Transcript Document

SEISMIC & WIND ANALYSIS OF
BRIDGES
 Based on Recommended LRFD guidelines for
Seismic design on Highway bridges (May 2006 ed.)
WIND ANALYSIS OF BRIDGES
 Pressure specified shall be assumed to be caused by a base design wind velocity
VB = 160 Km/hr.
 For bridges or parts of bridges more than 10,000 mm above low ground or water
level, design wind velocity VDZ should be adjusted according to:
Where VDZ = Design wind velocity at elevation Z (Km/hr)
V10 = Wind velocity at 10 000 mm above low ground or design water level (Km/hr)
VB = Base wind velocity of 160 Km/hr at 10 000 mm height
Z = Height of structure at which wind loads are being calculated as measured from
low ground or water level > 10 000 mm
V0 = Friction velocity a meteorological wind characteristic taken as specified in Table
below
Z0 = Friction length of upstream fetch a meteorological wind characteristic taken as
specified in Table below.
V10 may be established from basic wind speed charts available for various
recurrence intervals, site specific wind surveys or in absence of better criteria, the
assumption that V10 = VB = 160 Km/hr
WIND ANALYSIS OF BRIDGES

WIND PRESSURE ON STRUCTURES

The total wind loading shall not be taken
less than 4.4 N/mm in the plane of
windward chord and 2.2 N/mm in the
plane of leeward chord of truss or arch
components and not less than 4..4 N/mm
on beam and girder spans.
WIND ANALYSIS OF BRIDGES
 LOADS FROM SUPERSTRUCTURES
 Where wind is not taken as normal to the structure, the base wind pressures PB
for various angles of wind direction may be taken as specified in table below and
shall be applied to the single place of exposed area. The skew angle shall be
taken as measured from perpendicular to the longitudinal axis. The wind direction
for design shall be that which produces the extreme force effect on the component
under investigation.
 The transverse and longitudinal pressures shall be applied simultaneously.
WIND ANALYSIS OF BRIDGES
 FORCES APPLIED DIRECTLY TO THE
SUBSTRUCTURE
The transverse and longitudinal forces to be applied directly to the
substructure shall be calculated from an assumed base wind pressure of
0.0019 MPa. For wind direction taken skewed to the substructure, this force
shall be resolved into components perpendicular to the end and front
elevations of the substructure. The component perpendicular to the end
elevation shall act on exposed substructure area as seen in end elevation,
and the component perpendicular to the front elevation shall act on the
exposed areas and shall be applied simultaneously with the wind loads from
superstructure.
WIND PRESSURES ON VEHICLES
When vehicles are present the design pressure should be applied
on both structure and vehicles. Wind pressure on vehicles shall be
represented by an interruptible moving force of 1.46 N/mm acting
normal to and 1800 mm above the roadway and shall be
transmitted to the structure. When wind on vehicles is not taken as
normal to the structure, the components of normal and parallel
force applied to the live load may be taken as specified in the table
below
WIND ANALYSIS OF BRIDGES
Example:
WIND ANALYSIS
SEISMIC ANALYSIS OF BRIDGES
IMPORTANCE CATEGORIES
1. Critical bridges (open to all vehicles immediately after EQ. 2500 year return period)
2. Essential bridges (open to emergency / defense / security vehicles immediately after
EQ. 475 year return period event)
3. Other bridges
SEISMIC PERFORMANCE ZONE
SEISMIC ANALYSIS OF BRIDGES
SOIL PROFILE TYPE
SOIL PROFILE I: Rock either shale or crystalline in nature.
SOIL PROFILE II: Stiff cohesive or deep cohesion less soil.
SOIL PROFILE III: Soft to medium stiff clay and sand.
SOIL PROFILE IV: Soft clay or silt.
ELASTIC SEISMIC RESPONSE COEFFICIENT
SEISMIC ANALYSIS OF BRIDGES
•For bridges on Soil profile III & IV & in areas where A is not less than 0.3, Csm
need not exceed 2.0 A.
•For soil profiles III & IV and for modes other than fundamental mode that have
period less than 0.3 s, Csm shall be taken as
If period of vibration for any mode exceeds 4.0 s the value of Csm for that mode
shall be taken as
SEISMIC ANALYSIS OF BRIDGES
 As per AASHTO LRFD Bridges in seismic
zone 1 need not be analyzed for seismic
loads regardless of their importance and
geometry. However the minimum
requirements in sections 4.7.4.4 & 3.10.9
shall be applied.
 Seismic analysis for is not required for
single span bridges, regardless of the
seismic zone.
SEISMIC ANALYSIS OF BRIDGES
Seismic analysis required / not required
• * = No seismic analysis is required
• UL = Uniform load elastic method
• SM = Single mode elastic method
• MM = Multimode elastic method
• TH = Time history method
SEISMIC ANALYSIS OF BRIDGES
Regular & Irregular bridges
Bridges satisfying the above table requirements may be regarded as Regular
bridges otherwise Irregular bridges. Curved bridges comprise of multi simplespan shall be considered as irregular bridge if subtended angle in plan is greater
than 20o. Such bridges shall be analyzed by either multimode elastic method or
time history method.
SEISMIC ANALYSIS OF BRIDGES
A curved continuous girder bridge may be analyzed as if it
were straight, provided all of the following requirements are
meet.
1. The bridge is regular as defined in table above except
that for a two span bridge the maximum span length ratio
from span to span must not exceed 2.
2. The subtended angle in plan is not greater than 90 deg.
3. The span lengths of the equivalent straight bridge are
equal to the arc lengths of the curved bridge.
If the requirements are not satisfied then the curved
continuous girder-bridge must be analyzed using the actual
curved geometry.
SEISMIC ANALYSIS OF BRIDGES
Where
Po = uniform load arbitrarily set = 1 (N/mm)
Vs(x) = Deformation corresponding to Po (mm)
W(x) nominal unfactored dead load of the bridge superstructure and tributary
substructure (N/mm)
Then calculate the time period of the bridge structure
g = Acceleration due to gravity (m/sec2)