Railway truss bridges Lecture

8/2/2019 Railway truss bridges Lecture

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Railway truss bridges

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I – RAILWAY BRIDGES

General Remarks for all kinds of bridges:

1- The deck bridge is the best type because the compression chord of thetruss is braced by cross girders (X.G). [Lb out of upper compression chord

will be distance between cross girders].

2- There must be horizontal wind bracing at the level of the lower flange

of X.G.

3- There must be stringer bracing to carry lateral shock

There must be braking force bracing to carry braking force.

In Roadway there was no lateral shock & the braking force was

carried by slab to main girders to supports. But in case of Railway

Bridge, if there is no braking force bracing, there will be M y on X-

girders.




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4- What is the difference between pony & through bridge?1 - The X.G. in both lies at the level of the lower chord.

2 – Also both of them have lower wind bracing.

3 – Only the through bridge has

upper bracing

We use through bridge only if h

5.5 m, h is measured from the top of

the rail level h = H MG – h XG – 0.20

m (sleeper) – 0.15 m (rail) – 0.3

(height of upper member)

The through bridge is preferable than pony bridge because the

compression chord is supported by bracing.

In case of using through bridge, a closed frame or inclined

frames at both ends of the bridge must be provided to transfer

load from upper lever of upper bracing to bearings.




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Intermediate closed frame may be added if span is too big.

5- To draw plans, we have to take many sections:

a- The pony bridge has 2 plans, one shows the stringers and the stringer

bracing, while the other shows the wind bracing with braking force

bracing.

b- Deck and through bridges have 3 plans. Two of them are the same as

that of Pony Bridge, while the third is either lower bracing for Deck

Bridge or upper bracing for through bridge.

We can draw the first 2 plans in one plan only as shown below:




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6- The width of the bridge "B" is nearly constant because the width of

train is constant and is determined as follows:

- 3.5 m in deck bridges & 5.3 in pony & through bridges.

- For each additional track in any kind of bridges add 3.5 m

i.e. Deck bridge:

Single track = 3.5 m

Double track = 3.5 + 3.5 = 7 m

Triple track = 3.5 + 2 *3.5 = 10.5 m

Pony or through bridge:

Single track = 5.3 m

Double track = 5.3 + 3.5 = 8.8 - 9 m

Triple track = 5.3 + 2 *3.5 = 12.5 m

Note:

For long span bridges, L > 40 m, we use the main system as a

truss with height"H" =

108

L.

The shape of the M.G. is as follows the upper

chord is in the shape of " "- section while the

lower chord is in the shape of "Double T"




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It's preferable that number of panels is even

د د فع ا س م ل ا XG

7- The spacing between X.G "S" is taken mg H L

10so the angle of

diagonal of main truss will be equal 450.

The number of panels will be 8 or 10 or maximum 12 ( ج( و ز .

8- There is no concrete slab so we cannot make composite action.

9- There must be a stringer nearly under

each rail.

10- The spacing between stringers is 1.8 m

(constant) [the distance is not 1.5 m as

distance between rails for construction

requirement.[ ]

11- Distance between center lines of any 2 successive tracks is 3.5m.

12- There is no side walk (unless mentioned).

13- The braking force bracing is added at both ends & if span > 6 0 m we

add another bracing at mid span.




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14- Drawing of lateral shock bracing (stringer bracing), the bracing is a

horizontal truss to carry horizontal loads. It consists of stringer as upper

& lower chord, verticals and diagonals are added.

15- The distance between verticals of stringer bracing is 1.5 – 2.0 m (so

the diagonal is inclined at nearly 45 o)

Important: So the shape of stringer bracing is as shown (according to

spacing between X-G "S".

16- The level of braking force bracing is at the level of the lower

flange of X-G. The braking force is transmitted from level of Rail (upper




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flange of XG) to level of braking force bracing (lower flange of XG) by

one of the following 2 methods : -

Either Full depth stringer or bracket ( as discussed in plate girder)

:

braking force bracing

17- In Deck Bridge, the X.F is added each (15 – 18 m)

18- The X.F. is either X-system or V-system according to the angle (35o-

55o).

19- The upper bracing in through bridge and lower bracing in Deck

Bridge can be in the shape of either x-system or rhombic system.

20- In case of using semi-deck bridge, the cross-girder will be nearly at

midpoints of the vertical members which give concentrated loads at these

points. This means that half the member will be tension and the other will

be compression. It is preferable to use K-truss.




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21- There must be end bracket because there is no cantilever slab.

22- In case of single track deck bridges, the

bridge may compose of 2 main girders and

cross girder without stringers. (The main

girders are used at the position of stringers.)

1.50 ms

2.60 ms

H

1.7-1.8m




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23- The main dimensions of the train envelope ( ) is as shown:

4.62 ms

5 . 0

0 m s

3.16 ms0.80 m

2.31ms

3.16 ms

2.31 ms

5 . 0

0 m s

3.16 ms

3.495 ms

0.15 m

5 . 0

0 m

s

0.15m

0.80m

24- In some cases (when needed) especially in through bridge, we can

lower the stringer by maximum of 15cm in order to decrease the heightof construction and increase distance between rail & upper beam.




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25- The stringer may be added over the X.G. The actual height of

construction in this case will be hstringer + hx.g.+0.45

26- When to use Subdivided trusses

For long spans (70 m or more) with narrow bridge (single track)

In this case the span of stringer will be 7m or more while span of

X.G. will be 3.5m in case of deck or 5.5m in case of pony or through.

i.e. the depth of stringer will be bigger than depth of X.G.

So use subdivided to reduce span of stringer




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Very Very Important:

How to determine the type of bridge whether it is deck, pony,

through?

The choice of the type of bridge depends mainly on:

1- Whether the compression chord is supported or not.

From the point of supporting compression chord, Deck Bridge is the

best choice then through bridge & finally pony is the most expensive.

2- The relation between the available height of construction "H"

and the construction height of the bridge "Hc".

a- The construction height of the bridge "Hc" is measured from the top of

the bridge floor (the rail level) to the level of the bottom flange of the

lower chord.

ت ا ر ا ط ق )أ(ى رال ب و ك ل ا ى ف ة ط ق ن ل ف س أ ب و س ن م

- Height for deck bridge is htruss + 0.2(sleeper) + 0.15 (rail) +

(0.1 is the deflection)

- Height of construction for pony and through bridge is h XG +

0.45.

b- Available construction height "H" may be given or calculated as

follows:

H = Bank level – H.W.L – Δ required for navigation

The actual height of construction "Hc" must be less than or equal the

available height of construction "H". (HC ≤ H).




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If Htruss + 0.45 =1210

L+0.45 ≤ H So use deck bridge.

If Htruss + 0.45 =1210

L+0.45 > H So use through or pony.

We try first to make it through bridge, if not so pony bridge

How to determine whether the bridge is through or not?

If the height of the truss

(1210

L) – hxg – 0.35 (sleeper + rail) – 0.3 (height of upper beam)

5.5m

So we can use through bridge.




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Examples to determine the type of bridge:

Example 1:

2 tracks railway bridge crosses a river with level of H.W.L (10.00) and

L.W.L (8.00). The Rail level is (15.00) and the height required fornavigation is 3.00. The width of the river is 50 m.

Solution:

Width of river = span of bridge

H = Available height of construction = 15 – 10 – 3 = 2 m

H C deck =1210

50

+ 0.45 = 5.45

4.6 m > 2 m

So we cannot make Deck Bridge.

Since we will use pony or through bridge, so B = 5.3+3.5 = 9.0m

H XG =97

9

8

B= 1.0m

For through bridge: h = 5- 1 – 0.35 -0.3 = 3.35m 5.5m

So we cannot use through Bridge

So the bridge must be pony




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Example 2: Important

3 tracks railway bridge crosses a river with level of H.W.L (10.00) and

L.W.L (8.00). The Rail level is (18.00) and the height required fornavigation is 3.00. The width of the river is 50 m.

Solution:

H = Available height of construction = 18 – 10 – 3 = 5 m

H C = for deck =1210

50

+ 0.45 = 4.60 5.45 m

O.K. use Deck Bridge with H truss = 5- 0.45

= 4.55m

Example 3:

One track railway bridge crosses a river with level of H.W.L (10.00)

and L.W.L (8.00). The Road level is (15.00) and the height required

for navigation is 3.00. The width of the river is 70 m.

Solution:

H= 15 – 10 – 3 = 2 m

H C for deck =1210

70

+ 0.45

= 6.3m 7.45 m > 3 m

So it is either pony or through

Since this bridge is through or pony

So B of X.G = 5.3m

H XG =8

3.5= 0.65 m Take HMG = 0.65 + 0.45 + 5.5 + 0.3 = 7.0 m

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Railway truss bridges Lecture