1. BASIS OF DESIGN OF
COMPOSITE STEEL
BRIDGES
Ramy Gabr, M.Sc. , P.E.

2. BASIS OF DESIGN OF
COMPOSITE STEEL
BRIDGES
Parsons, Doha
 The discussion
presented herein is
based on the bases of
design of the AASHTO
LRFD 2010 Section 6 –
Steel structures.

3. AGENDA
1. Introduction (Types, Pros and Cons)
2. Limit State Design
3. Section Properties
4. Service Limit State
5. Strength Limit State
6. Fatigue and Fracture Limit State
3

4. AGENDA
1. Introduction (Types, Pros and Cons)
2. Limit State Design
3. Section Properties
4. Service Limit State
5. Strength Limit State
6. Fatigue and Fracture Limit State
4

5. Introduction
 Types of Steel Bridges
1. Beam Bridges (15m – 100m)
a) Multi-girder
b) Ladder deck
2. Box Girder Bridges (45m – 180m)
3. Truss Bridges (40m – 500+m)
4. Arch Bridges (30m – 500m)
5. Cable-stayed Bridges (200m – 850+m)
6. Suspension Bridges (>850m)
5

6. Introduction
 Advantages:
• Pleasing aesthetics offering a
smooth cross section.
• Structural and nonstructural
components are typically
hidden.
• Better torsional resistance.
• More economical (steel
weight) due to the increased
bending strength offered by
their wide bottom flanges.
 Disadvantages:
• Box girders should be no less
than 1.5m deep to allow
access for maintenance and
inspection.
• Higher fabrication and
erection costs.
• Requires skilled workers
during fabrication.
• Risks associated with working
in enclosed spaces during
maintenance.
6
Advantages and Disadvantages of Bridge Types
Box-Type Bridges

7. Introduction
 Advantages:
• Simpler design
• Lighter sections
• Ease of fabrication and
erection
• Ease of maintenance
 Disadvantages:
• Accumulation of debris and
acids on top of the flanges
• Aesthetics is not preferred
• Coating is more difficult than
box girder types
7
Advantages and Disadvantages of Bridge Types
Beam-Type Bridges

8. AGENDA
1. Introduction (Types, Pros and Cons)
2. Limit State Design
3. Section Properties
4. Service Limit State
5. Strength Limit State
6. Fatigue and Fracture Limit State
8

9. Limit State Design
 Limit State: a condition of a structure beyond which it no
longer fulfills the relevant design criteria.
 Specified limit states to achieve the objectives of
constructability, safety and serviceability.
 Other design provisions address inspectability, economy and
aesthetics (Section 2.5). However, these issues are not part of
the limit-state design philosophy.
 Limit state in LRFD:
is the load or stress level
is the load factor (load combinations)
is the load modifier (ductility, redundancy, and importance)
are the nominal and factored resistance respectively
is resistance factor
9
rn RRQ 
Q



rn RR &

10. Limit State Design
 Similarly, limit state in ASD:
is the load or stress level
is the load modifier (ductility, redundancy, and importance)
are the nominal and factored resistance respectively
is safety factor
 Limit states in AASHTO LRFD – Section 6:
• Service Limit State
• Strength Limit State
• Fatigue and Fracture Limit State
• Extreme Event Limit State
10
r
n
R
R
Q 


Q


rn RR &

11. Limit State Design
 Limit states in AASHTO LRFD – Section 6:
• Service Limit State
• The service limit states ensure the durability and
serviceability of the bridge and its components under
typical everyday (i.e. service) loads.
• The AASHTO LRFD includes four service limit state load
combinations of which only two are applicable to steel
bridges.
• Strength Limit State
• Ensure strength and stability of the bridge and its
components under the statistically predicted maximum
loads (i.e. factored) during the life span of the bridge.
• The strength limit states are not based upon durability or
serviceability.
11

12. Limit State Design
 Limit states in AASHTO LRFD in Section 6:
• Fatigue and Fracture Limit State
• Represents a more severe consequence of failure than
the service limit states (brittle failure), but not
necessarily as severe as the strength limit states (many
passages trucks may cause a critically-sized fatigue crack
while only one heavy truck can lead to strength limit
state failure).
• The fatigue and fracture limit state is only applicable
where the detail under consideration experiences a net
applied tensile stress under FATIGUE load (S-N Curve).
• Using FATIGUE I (=1.5, infinite life) and FATIGUE II
(=0.75, finite life) depends on comparing the expected
ADTTSL against ADTTSL for 75-year equivalent to infinite
life specified in AASHTO.
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13. Limit State Design
 Limit states in AASHTO LRFD in Section 6:
• Extreme Event Limit State
• Represents less frequent events such as earthquakes and
vehicular collisions.
• These loads or events of such great magnitude that if
designed for the levels of reliability or failure rates of the
strength limit states would be economically prohibitive
or unfeasible.
13

14. AGENDA
1. Introduction (Types, Pros and Cons)
2. Limit State Design
3. Section Properties
4. Service Limit State
5. Strength Limit State
6. Fatigue and Fracture Limit State
14

15. Section Properties
 Bridge deck sections are divided into:
• Steel section (before composite action) (DC1 and CLL).
Applicable for bridge during erection and pouring of deck.
Used to compute stresses for DL= OW+Concrete Deck
• Composite section beff: Table 4.6.2.6.4-1
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STEEL SECTION
1
4
EFFECTIVE WIDTH
beff beff
a = 0.8w - 1.2ww

16. Section Properties
 Bridge deck sections are divided into :
• Composite section:
• Short term(n): used with transient loads (LL+I)
• Long term (3n) due to creep and shrinkage – used with
permanent loads (DC2 and DW) (6.10.1.1.1)
16
beff
b/3n
EFFECTIVE SECTION
Long Term
Composite (3n=24)
beff
b/n
EFFECTIVE SECTION
Short Term
Composite (n=8)

17. AGENDA
1. Introduction (Types, Pros and Cons)
2. Limit State Design
3. Section Properties
4. Service Limit State
5. Strength Limit State
6. Fatigue and Fracture Limit State
17

18. Service Limit State
 Service limit state in AASHTO LRFD (Sections 6.10.4 and 6.11.4)
includes:
a) Elastic deformations: control of live load deflections
b) Permanent deformations: prevent permanent deflections
due to expected severe traffic loadings that would harm
rideability.
• It includes satisfaction of flange stresses not exceed:
composite sections
noncomposite sections
• These checks are to be made under the SERVICE II
combination.
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800
span
LLall  
yfh FR95.0
yfh FR80.0

19. AGENDA
1. Introduction (Types, Pros and Cons)
2. Limit State Design
3. Section Properties
4. Service Limit State
5. Strength Limit State
6. Fatigue and Fracture Limit State
19

20. Strength Limit State
 Strength limit state in AASHTO LRFD includes:
a) Constructibility checks (Sections 6.10.3 and 6.11.3):
• Involves checking the steel cross section before
composite action development (only DC1 and CLL).
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21. Strength Limit State
21

22. Strength Limit State
22

23. Strength Limit State
23

24. Strength Limit State
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25. Strength Limit State
 Strength limit state in AASHTO LRFD includes:
a) Constructibility checks (Sections 6.10.3 and 6.11.3):
• Involves checking the steel cross section before
composite action development (only DC1 and CLL).
• Flexural checks (Top Flange):
o Vertical bending stress (fbu)
o Lateral bending stress (fl)
• Wind loads
• Horizontal component of web shear
• Deck overhang loads
• Amplification factor to account for second-order nonlinear effects (geometry)
o Yielding (Fy)
o Local buckling and Lateral torsional buckling (Fnc)
o Web bend-buckling resistance (Fcrw)
25
ychflbu FRff 
crwfbu Ff 
ncflbu Fff 
3
1

26. Strength Limit State
 Strength limit state in AASHTO LRFD includes:
• Flexural checks (Bottom Flange):
o Vertical bending stress (fbu)
o Yielding (Fy)
is function of St. Venant torsional shear (1.0 in
straight bridges, <1.0 in curved bridges)
• Shear checks (Web):
o Usually shear during construction is not governing
26
 yfhfbu FRf 


27. Strength Limit State
 Strength limit state in AASHTO LRFD includes:
b) Strength limit state checks (Sections 6.10.6 and 6.11.6):
• Flexural checks (positive moments)
o Compact section
Where Mn is based on My & Mp
o Noncompact section
, is based in yield and torsion
o Sections in horizontally curved steel girder bridges
shall be considered as noncompact sections
• Flexural checks (negative moments)
o depends on the presence of longitudinal stiffeners
and torsion, , depends on yield
27
nfxtlu MSfM 
3
1
ncfbu Ff  ntflbu Fff 
3
1
ncflbu Fff 
3
1
ntflbu Fff 
3
1
ncF
ntF
nF

28. Strength Limit State
 Strength limit state in AASHTO LRFD includes:
b) Strength limit state checks (Sections 6.10.6 and 6.11.6):
• Shear checks
o Unstiffened webs
is based on elastic shear yielding or shear
buckling resistance
o Stiffened webs
End panels: is based on shear yielding or shear
buckling resistance taking into account stiffener
spacing
Interior panels: is based on shear yielding or shear
buckling resistance with tension field action if
satisfying its requirements
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pnvu CVVV  
nV
nV
nV
Interior panel
End
panel
Ddo 3 Ddo 5.1
D

29. AGENDA
1. Introduction (Types, Pros and Cons)
2. Limit State Design
3. Section Properties
4. Service Limit State
5. Strength Limit State
6. Fatigue and Fracture Limit State
29

30. Fatigue and Fracture Limit State
 Fatigue and fracture limit state in AASHTO LRFD (Sections
6.10.5 and 6.11.5) includes:
a) Fatigue checks
• Load induced fatigue – compression and tensions flanges
and shear studs (and any component produce net tensile
stresses)
is the nominal fatigue resistance and it depends on the
detail category of each component (A, B, B’, C, C’, D, E, E’)
• Distortion induced fatigue (detailing)
o Distortion-induced fatigue occurs in the web near a
flange at a welded connection plate for a cross-frame
where a rigid load path has not been provided to
adequately transmit the force in the transverse
member from the web to the flange.
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nFf )()( 
nF)(

31. Fatigue and Fracture Limit State
 Fatigue and fracture limit state in AASHTO LRFD (Sections
6.10.5 and 6.11.5) includes:
a) Fatigue checks
• Distortion induced fatigue
o To control web buckling and elastic flexing of the web
(i.e. rigid load path), transverse connection to be at
top and bottom flanges and the provision of “Special
fatigue requirements for webs” shall be satisfy
• Special fatigue requirements for webs (Section 6.10.5.3)
o Interior panels of webs with transverse stiffeners,
shall satisfy shear strength limit state under FATIGUE
load combination (with twice the factored fatigue
load considered). This involves checking maximum
web shear-buckling instead of the stress ranges.
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32. Fatigue and Fracture Limit State
 Fatigue and fracture limit state in AASHTO LRFD (Sections
6.10.5 and 6.11.5) includes:
b) Fracture requirements (should be mentioned in the specs)
• All primary superstructure components and connections
sustaining tensile force effects due to STRENGTH I load
combination, require mandatory Charpy V-notch fracture
toughness.
• Fracture-critical members
to be tested in conformance
with AASHTO T 243M
(ASTM A 673M).
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33. Fatigue and Fracture Limit State
Thank You
Q & A
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