Chapter 14
Roadway Bridge
1
contents
• Roadway Bridge Floor
•
Side walks and Railings
• Bridge Bracings
• Design of lateral support at top chord of through
pony bridge
• Cross Sections for wind Bracing
• End X-frame in deck bridges
• Transmission of the braking forces the bearing
2
Truss Bridges
A.
Types of bridge trusses
B- Determination of forces in truss members
c. Proportioning of truss members
D- Box section for bridge trusses Top chords
Lacing bars, batten plates
Bottom chords
Diagonals
Verticals
Design of compression member
Design of Tension Members
3

Design of Bolted Joint
 Design of Battens and Diaphragms
 Design of End Portals
4
Roadway Bridge Floor

The floor of a Roadway Bridges consists of:
 1.
A wearing surface or Roadway Covering.
 2.
Sub floor transmitting the loads to the
stringers and X-girders.
 The sub floor is similar to the solid floor of a
ballasted Railway Bridges. It may be timber, steel
floor or R.C. floor.
Back
5
•·
Timber floor (Type 1)
•For bridges, generally two layers of flanks are provided.
For calculating these flanks we assume that the maximum
wheel load is distributed over two flanks.
P
5-6 cm
1-2 cm
X.G
6
·
Reinforced concrete floors (R.C. floor)
It may be supported by the main girders only, the X.G.
only or by stringers and X. girders. The span of the slab
may be 2.5 to 3.5 m, and thickness of slab to be 20 cm
nearly. The R.C. slab reinforcement, generally 12 bars
are used at least per one meter.
Slab on X.G only
A
X.G
X.G
A
(2.5-3.5) m
X.G
(2.5-3.5) m
Sec A-A
(1.2-1.5)m Rail way
7
Slab on M.G only
U.W.Br
M.G
M.G
M.G
M.G
L.W.Br
(2.5-3.5) m
8
Stringer
Side walk stringer
Truss bracket
M.G
X.F
Member to prevent
lateral buckling of web
Span of slab
X.G (truss)
Kafre El-ziat Roadway Br.
9
Stringer
X.G
U.W.Br
M.G
X.F
L.W.Br
10
5 cm Asphalt
X.G
X.G every 150 cm
·
Wearing surface
·
The wearing surface for roadway covering consists
of timber blocks, hard bricks, asphalt bricks, stone blocks,
asphalt or concrete.
The choice of material depends on the traffic, the span of
bridge, the cost and climate
11
Side walks and Railings
The side walks are placed either inside or outside of the
main girder. If they are arranged outside, they must be
supported on cantilever brackets situated in the plane of
the X.G. so that the –ve bending moment of the bracket is
transmitted to the X.G. The floor of these side walks
should be a precast R.C. slab (6cm) thick resting on the
side walk stringers. The wearing surface is a 2 cm layer of
asphalt. In through bridges the curb should be at least 50
cm inside the main girder
Back
12
Hand railings and brackets withstand the effect of a
transverse horizontal force of 150 kg/ m’ in cases of
Railway bridges, Roadway bridges, and foot bridges,
supposed acting at top level of hand rail. This horizontal is
transmitted from the hand rail to the main posts and from
their connections to the cantilever brackets.
Side walks parts
1.
Slab
Take strip 1 m and statical system as continuous beam
supported on side walk stringer (one way slab), take t = 8
cm and get As. The applied loads are considered 500
kg/m2 or one concentrated load 5 t.
13
110 cm
3
1
4
2
5
Stringer
6
X.G
1.
Side walk stringer
Simple beam span distance between two brackets (take channel X.
sec.)
2.
Hand rail
Simple beam span distances between two brackets (take angle or
channel X. sec.).
14
OR
OR
2
M=W.L /8
4
150 Kg/m2
Post
Cantilever beam (take 2-angle or 2-channel X. sec.).
P = 150 L
M = P y
Kg
OR
OR
15
5
Connections
Double shear bolts
P = 150 L Kg
M = P y
F1 =
F2
M
M  d1
d
F1
F2
F1
2
16
6
Bracket
Calculate M& N& Q at center of bracket.
In case of beam loaded alone we must calculate Fl.t.b and
the check that the actual stress Fc is less than the
+
+
S.F
B.M
N.F
17
allowable stress Fp.b.
For ST 37
Fc  1.40  0.0000652
7500
Fc  2 t / cm 2

If   100
If   100
 = l/ i ,
where I for compression flange only.
·
for bracket l/ b  2 l/ b
·
assume unequal angle 80120
·
Bolts subjects to shear
F1 =
M  d1
d
2

2
2
2
d
=
2
d

d
1
2


18
Bolts subjects to tension
M y
t =
I N.A
 0.8 Ft
F1
d1
y
d1
N
A
F1
19
Bridge Bracings

The bridge is provided with horizontal and
vertical bracings: 1. The stringers are connected together by
stringer bracing given before.
 2. The chords of the main girders are jointed
together by an upper and lower horizontal
bracing called wind bracings.
20
•These transmit to the bearings of the bridge;
•a.
The lateral forces due to wind.
•b. The lateral shock 6t.
•c.
Centrifugal force.
3 - Special horizontal bracings for the braking forces.
4-Two vertical and transverse bracing called X-frames or
portals (in case of through) transmitting reaction of the
upper lateral bracing to the bearings of bridge.
5- Some intermediate vertical transverse bracings called
intermediate X-frames or intermediate portals for the
rigidity of the structure.
It isn’t necessary to find all these bracings in every bridge,
there existence depend upon the type of the bridge, the
21
span and the floor.
I-The Deck Bridge
U.W.Br
End X Fram
L.W.Br
Deck Bridge.
The upper wind bracing transmits the wind
pressure WT on the train & WF on the floor & ½ WG
on the wind ward side of the main girder.
22
The lower wind bracing transmits the wind pressure ½
WG on the wind ward side of the main girder.
W = 100 kg/ m2
in case of loaded bridge
W = 200 kg/ m2
in case of unloaded bridge
* The wind pressure WT on the train produces in addition
to horizontal loading of the upper wind bracing.
a vertical loading, V =  WT  e to the main girder.
b
23
W.T
3.5 m
V
e
V=WT .e
b
W.F
X.G
U.W.Br
W.G
M.G
End X Fram
L.W.Br
b
Dick
24
In case of a truss bridge, only the exposed area of the
members is considered. This area is equivalent to 40 % of
the hole area of the surface of the truss. In all bridges with
an upper and a lower wind bracing, their shall be provided
at each end a X-frame
to transmit to the
bearings,the
X.G U.W.Br
horizontal reactions of
the upper wind
bracing. The
M.G
horizontal reactions of
End X Fram
the lower wind
bracing are
L.W.Br
transmitted directly to
25
the bearings
·
The end X-frames in deck bridges shall be of rigid
type. In all railways and in roadway deck bridges there
shall be intermediate transverse bracing at least at every
third panel point to increase the stiffness of the bridge.
These intermediate X-frames will release the end X-frame
from a part of the horizontal reaction of the upper wind
bracing. Yet it is recommended not to consider that release
unless the bridge as the space structure.
26
ii-The Through Bridge
U.W.Br
Portal Fram
L.W.Br
Through with upper bracing Bridge.
27
In through bridge two horizontal wind bracings should
be arranged if possible. In the plate girder through
bridges we can’t arrange an upper wind bracing in the
bony truss Roadway Bridge we have only a lower wind
bracing which transmits all the wind loads to bearings.
The force WF on the floor will be considered to act on a
solid surface as the plate girder. The through Railway
Bridge shown above is provided with two horizontal
wind bracings.
28
U.W.Br
W.G
W.G
W.T
2.5 m
W.F
W.T
3.5 m
W.F
L.W.Br
Pony truss bracing
L.W.Br
Dick
29
The upper wind bracing transmits the wind pressure ½
WG on the wind ward side of the main girder.
The lower wind bracing transmits the wind pressure ½
(WG), WT on the train & WF on the floor & on the wind
ward side of the main girder.
W.T
At the connection of
each X-G to the main
girder,
stiffness
bracket
shall
be
arranged.
3.5 m
W.G
X.G
L.W.Br
Through
Back
30
Design of lateral support at top
chord of through pony bridge
C = force in flange = AfFt
 The U-frame formed by the two vertical stiffeners
and the horizontal stiff X-girder is acted upon by a
horizontal transverse force = C/100 at the centroid
of compression flange as well as the wind pressure
between two consequence X-girders. The
maximum stressed section is mn. The compression
stress at point n ≯ Fltb. If the stress isn’t safe, we
either increase the thickness of the bracket plate or
add a stiffening angle.

31
The connection between the X-girder and bracket is
designed on the shearing force A that between X-girder
and the bracket and horizontal gusset of wind bracing on
force B. if the X-G is built up section the bracket
connection is designed as a web splice.
32
n
C.G
m
n
Chord
C
1/100 C
C
m
X.G
Cos =1/200
2x1/200xC=1/100xC
33
1/100 C
W.G
A
A
X.G (Rolled section)
B
X.G
B
Back
34
Cross Sections for wind Bracing

The diagonal of wind bracing
The diagonal of wind bracing shall have
stiff section to prevent vibration and to help
in reducing the deflection of main girder
due to eccentric loading (space frame
treatment). The section should have a depth
not less than L/40. The recommended
sections are given in Fig.(5.).
35
h < L/40
h
36
The choice of the section depends more or less upon the
span of the diagonal, the two channel section is convenient
for too long spans executed in Banha and Samanood
bridges. The two channels are connected together by
latticing or batten plates.
≯ in compression & ≯ in tension
Gusset for U.W.Br.
Lacing
h
End patten plat
37
In case of one diagonal member only
Case of the warren system which designed on a force S;
Q
 Fall
D.F= +Q/sin
Sin
Fall  0.85 Ft
for tension force
S= 
Fall  0.85 Fc
for compression force
38
Case of the N-system which designed on a force S;
D.F= +Q/sin
Q
S= 
 Fall
Sin
Fall  0.85  Ft
for tension force
39
Case of the K-system , Rhombic And Multiple which
designed on a force S;
K- system
Rhombic system
Multiple without vertical system
40
Q
S= 
 Fall
2Sin
If the bracing is made up of crossed diagonal and struts,
the calculation is made under the assumption that the
tension diagonal are only acting. The struts here receive
compression force. If the multiple systems of wind bracing
a further reduction of 20 % in the allowable stresses given
before, shall be made to account for approximation of
solution that both systems (tension and compression
member) equally share the lateral loads. in case of one
angle in compression the allowable compression stress
shall be reduced by 40% of Fc.
41
Hence, the approximations in allowable stresses are;
Fall  0.85  0.8  Ft
for tension force
Fall  0.85  0.8  0.6  F p.b
for compression force & A eff
& A eff

3 A1

  A1 
3 A1  A2

 Agress
Reduce stresses0.85due to space effect
Reduce stresses0.80due to approximation for indeterminancy
Reduce stresses0.6 for one angle action(unsemmetric sectionsonly)in compression
Back
42



End X-frame in deck bridges
The compression diagonal is assumed in acting and we design the
tension diagonal, also we assume that the X-frame is resting at a
movable support at one end and the hinge support at other end.
U.W.Br + 6 t
L.W.Br
X.G
Ru+Rl+6 t
Back
43
Transmission of the braking forces the bearing
In Railway bridges especial bracing should be arranged to
transmit the longitudinal forces from the stringers to the
panel points of the main girder, hence; they are transmitted
through the main girder to the hinged bearings. Some times
a bracing is arranged at every panel point. But generally
two bracings at the quarter points of bridge are sufficient.
The braking bracing system shown in sketch is statically
indeterminate but the loads are symmetric about
perpendicular axes to X-X. Therefore the diagonal B’n &
c’n are zeros since they correspond to themselves. Also, the
loads are antisymmetric about axes Y-Y and thus members
mb’ & mc’ & nb & nc are zeros, If special bracing of the
longitudinal forces is omitted, these forces are transmitted
44
from stingers to main girder by bending of the X-girder.
B/4
2
B/4
B/8
B/8
B/8
B/8
B/8
B/8
B/8
B/8
7
B/4
B/4
M.G
1
2
3
4
5
6
7
9
8
B/4
B/4
y
My/Zy
B/8
c' B/8
b'
B/8
x
x
B/8
B/8
X.G
B/8
X.G
Back
c
b
45
y
Truss Bridges
b  L/ 20,
b  h/ 3
Where, b = bridge width = distance between center lines
of two main girders
L = span of bridge
The depth of trusses shall be chosen in such away that the
elastic deflection due to L.L (without dynamic effect)
shouldn’t exceed L/800 for Railway bridges and L/600
for Roadway bridges.
Back
46
h
X.G
b
b > L/20 , b<h/3
h≮
span
10
(simple ) &
span
h≮
(simple ) &
8
span
12
span
10
(continue)
Road.
(continue)
Rail.
47
A.
Types of bridge trusses
Either both chords are straight and parallel or only one
of them. In a through bridge the upper chord may
polygonal, in a deck bridge the lower chord may be
polygonal. Curved chord should not be used in bridge
trusses on a account of the additional bending stresses.
The loads are transmitted to the panel point of the truss
by a system of stringers and cross girder. No load except
the own weight of the truss members should act between
the panel point.
48
1.Trusses with horizontal chords
They suitable for span up to 60 m. the joints are
simpler than in trusses with polygonal chords. The depth
is h  L/ 8 for Railway Bridge, or h  L/ 10 for Roadway
Bridge. For continuous and cantilever trusses the depth
may be taken h  L/ 10 for Railway bridges, h  L/ 12 for
Roadway bridges. Some times a greater depth is used to
allow an upper wind bracing. The arrangement of web
members may be N-system or warren system. The warren
system trusses require generally less material than the Nshaped trusses, since the vertical members have smaller
forces, the number of joints and changes of cross section
in warren system are also less. Shop work for warren
trusses will be cheaper.
49
h
Warrren
h
N shaped
N through
N through
50
2-Trusses with polygonal chords
They are used for spans up to 60 m. the economical depth
at middle is h = L/7. The web system is either N-shaped
or warren. The economical inclination of the diagonal to
horizontal =  = 40 - 60. A polygonal chord trusses
lighter than a truss with horizontal chords since the forces
in the diagonals are smaller. On the other hand the shop
work is more complicated which means a higher cost.
h=1/7 L
(0.5-0.7)h
= 40-60
N or Warren
51
3-Trusses with subdivided panels
Rhombic diagonal (f) and K-system
(e),
with
These kinds are economical for span over 80 m. The panel
length is reduced in all this system and thus the cost of the
floor is less, but the increased number of joint increase the
cost of shop work. A truss with Rhombic diagonal has a
good appearance; a truss with subdivided panels has big
secondary stresses. K-system trusses have the smaller
secondary stress.
52
e- Subdived system
Rhombic system
K- system
53
4- Trusses with multiple web system
These were used in past where the tension diagonals
consist of flat bars. Now they are again used for main
girders, but type h with crossed diagonals is frequently
used for wind bracing. For approximate calculation, the
common assumption is that the truss may be divided up
into two or more component trusses with the same chords
but with different web system. The loads also are divided
and placed upon this component trusses. Then the stress in
a web member is determined as its stress calculated in the
truss of which it is a part. The chords are a part of all
component trusses, hence the stresses in a chord member is
obtained by adding it partial stress from each component
truss.
54
=
+
5- Trusses with 3 chords
The arch truss with a tie (k) and the truss reinforced by a
hinged arch (Pow string truss) are supported on a hinged
bearing at one end and a movable bearing at the other.
They are therefore externally static determinant but
internally they are static indeterminate. These trusses
have good appearance but they more expansive than
trusses with two chords.
55
Arch with atie
Row String
Back
56
b- Determination of forces in truss members
We determine the forces in the truss members on the
assumption that the member are connected by hinges, so that
loads applied at the panel point produce only axial forces in
the truss member. The secondary stress which are the
bending stress induced by the rigidity in connection, are
generally neglected. In our specification it is required to
calculate the secondary stress in the following cases:
1- For trusses with subdivided panels.
2-For member whose width in the plane of the truss is
more than 1/10 of its length.
3-For loads acting between the panel point.
Back
57
c. Proportioning of truss members
For the chord member we can use either sections with one
web plate (T section) or sections with two web plates
(Box section). T-sections are used only for small bridge.
Box sections have grates moment of inertia about axis y-y
and are better used for the connection of gusset plate. The
sections of all chords and web member should be
symmetrical about axis y-y in the plane of the truss.
Diagonals and verticals are usually symmetrical about axis
x-x also. The required area of the chords change at every
panel point of the truss and in choosing the different cross
section we must try to get simpler connection and splices
at the panel point.
Back
58
y
y
x
x
y
x
x
Chord members
y
D- Box section for bridge trusses
Top chords
The minimum section consists of a horizontal plate and 2
channels or a horizontal plate, 2 vertical plates and four
angles.
Depth of top chord h = (1/12 – 1/15) of the panels length ≯
Width a = (0.75 – 1.25) h
59
L
10
h
min
a
max
a
(3/4 - 5/4)h
To avoid local buckling, the minimum thickness of web
and cover plates should be as follows;
The unsupported width of a plate measured between
adjacent lines of rivets or welds connection the plate to
other parts of the section should not exceed:
60
t = thickness of a single plate or of 2 or more plate
provided that this plates are adequately tacked together.
b
64

t
FY
b
t
t
b
max
(3/4 - 5/4)h
max
(3/4 - 5/4)h
61
b
t
t
d1
max
(3/4 - 5/4)h
d1 30

t
Fy
Only excess over this width should not be included in the
effective sectional area in computing direct compressive
stresses. The center of gravity axis x-x for the different
section should not change too much. In drawing the truss
we use an average value y = (y1 + y2 + y3 + ....)/ n.
62
It is good practice to use a cover plate over the whole
length of the top chord even if the end members have
excessive cross section.
y
y1
x
y
x
y
y2
x
y
x
y
y3
x
x
y
Back
63
Lacing bars, batten plates
The two plates of the compression members shall be
connected together by diaphragms and the open side of the
box section shall be provided with batten plate close to the
gusset plate and with intermediate batten plates or lacing
bars to avoid lateral buckling of their component parts.
The slenderness ratio of each component part between
consequent connections of lacing bars or batten plates shall
be not more than 50 (and 2/3 l/i of the whole section).
64
y
z
batten pl.
z
y
A
Lz
View A
65
Bottom chords
The depth of the bottom chord is equal to that of the top
chords h = (1/12 – 1/15) of the panels length, or slightly
more (2 – 4 cm). No horizontal plate is provided at the
bottom of the section to avoid water packets. In
continuous and cantilever bridges where some bottom
chord members are in compression, horizontal plate may
be used and it must be provide with drainage holes (4 – 5
cm) . The two component parts of tension member shall
be connected together by diaphragms and batten plates
similar to these of the compression members, but their
thickness may be taken 25 % lighter (t2  lmember/ 15).
66
h
h
bottom with cover plate (holes)
Diagonals
For appearance the width of the diagonal should not be
more than that of the top chord and should decrease from
the end to the middle of the bridge. The compression
diagonal at end of the warren truss has a section similar to
that of the top chord.
Back
67
Verticals
In trusses with a N-shape web system, the vertical have
similar sections as diagonals. In warren trusses the vertical
may consist of a web plate + 4 flange plates or an I-beam
(B.F.I.B).
For diagonal or vertical tension member;
t2  lmember/ 15
(D&V)
t2  (lmember)/ 30
Railway bridges
t2  (lmember)/ 35
Roadway bridges ( D & V& C )
t2  (lmember)/ 10
(C )
D → Diagonal
& V→ Vertical
(D&V&C)
&
C → Chord
68
t2
t2
t2
t2
< 10 mm
Back
69
Design of compression member
The slenderness ratio l/ i of compression member of main
girder shall not exceed 90 for Railway bridges and 110 for
Roadway bridges.
E. Effective buckling lengths
Table 4.5
70
Table (4.5)
Member
Chords
Diagonals
In plane
0.85 l
0.70 l
Out of plane
Compressio
n chord
Effectively
braced
Compression chord
0.85 l
0.75 span
(Clause 4.3.2.2
or equation 4.2 if
using U-frames)
0.85 l
1.20 l
Unbraced
- Single
triangulated
web system
71
- Multiple
Intersected
web
rectangular
system
adequately
connected
- K-system
0.85 l/2
0.75 l
l
0.90 l
1.20 l
1.50 l
Vertical
members
- Single
triangulated
web system
- Kintersected
web system
0.70 l
0.85 l
1.20 l
(0.75+0.25N
s/Nl)L
(0.9+0.3Ns/
Nl)l
0.35l
72
Effective buckling lengths
x
x
0.85
0.7
0.85
0.7
y
0.9
y
0.7
y
Lb in plan
y
bracing
y
y
0.85
0.85
0.75
1.2
x
0.85
x
x
Lb out-plan
(with upper bracing)
x
Ns/N
y
4
2.5 EI a
y
1.0
1.2
1.5
x
1.2
x
x
x
Lb out-plan
(No- bracing)
73
Ns/N
Unbraced compression chords
a- For simply supported truss, with laterally
unsupported compression chords and with no cross-frames
but with each end of the truss adequately restrained
(Figure 4.1), the effective bucking length (kL), shall be
taken equal to 0.75 of the truss span, (clause 4.3.2.2).
b- For a bridge truss where the compression chord is
laterally restrained by U-frames composed of the cross
girders and verticals of the trusses, the effective buckling
length of the compression chord (Lb) is:
L b = 2.50 4 E . I y .  . a  a
74
y
y
d1
d2
I1
I2
B
E = the Young’s modulus = 2100 t/cm2
Iy = the moment of inertia of the chord member about the YY axis.
a = the distance between U-frames (cm) = S
75
d13
d 22 * B
=

3 * EI1 2 * EI2
d1 = the distance from the centroid of the compression
chord nearest face of the cross girder of the U-frame = dw
– Hx.G.
d2 = the distance from the centroid of the compression
chord to the centroidal axis of the cross girder of the Uframe = dw – Hx.G./2
I1 = the second moment of area of the vertical member
forming the arm of the U-frame about the axis of bending.
76
I2 = the second moment of area of X-G about the axis of
bending = IX
B = the distance between centers of consecutive Main
Girders connected by the U-frame
Back
77
Design of Tension Members
Tension members shall always be of rigid construction
and their slenderness ratio l/ i shall not exceed 160 for
Railway bridges and 180 for Roadway bridges. The
effective net section area shall be taken for all tension
members. This area shall be the least that can be
determined from any plane or planes cutting each
component plate or sections  to its axis,
78
diagonally or following zig-zag line through adjacent
rivet holes. In each case all holes of line to meet with
shall be deducted from the gross sectional area where any
portion of the sectional area is measured for a diagonal
plane adding (S2/ 4g) for each gauge space. The minimum
sectional area should not be less than that obtained by
assuming all the holes to be in one perpendicular plane.
Back
79
Design of Bolted Joints
Connection of web member to gusset plate “and splices of
chord member” shall have a strength equal to the
maximum strength of the connected members. The bolts
shall be arranged symmetrical about the center line of the
member. The connection to either direct or part of it is
indirectly connected by:1- Splice plates or lug angles.
2- If breaking is along section S-S bolts (1 single + 2
double + 3 double) shear, must carry the load.
3- The strength of the splice plate should be enough to
carry a force corresponding to bolts (4 + 5) single shear.
80
Lug angle
Lug angle
10 % more
20 % more
20 % more
Symmetrical
40 % more
Unsymmetrical
81
gusst pl.
S
S
Splice pl.
gusst pl.
Sec S-S
82
Connections for members
The connection shall be designed for a capacity based on
the maximum of:1- The average between the actual force and the
maximum strength of the member of not less than 0.75 the
maximum strength of the member.
2 The bolts between the chord and the gusset plate must
correspond to the algebric sum of the horizontal
components of the strength of the diagonals S1, S2
S1
S2
L = S1Cos  + S2Cos 
83
3. The number of bolts should correspond to the
effective strength of the two diagonal, i.e., the number of
 = (number of bolts in member 1 + number of bolts in
member 2)Cos .
84
Example
S1
S3
94 t
S2
S4
126 t
1/tan
70 t
196 t
I.L.S1
L=750
1/tan
I.L.S1
In a lower chord panel, if the maximum force in chords
and diagonals are as given, design suitable cross section,
connections and splices.
85
Member S1 = + 94 t
2 [ No 24
Bolts M22,  = 24
Anet = 2(42.3 – 2.40.95 - 22.41.3) = 67.56 cm2
Fact = 94/ 67.56 = 1.39 t/ cm2 < 1.6 t/ cm2
Maximum force = 67.561.60 = 108.10 t
Rleast = Rs. sh
qb = 0.25 Fub
For bolts of grade ( 8.8),
Rs. sh = qb As n = 0.258.03.03 1 = 6.06 t
(n = No. of shear planes)
86
Bolts connecting diagonal to gusset = (Maximum force/
Rleast) 1.15
= (108.1/ 6.06)1.15 = 20.52 bolts
Use 24 bolt M22
y
Member S2 = - 70 t
2 [ No 22
& Bolts M22,  = 24
Agross = 2(37.40) = 74.80 cm2
x
x
y = ly/ iy = (0.85750/ 11) 1.20 = 69
(1.20 due to lacing bars)
x = lx/ ix = (0.70750/ 8.5) 1.20 = 61
y
87
max = 69  Fp.b = 1.60 – 0.000085 (l/ i)2 = 1.119 t/ cm2
Fact = 70/ 74.80 = 0.94 t/ cm2 < 1.119 t/ cm2
Maximum force = 74.801.119 = 83.70 t
Rleast = Rs. sh
qb = 0.25 Fub
For bolts of grade ( 8.8),
Rs. sh = qb As n = 0.258.03.03 1 = 6.06 t
(n = No. of shear planes)
Bolts connecting diagonal to gusset = (Maximum force/
Rleast) 1.15
= (83.7/ 6.06)1.15 = 15.88 bolts
Use 16 bolt M22
88
Member S3 = + 126 t
2 [ No 30
& Bolts M22,  = 24
Anet=2(58.80 – 22.41.00 - 22.41.6) = 92.64 cm2
Fact = 126/ 92.64 = 1.36 t/ cm2 < 1.6 t/ cm2
y
x
x
y
89
Member S4 = + 196 t
2 [ No 30 + 2 PL 24012 & Bolts M22,  = 24
Anet for 2[ = 2(58.80 – 22.41.00 - 22.41.6) = 92.64 cm2
Anet for 2PL = 2(24 – 22.4) 1.2 = 46.08 cm2
y
Anet for 2[+2PL = 92.64 + 46.08 =
138.72 cm2
Fact = 196/ 138.72 = 1.413 t/ cm2 x
< 1.6 t/ cm2
x
y
90
Force to be transmitted from gusset plate to bottom chord
= Fmax
Fmax = (S1 + S2)Cos  = (67.561.6 + 74.801.119) 1/2
= 135.62 t
Bolts connecting diagonal to gusset = (135.62/ 6.06)1.15
= 25.74 bolts
Use 28 bolt M22
The bolts in the framing angle are not counted as they used
to transmit the reaction of the cross girder.
91
Splice of chord
Splices of tension or compression chord shall be
designed on the maximum strength of the member. For
straight chord the splice shall be outside the gusset
plate. For broken chord the splice will be within the
gusset plate.
Member S3
2 [ No 30
& Bolts M22,  = 24
Net area of flange = (10 – 23) 1.60 = 12.30 cm2
Net area of splice plate = (10 – 23) 1.60 = 12.30 cm2
Number of single shear field bolts =
(12.301.6/ 6.06) 1.15 = 3.25 bolts
92
Net area of web = (30 – 21.60) 1.00 = 22.20 cm2
Use 2 splice plates = 300.80 + 240.80
Net area of 2 splice plates = (30 – 22.3) 0.80 + (24 22.30) 0.8 = 35.80 cm2
Rleast = RD. sh or Rb
Rb = Fb  d  min t
Fb =   Fub
 = 0.60 for end distance (S1.5d), (Table 6.2)
Fb = 0.60  8.00 = 4.80 t/ cm2
93
Rb = 4.80  2.20  1.00 = 10.56
t
qb = 0.25 Fub
For bolts of grade ( 8.8),
y
x
Rs. sh = qb As n =
0.258.03.03 2 = 12.12 t
x
y
Splice pl.
94
Rb = 10.56 t
Number of double shear field bolts = (22.201.6/ 10.56)
1.15 = 3.87 bolts
Use 4 bolt M22
The splices in the chords are placed in the side of the:-
1. Smaller cross section except in cases where the erection
is done by the cantilever method.
2. Gusset plate; in a polygonal chord in order to avoid the
bending of plates, angles and channels the splice plate is
placed at the brake of the chord on the gusset plates.
3.Gusset shall be proportion to withstand the effective forces
in the web member
95
The thickness of the gusset is determined from the critical
section abcd. This section should be at least 15 – 20 %
stronger than the diagonal it self generally all gusset are
made of the same thickness. The thickness of gusset plate
shall be at least 12 mm in Railway bridges and 10 mm in
Roadway bridges.
96
2 NO.220
2 NO.240
d
12M22(8.8)
a
b
c
8M22(8.8)
2 Pl. 240x12
4M22(8.8)
4M22(8.8)
14M22(8.8)
Bolts for framing
only.
Gusset pl.
for L.W.Br.
2 NO.300
97
98
99
4- At the chord panel point the cover plate should be
connected to a gusset plate by special connection angle to
make the center of gravity of the rivet group between gusset
plate and top chord nearer to the center of gravity of the top
chord.
100
A
S
S
1
2
3
4
A
Sec A-A
1
3
2
4
n
n
n
A
S
S
1
2
3
A
Sec A-A
Back
2
3
1
101
Design of Battens and Diaphragms
The two parts of the box section must be connected
together in such away that they act ass one unit. For
compression member stronger details are necessary than
for ten member.
Diaphragms
Diaphragms are transverse plates or channel connected to
the two webs of the box section by angles. They are
necessary to assume the rectangular shop of the box
section. For the chords wee have at least one diaphragm
between the two panel points. In the diagonals, we arrange
at least one diaphragm near each end.
102
Batten plate
One the open sides of the box section we have batten plates
as close to the gusset plate as possible one intermediate
batten plate or lacing bars to avoid lateral buckling of the
unsupported flange for the calculation of the lattice bars of
a compression member we assume a transverse force = 2 %
of the longitudinal force in the member. If there is a
continuous plate at the upper side of the box section,
latticing will be in the lower side only and transverse force
will be according to cross section of the lower side only. In
tension member a lattice system and batten plates 25 %
lighter is used.
103
A
S
S
Sec A-A
A
Sec S-S
104
B
B
B
D
D
Battening of compression member
The number of batten is such that we get at least 3 bays
batten shall be of plates, channels, I-section bolted or
welded to resist the following forces:The member as a whole can be considered as a
vierandeen girder or we can assume hinges at mid
distances and change it to statically determine system.
Shear in batten plate = Qd/ a
105
Bending moment in batten = Qd/ 2
Q/2
Q/2
d/2
M=Q.d/2
Q.d/4
d/2
Q/2
Q/2
Q/2
Q/2
a
Q.d/a
F1
Q.d/a
t
Q.d/2
F1
106
Q.d/a
d
batten pl.
Back
Design of End Portals
End transverse bracing are called portals. The portals are
placed either in the vertical plan of the end post (1), in
the plan of the first vertical (2), or in the inclined plan of
the end diagonal (3). In case (2) the first panel of the
lower wind bracing is affected by the reaction
transmitted by the end portals. The arrangement of end
portal in case (2) is stiffer than in case (3). The portal
must not interfere the clearance line.
107
The shop of the portal depends on the depth and on the
clearance line. The portals are generally static
indeterminate closed frames in which the post over subject
to bending stresses.
1
3
2
108
W1
Approximate calculation of portals
The point of inflection of the post are situated according
to the relative stiffness of the cross girders, post, and the
upper strut at height between 1/3 – 1/2 of the force height
h’. We can replace the point of inflection by two hinges
at C and C’ each of them transmits ½ W1. Then the portal
can be calculated as static determine frame. If the portal
is in the plan of inclined end diagonal, the points of
inflection C, C’ should net be more than h’/ 3 from A and
A’.
109
W1
W1
h'
h'
h
h
X.G
W1.h/b
b
X.G
W1
W1.h/b
W1
b
W1.h/b
W1.h/b
W1.d/2+W1.e
W1.d/2
W1
e
W1.d/2
W1.d/2
d
W1/2
W1/2
W1/2
W1/2
f
W1.f/2
W1.f/2
X.G
W1.f/2
g
W1.f/2+W1.g
W1.h/b
Back
W1
110
b
W1.h/b