These slides give a short summary for the design of a truss bridge as a part of Steel Structures Design Sessional for the Course CE 320.

1. CE320: Steel Structures Design Sessional

2. Warren Truss Bridge

3. Longitudinal Elevation
Top/Bottom Bracing
Deck System
Cross Bracing
Diagonal Bracing
Floor Beam
Stringer
Top/Bottom/Diagonal Cord
10 @ 28 ft = 280 ft

4. Double Angle
Portal Bracing
32 ft
30 ft
4 @ 6 ft
24 ft
3 ft 3 ft
Cross Section
Built-up Box Section
(Truss Member)
Built-up W Section
(Floor Beam)
Rolled W and I
Section
(Other Elements)

5. Bridge Deck Precast Concrete
Cast-in-situ Concrete
Precast Concrete Deck
Cast-in-situ Concrete
Floor Beam

6. Loads on the Bridge
Dead Load (considered for this class):
 Wearing surface (30 psf)
 8 in Concrete deck (100 psf)
 Sidewalk (250 lb/ft) and Railing (25 lb/ft)
 Self weight of Stringer (Rolled W Section)
 Self weight of Floor Beam (Built-up W Section)
 Self weight of Truss Element (Built-up Box Section)
 Self weight of Top & Bottom Bracing System (Rolled W Section)
Live Load (considered for this class):
 Vehicular Load
 Wind Load (As point load according to BNBC)
 Live Load Impact (Impact Faction, I = 50/(125+L) ≤ 0.3 )

7. Vehicular Load
Design Truck Load (HS 20)
Design Lane Load (HS 20)

8. Wind Load as per BNBC
• BNBC Chapter 6, 2.4
• Table 6.2.20 –”Overall Pressure Coefficient CP
for Trussed Towers”
• Lohalia river, Patuakhali, 260km/h (Odd rolls)
• Surma river, Kanaighat, Sylhet, 195km/h
(Even rolls)

17. Modeling in SAP

18. Plan at top level
Plan at bottom level

19. Arrangement
of Portal

20. Initial sections to start: Truss Member
• Material grade: Fy=345MPa
• Top chord: Box section 600x450
» Thickness 16-30mm (assume 25mm initially)
• Bottom chord: Box section 600x450
» Thickness 16-30mm (assume 25mm initially)
• Diagonal: Box section 600x450
» Thickness 16-30mm (assume 25mm initially)

22. Initial sections to start: Portal
Portal: Double angle 150x150x10

23. Initial sections to start: Bracing
Bracings: I section, depth 300, width 250, thickness 10-12mm

24. Initial sections to start: Stringer
Dead Load Moment, MDL = wL2/8
Dead Load Shear, VDL = wL/2
w is calculated as the dead load of the tributary
Area and the self weight of the stringer (Assume
self weight as 140 plf)

28. M = MDL + MLL + MIL V = VDL + VLL + VIL
Sx = M/Fb
Fb = 0.66Fy
Identify W section based on Sx
Check: fv = V/Aw ≤ 0.3Fy
Aw is the area of the web

32. Initial sections to start: Floor Beam

43. Design

49. FATIGUE LOADS
1. Under service load conditions, majority of trucks do not exceed the legal weight limit. So it
would be unnecessary to use the full live load model. Instead it is accommodated by using a
single design truck with the variable axle spacing of 9m and a load factor of 0.75 as
prescribed in table.[A3.4.1.1].
2. The number of stress load cycles is based on traffic surveys. In lieu of survey data, guidelines
are provided in AASHTO [A3.6.1.4.2]. The average daily truck traffic (ADTT) in a single lane
may be estimated as
ADTTSL = p(ADTT)
Where p is the fraction of traffic assumed to be in one lane as defined in table4.3.

50. PEDESTRIAN LOADS
• The AASHTO pedestrian load is 3.6 x 10-3 MPa, which is applied to sidewalk that are integral
with a roadway bridge.
• If load is applied on bridge restricted to pedestrian or bicycle traffic , then a 4.1 x 10-3 MPa is
used.
• The railing for pedestrian or bicycle must be designed for a load of 0.73 N/mm both
transversely and vertically on each longitudinal element in the railing system.[A13.8 and A18.9].
• In addition as shown in the figure , the railing must be designed to sustain a single
concentrated load of 890 N applied to the top rail in any direction and at any location.

51. DECK & RAILING LOAD
• The deck must be designed for the load effect due to design truck or design tandem ,
whichever creates the most extreme effect.
• The deck overhang, located outside the facia girder and commonly referred to as the
cantilever is designed for the load effect of a uniform line load of 14.6 N/mm located 3m from
the face of the curb or railing as shown in the figure.
• The gravity load for the deign of deck system are outlined in AASHTO[A3.6.1.3.3].
• The vehicular gravity loads for decks may be found in AASHTO [A3.6.1.3].

54. Load
Combination
Limit State
DC
DD
DW
EH
EV
ES
LCE
BR
PL
LS
WA WS WL FR
TU
CR
SH
TG SE
Use one of these at a time
EQ IC CT CV
STRENGTH – I γp 1.75 1.00 - - 1.00 0.50/1.20 γTG γSE - - - -
STRENGTH - II γp 1.35 1.00 - - 1.00 0.50/1.20 γTG γSE - - - -
STRENGTH - III γp - 1.00 1.40 - 1.00 0.50/1.20 γTG γSE - - - -
STRENGTH – IV
EH, EV, ES, DW, DC
ONLY
γp
1.5
- 1.00 - - 1.00 0.50/1.20 - - - - - -
STRENGTH – V γp 1.35 1.00 0.40 0.40 1.00 0.50/1.20 γTG γSE - - - -
EXTREME EVENT – I γp γEQ 1.00 - - 1.00 - - - 1.00 - - -
EXTREME EVENT – II γp 0.50 1.00 - - 1.00 - - - - 1.00 1.00 1.00
SERVICE - I 1.00 1.00 1.00 0.30 0.30 1.00 1.00/1.20 γTG γSE - - - -
SERVICE – II 1.00 1.30 1.00 - - 1.00 1.00/1.20 - - - - - -
SERVICE - III 1.00 0.80 1.00 - - 1.00 1.00/1.20 γTG γSE - - - -
FATIGUE – LL, IM,
AND CE ONLY
- 0.75 - - - - - - - - - - -
AASHTO (2007) LOAD COMBINATION AND LOAD FACTORS

55. • Service I : DL +(LL+IM) +0.3W +W on live
• Service II: DL + 1.3 (LL+IM)