bridge ch2 prelimdesign

8/2/2019 Bridge Ch2 PrelimDesign

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Chapter 2: Preliminary Design

Dr. Adel Al-Assaf

Bridge Engineering

0


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LRFD Philosophy

Dr Adel Al-Assaf1

Qirepresents the demand actionforces, due to applied loads, atcertain section.

Rn represents capacity/resistingforces of the structural element at

certain section.


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Load Modifiers

Dr Adel Al-Assaf2

Strength Service Fatigue1.05 1.00 0.95 1.00 1.00

D Nonductilecomponents andconnections

conventional

designs and detailscomplying with

AASHTO

Enhanced Ductility,beyond AASHTO

- -

R nonredundantmembers

conventional levels

of redundancyexceptional levels

of redundancy- -

I important bridges typical bridges relatively lessimportant bridges.

- -


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Load Combinations

Dr Adel Al-Assaf3

Four main types of design limit states

1. Strength Limit State: Yielding of steel / Crushing in concrete

2. Extreme Limit State: Ductility limits

Yielding in steel / Crushing in concrete

3. Service Limit State: to check the following: Service stress limits Crack width Deflection

Stability: Soil-structure interaction, slope-stability

4. Fatigue Limit State: Check the failure of steel elements, components, and

connections under cyclic tensile load due to a single designtruck.


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Load Combinations - Strength

Dr Adel Al-Assaf4

STRENGTH IBasic load combination relating to the

normal vehicular use of the bridge without wind.

STRENGTH IILoad combination relating to the use of thebridge by Owner-specified special design vehicles,evaluation permit vehicles, or both without wind.

STRENGTH IIILoad combination relating to the bridgeexposed to wind velocity exceeding 90 km/hr.

STRENGTH IVLoad combination relating to very high

dead load to live load force effect ratios.

STRENGTH VLoad combination relating to normalvehicular use of the bridge with wind of 90 km/hr.velocity.


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Load Combinations - Extreme

Dr Adel Al-Assaf5

EXTREME EVENT I: Load combination includingearthquake.

EXTREME EVENT II: Load combination relating toice load, collision by vessels and vehicles, andcertain hydraulic events with a reduced liveload other than that which is part of the

vehicular collision load, CT.


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Load Combinations - Service

Dr Adel Al-Assaf6

SERVICE ILoad combination relating to the normal operational use of thebridge with a 90 km/hr. wind and all loads taken at their nominal values. Also

related to deflection control in buried metal structures, tunnel liner plate, andthermoplastic pipe, to control crack width in reinforced concrete structures,and for transverse analysis relating to tension in concrete segmental girders.This load combination should also be used for the investigation of slopestability. (Used to check compression in prestressed concrete componentsand tension in prestressed bent caps are investigated using and to checkdeflection).

SERVICE IILoad combination intended to control yielding of steel structuresand slip of slip-critical connections due to vehicular live load (Applicable onlyto Steel Structures).

SERVICE IIILoad combination for longitudinal analysis relating to tension inprestressed concrete superstructures with the objective of crack control andto principal tension in the webs of segmental concrete girders (should be usedto investigate tensile stresses in prestressed concrete components).

SERVICE IVLoad combination relating only to tension in prestressed concretecolumns with the objective of crack control.


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Load Combinations - Fatigue

Dr Adel Al-Assaf7

FATIGUEFatigue and fracture loadcombination relating to repetitive gravitationalvehicular live load and dynamic responsesunder a single design truck having the axlespacing specified in Article 3.6.1.4.1.


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Loads (Definitions)

Dr Adel Al-Assaf8

Design LaneA notional traffic lane positioned transversely on the roadway.

Dynamic Load AllowanceAn increase in the applied static force effects toaccount for the dynamic interaction between

the bridge and moving vehicles.

Influence SurfaceA continuous or discretized function over a bridge deckwhose value at a point, multiplied by a load

acting normal to the deck at that point, yields the force effect being sought.

LaneThe area of deck receiving one vehicle or one uniform load line.

Lever RuleThe statical summation of moments about one point to calculatethe reaction at a second point.

LoadThe effect of acceleration, including that due to gravity, imposeddeformation, or volumetric change.

Roadway WidthClear space between barriers and/or curbs.

TandemTwo closely spaced axles, usually connected to the same under-carriage, by which the equalization of load between the axles is enhanced.

WheelSingle or dual tire at one end of an axle.

Wheel LineA transverse or longitudinal grouping of wheels.


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Loads Categories

Dr Adel Al-Assaf9

PERMANENT LOADS Dead Loads: DC, DW, and EV

Earth Loads: EH, ES, and DD LIVE LOADS

Gravity Loads: LL and PL Dynamic Load Allowance: IM Centrifugal Forces: CE Braking Force: BR

Vehicular Collision Force: CT WATER LOADS: WA

WIND LOAD: WL AND WS

ICE LOADS: IC

EARTHQUAKE EFFECTS: EQ

EARTH PRESSURE: EH, ES, LS, AND DD

FORCE EFFECTS DUE TO SUPERIMPOSED DEFORMATIONS: TU, TG, SH, CR, SE

FRICTION FORCES: FR

VESSEL COLLISION: CV


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Bridge Loadings

Dr Adel Al-Assaf10

Besides the commonly known basic load cases, such as, dead load, earthpressure, wind load, Earthquake loads, ..etc), the following should be considered:

BR = vehicular braking force

CE = vehicular centrifugal force

CT= vehicular collision force

CV= vessel collision force

FR = friction

IM = vehicular dynamic load allowance

LL = vehicular live load LS = live load surcharge

PL = pedestrian live load

WA = water load and stream pressure

WL = wind on live load

Note! AASHTO-BDS-LRFD Loads and Load Combinations are applicable for bridges

with span not more than 60 meters. Special loading provisions shall beconsidered otherwise.

In case of flexible superstructures with relatively long fundamental naturalperiod, special analysis schemes shall be considered for vehicular and windloads.


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AASHTO Load Combinations & Load Factors

Dr Adel Al-Assaf11


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AASHTO Load Factors

Dr Adel Al-Assaf12


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Dead Loads

Dr Adel Al-Assaf13

Dead load shall include the weight of allcomponents of the structure, appurtenancesand utilities attached thereto, earth cover,wearing surface, future overlays, and plannedwidenings (Including DC, DW and EV)

In the absence of more precise information, thedensities, specified in Table 3.5.1-1 (Densities),

may be used for dead loads.

Use g= 9.81 m/s2, to convert mass density toweight.


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Dead Loads - Densities

Dr Adel Al-Assaf14


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ExampleCompute DC & DW

Dr Adel Al-Assaf15

Find the total dead load for both the exterior and interior girders.

Assume the following:

AASHTO Girder V,A = 250mm, Taxes Constant Slope Barrier Profile Do not include the diaphragm weight

Assume 50 mm future wearing layer.

Assume concrete strength = 55 MPa for girders and 35 MPa for deck slab.


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Dr Adel Al-Assaf16


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Solution

Dr Adel Al-Assaf17


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Live Load

Dr Adel Al-Assaf18

Lane 3000 mm to 3600 mm.

Number of design lanes = INT(w/3600); such that w is theclear roadway width in mm between curbs and/or barriers.

Each lane shall be loaded by HL-93 Load.

A multi-presence factor (m) shall/may be applied asnecessary.

Dynamic Load Allowance (IM) shall be applied to

truck/tandem loads only.

Assigned live loads shall be distributed to girders as perArticles 4.6.2.2 and 4.6.2.3 or by means of 3-D influence linemethods.


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HL-93 Load

Dr Adel Al-Assaf19

HL-93 Load consists of Design truck or design tandem, and

Design lane load.

The loads shall be assumed to occupy 3000 mm transversely within a designlane.

Design Lane Load shall be 9.3 kN/m along the traffic, and shall be distributedover 3000mm width in the transverse direction.

Design Truck (3.6.1.2.2)

Design Tandem Load (3.6.1.2.3)
http://www.enggpedia.com/images/Bridge/tandem%20+%20lane.pnghttp://www.enggpedia.com/images/Bridge/Design%20Lane.png


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Live Load

Dr Adel Al-Assaf20

Multiple Presence of Live Load (AASHTO 3.6.1.1.2)

Consider each possible combination of numberof loaded lanes to maximize the effect

This factorshall be applied to one lane loaded,and may be used otherwise.

These factors are included in the distributionfactors (Articles 4.6.2.2 and 4.6.2.3), exceptwhen using Lever Rule is indicated.

For fatigue limit state, these factors shall betaken out; that is, where the single-laneapproximate distribution factors in Articles4.6.2.2 and 4.6.2.3 are used, other than the leverrule and statical method, the force effects shallbe divided by 1.20.


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Live Load

Dr Adel Al-Assaf21

Dynamic Allowance Factor (AASHTO 3.6.2)

Multiply the load by (1+IM/100)

Shall be Applied to: Truck and Tandem Loads Short to Medium long spans

Need Not to be applied to: CE, BR and LS loads Foundations of structures completely buried in soil

Can be recued in case applied on fill above deckslab,IM = 33(1.0 - 4.1x10-4 DE) 0.0;

Where, DE= the minimum depth of earth cover above thestructure (mm)


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Application of Live Load

Dr Adel Al-Assaf22

Unless otherwise specified, the extreme force effect

shall be taken as the larger of the following: The effect of the design tandem combined with the effect

of the design lane load, or

The effect of one design truck with the variable axlespacing specified in Article 3.6.1.2.2, combined with the

effect of the design lane load, and

For both negative moment between points of contraflexure under a uniform load on all spans, and reactionat interior piers only, 90 percent of the effect of twodesign trucks spaced a minimum of 15 000 mm between

the lead axle of one truck and the rear axle of the othertruck, combined with 90 percent of the effect of thedesign lane load. The distance between the 145 000-Naxles of each truck shall be taken as 4300 mm.


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Deck Overhang Load (3.6.1.3.4 )

Dr Adel Al-Assaf23

For the design of deck overhangs with a cantilever,not exceeding 1800 mm from the centerline of the

exterior girder to the face of a structurally continuousconcrete railing, the outside row of wheel loads maybe replaced with a uniformly distributed line load of14.6 N/mm intensity, located 300 mm from the face ofthe railing.

Horizontal loads on the overhang resulting from vehiclecollision with barriers shall be in accordance with theprovisions of Section 13.


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Live Load

Distribution

Procedure

Dr Adel Al-Assaf24

Use thisprocedure forSuperstructure

design


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Lever Rule

Dr Adel Al-Assaf25

Example 1

Example 2


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Dr Adel Al-Assaf26


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Substructure Design

Dr Adel Al-Assaf27


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Fatigue Load (3.6.1.4)

Dr Adel Al-Assaf28

Consider one HS-20 Truck with constant rear axel spacing = 9.00meters.

Dynamic Impact factor = 1.15

Use m = 1.00.

Use distribution factors in Article 4.6.2 or refined method in Article4.6.3

A single design truck shall be positioned transversely andlongitudinally to maximize stress range at the detail underconsideration, regardless of the position of traffic or design laneson the deck.

Estimate the number of stress cycles over the bridge life, that is,estimate theADTTSL. This can be used in estimating the allowablefatigue stress (for metals) using the S-N curve. In this case use thetotal number of stress cycles during the design life of the bridge.


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FatigueEstimate Capacity

Dr Adel Al-Assaf29

Use the S-N Curve for steel and other metal

structures Q: How to estimate the number of cycles?

A: Refer to ASHTO-BDS-LRFD, and Discuss ADTT

For Concrete Structures use the equation ff=145 - 0.33 fmin + 55(r/h)


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S-N Curve

Dr Adel Al-Assaf30

bridge ch2 prelimdesign

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