
0.4 |
Electric power, telephone cables |
1.0 |
1.0 |
Zinc spraying and painting |
2.1 |
2.1 |
Total dead load |
153.4 |
209.4 |
For the purposes of this study, the following breakdown of main span dead load is assumed:
Road way deck |
= |
2.13 kN/m2. |
} |
|
Roadway surfacing |
= |
1.00 kN/m2 |
} |
Based on 2 x 7.3 m of roadway |
Footway deck |
= |
1.22 kN/m2 |
} |
and 2 x 4.7 m of footway |
Footway surfacing |
= |
0.10 kN/m2 |
} |
It is assumed that the breakdown of load between parapets, barriers and grillages is as follows:
4 no. pedestrian barriers @ 0.15 kN/m |
} |
|
4 no. roadway barriers @ 0.7 kN/m |
} |
4.8 kN/m total. |
1 no. central grillage system @ 1.4 kN/m |
} |
It is further assumed that the two footway cantilever brackets weigh approximately 1/3 rd of the footways @ 4.2 kN/m
3.2 Design (1964) Traffic and Pedestrian Loading
The bridge was designed to carry B.S.153 Part 3: 1954 loading comprising of HA UDL and KEL, footway load and 45 units of HB load. The cycle tracks were assumed to carry half that specified for footway loading. HA UDL + KEL was applied in full to two lanes and 1/3 rd to the other two lanes. Thus, a total of 2 and 2/3 rd lanes were applied plus KEL's. In the HB plus HA combinations, one full carriageway of HA was replaced by the HB vehicle.
The application of the design HA + footway + cycleway loading is summarised (in terms of the lane load W) in the following figure taken from the ICE report:
The distribution of W with length was approximately as follows:
Loaded Length |
Lane Load W |
Total Traffic Load |
Total Load |
|---|---|---|---|
(x1.0) |
(x 2.66) |
(x 3.43) |
|
100 |
16.5 |
43.9 |
56.6 |
200 |
13.1 |
34.9 |
45.0 |
300 |
10.7 |
28.5 |
36.7 |
408 |
9.2 |
24.5 |
31.6 |
1006 |
5.9 |
15.7 |
20.2 |
1414 |
5.9 |
15.7 |
20.2 |
1823 |
5.9 |
15.7 |
20.2 |
3.3 Design (1964) Environmental Loading
3.3.1 Temperature Range
According to the ICE report, the bridge was designed to accommodate the following range of temperature:
20 deg C + 16.7 / - 33.3 deg C (50 degree C range).
3.3.2 Wind Load
The design wind speed was 110 mile/hr (49.2 m/s) at deck level. (see p 377 of the ICE report for loads derived from this base wind speed). It is believed that this was for a 3 sec gust (to be confirmed).
3.4 Traffic Loading for the Study
3.4.1 Design Code Loading Regimes
(a) Original and current British Standard Loading
The original design was checked for B.S.153 Part 3: 1954 loading and included both HA and HB types. At the time of design, the HA UDL in lane 1 was between one half (short loaded lengths) and a third (long loaded lengths) of current BS design loading. The KEL was similar to current BS loading, at 120 kN. The following graph extracted from Reference 2 (See Appendix B) illustrates the variation in the UDL lane loading:
The definition of HB was similar to current definition. The bridge was designed for 45 units of type HB, i.e. a 1800 kN vehicle. The vehicle was assumed to occupy one lane without accompanying HA load in the same lane.
The application of loading is described in some detail in the ICE report, an extract from which is given above. In general it was assumed that 2.66 lanes (= 1.0 + 1.0 + 0.33 + 0.33) would apply.
In comparison, BD37 loading is assumed to be 2.87 lanes (= 1.0 + 0.67 + 0.6 + 0.6). Thus, both intensity and number of loaded lanes considered has increased.
(b) Current Eurocode Loading (UK NA)
The UK NA version of the Eurocode loading is 5.5 kN/m2 between barriers regardless of loaded length, or width. Tandem axle loads of 600, 400 and 200 kN would be applied in three of the notional lanes.
(c) Reduced Eurocode Loading for 'Controlled' use
Eurocode LM1 loading includes an adjustment factor αq which may be used to define a controlled (reduced) loading regime. The reduction factor is αq1 = 0.8 in cases where lighter loading is anticipated, but not controlled. The factor is used to reduce the lane 1 loading of 9.0 kN/m2 to 7.2 kN/m2. Lane 1 will be 3.0 m nominal width. All other areas are loaded at 2.5 kN/m2. This is the minimum reduced case recommended by the Eurocode unless traffic control (i.e. signing etc) is employed - so a further reduction is possible for this bridge under a controlled traffic regime.
(d) Application of Design Code Loading
The above current code intensities are useful for comparison purposes. However, given the nature of the state of the existing bridge and history of assessments based on actual traffic flow, it is not considered desirable to consider any of the above loading conditions in the comparative study. The study will, therefore, proceed on the basis of the Bridge Specific Assessment Live Loading (BSALL).
3.4.2 BSALL Loading on Four Lanes
A significant amount of work has been carried out in recent years to estimate the actual road traffic loading on the bridge. This work, covered in detail elsewhere, is not reported further here. (See references 3 and 4 in Appendix B). For the purpose of this study, the following assumptions regarding application of this loading have been made:
i) Unless noted otherwise below, the loading is applied in accordance with BD37/01.
ii) The intensity of the nominal (unfactored) BSALL UDL is as follows:
Loaded Length |
Nominal BSALL |
|---|---|
100 |
20.3 |
200 |
16.7 |
300 |
15.2 |
408 |
14.3 |
1006 |
12.8 |
1414 |
12.5 |
1823 |
12.4 |
Note: nominal loads are equivalent to loads having a 2400 year return period divided by approximately 1.2
iii) The lane reduction factors are as follows:
Lane 1: 1.00
Lane 2: 0.67
Lane 3: 0.33
Lane 4: 0.33
iv) The nominal KEL is 120 kN
v) Footway loading is assumed to be 0.3 kN/m for all loaded lengths
3.4.3 BSALL Loading for other patterns of Lanes
Previous BSALL's used in assessment of the Forth Road Bridge were derived in 1985, 2002 and 2005. The most recent publicly available BSALL was derived in 2006. This was based on three weeks of traffic data recorded in late August and early September of that year. This was deemed to be a 'neutral' period (i.e. no public holidays or especially adverse weather). It avoided lane closures, and previous BSALL's had been derived from data recorded at the same time of year (which allowed trends to be identified).
Further BSALL’s have been estimated using typical UK vehicular traffic data. They represent three possible traffic operating scenarios. All are considered to be safe-sided, and might be somewhat reduced if current Forth Bridge traffic data were to be obtained and analysed. :
a) 2 lanes of unrestricted traffic*
b) 4 lanes of traffic restricted to twin-axle vehicles **
c) 2 lanes of traffic restricted to twin-axle vehicles
* Scenarios for future usage may include this case, but it is not considered further in this report..
** The 4 lane case is to be considered for design, if BS5400 were to be the relevant specification document. BS 5400 (BD37/01) requires that two notional lanes be considered for carriageways having a width of 5.00 top 7.50 m. However, for the purpose of comparison in this study, item c is used.
The results of the study are reported in the section on design loading below.
3.5 Footway/Cycleway Loading
It is understood that recent assessments used 0.3 kN/m total loading; this figure has been adopted.
3.6 Design Traffic and Footway Loading
The nominal (unfactored) loads for use in this study to compare various options are summarised in the following table.
Nominal Design Loads 1, 2, 3 :
Loaded |
1964 design: |
BSALL: |
BSALL: |
BSALL: |
BSALL: |
|---|---|---|---|---|---|
100 |
56.6 |
47.3 |
30 |
19 |
17 -15 |
200 |
45.0 |
38.8 |
28 |
19 |
17 - 12 |
400 |
31.6 |
33.3 |
27 |
19 |
16 - 12 |
1000 |
20.2 |
29.8 |
26 |
19 |
13 - 12 |
1400 |
20.2 |
29.1 |
25 |
19 |
13 - 12 |
1800 |
20.2 |
28.8 |
25 |
18 |
12 |
Notes:
1. Footpath load included in all cases.
2. In 1964 design, footpath load was approximately 29% of the traffic loading.
3. In BSALL loads footpath loading is assumed to be 0.3 kN/m regardless of loaded length.
4. 2 lane BSALL and 2 axle BSALL are more approximate than the 'all inclusive' values.
5. The range reflects the approximation. The higher figure has been used in this report.
3.7 Railway Live Loading
3.7.1 General intensity of loading
The existing bridge was not intended to carry rail load - so this aspect is not covered in the ICE report. For the purpose of this study, a notional load based on Load Model 71 of the Eurocode (refer to EN 1991-2 6.3.2) has been used. LM 71 is intended to represent 'normal rail traffic'.
LM 71 loading consists primarily of an 80 kN/m/track distributed load which is displaced over a length of 6.4 m by four axles of 250 kN.
This 'characteristic loading' is modified by a partial factor '_' and termed 'classified vertical loads'. It may be reasonably assumed that a factor α = 0.5 would conservatively account for most typical passenger railway loads likely in the UK. This factor could be reduced to α = 0.25 if only Light Rail and/or tram usage were permitted. Since only light rail or tram type loading is to be considered for the bridge, α = 0.25 is adopted here. (For confirmation of this assumption, see Section 4 below).
3.7.2 Loaded length
(a) Normal operation
The loaded length in the Eurocode is not limited (The loading is required to cover both goods and passenger service). This is not considered reasonable for light rail passenger operations where multiple car units are typically less than 100 m, trams typically less than 50 m, and separated by significant distances using signal control. For example, the recently introduced Dockland Light Railway (DLR) three car trains are approximately 90 m in length. The proposed Edinburgh Tram train is less than 50 m in length. (For further information on typical train characteristics, see Section 4 following). Thus, for the purposes of this study, the loaded length has been assumed to be up to 50m (similar to the Edinburgh Tram system), although a 100 m loaded length has also been considered in order to assess the sensitivity of the response to loaded length.
At α = 0.25 The total load on a 50 m length would equate to approximately 1,125 kN. It is worth noting that current DLR loading gives a similar result.
(Note: For spans of this dimension (400 and 1000 m), Dynamic Amplification is not required by the Eurocode.)
(b) Emergency operation
In emergency conditions, two trains may be coupled together to withdraw a broken-down train. In this situation, the 50 m loaded length would be increased to 100 m, although on one track only.
3.7.3 Combination with Traffic Loading
The probability of fully loaded trains in normal operation occurring in combination with characteristic traffic loading is reasonably high. Thus, in the study, the full nominal rail loading has been added to the full nominal traffic loading without reduction. (Note. The Eurocode philosophy would allow a degree of reduction to the traffic loading in combination with rail loading - but this has been conservatively ignored at this stage.) On the other hand, the probability of the breakdown situation occurring simultaneously with full traffic loading is less. Furthermore, the structural requirements (for instance, fatigue demand and so on) would be less onerous.
This study, therefore, concentrates on comparisons using the probable normal operation loading configurations in combination with full traffic loading.
3.7.4 Design Railway Loading for the study: Summary
The loading is taken as 0.25 times LM 71 loading: approximately 20 kN/m/track.
The study considers the following permutations:
a) Either one or two tracks of rail loading.
b) 50 m loaded lengths with a comparative check for 100 m loaded lengths.
All permutations are combined with traffic loading without reduction for combination effects.
3.8 Dead Loading Associated with the Railway
Main bridge (steel deck):
The following additional loads associated with the dead load of one additional railway track are assumed:
Four rails at 49 kg/m (including check rails) |
1.92 kN/m |
Softwood timber sleepers at approximately 0.70 m c/c |
0.70 kN/m |
Chairs, fixings, resilient pads etc |
0.80 kN/m |
Steel stringers (2 no. plus bracing etc) |
2.25 kN/m |
Lightweight decking |
1.00 kN/m |
Catenary and associated details |
1.00 kN/m |
7.67 |
Thus, allow 8 kN/m/track.
Approaches (concrete deck):
The following additional loads associated with the dead load of one additional railway track are assumed:
Four rails at 49 kg/m (including check rails) |
1.92 kN/m |
Chairs, fixings, resilient pads etc |
0.80 kN/m |
Concrete upstands (2 no.) |
3.00 kN/m |
Waterproofing plus screed protection |
1.50 kN/m |
Catenary and associated details |
1.00 kN/m |
8.22 kN/m |
Thus, allow 8 kN/m/track.