Presented to AASHTO T-8, May 16, 2011

AASHTO SPECIFICATIONS FOR SPAN LOCK DESIGNBy James M. Phillips III, PEE.C. Driver

PRESENTATION OUTLINE

Span Lock Synopsis Standard and LRFD Specification Design

Requirements Example Applications Evaluation Recommendations

SPAN LOCK SYNOPSIS

AASHTO specifications require span locks for bascule bridges (6.8.1.5.1) and suggest them for vertical lift bridges (6.8.3.7.1)

Double-Leaf Bascule Bridges require “center locks” Span Locks on double-leaf bascule bridges are one of

the most maintenance prone mechanical components of any movable bridge.

This presentation focuses on the design of lock bar type, center locks and inconsistencies between the requirements and results obtained using the LRFD specifications versus the Standard Specifications

DOUBLE-LEAF BASCULE SPAN LOCKS

Typical Florida Double-Leaf Bascule

Span LockC Between Bascule LeavesL

DOUBLE-LEAF BASCULE SPAN LOCKS

TYPICAL LOCK BAR TYPE SPAN LOCK

LOCK BAR TYPE SPAN LOCK

CALCULATING SPAN LOCK LOADS

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CODE REQUIREMENTSSTANDARD / LRFD SPECIFICATIONS

Bridge Specifications

Standard Specifications (1988) LRFD Specifications (2007)

Live Load HS-20, Alternate Lane Loading

Live Load Notional LoadHL-93 Truck + Lane LoadSpecial Fatigue Truck Load

3.8.2 Impact Formula

3.6.2 Dynamic Load Allowance (Impact)

IM = 76% Deck JointsIM = 15% for FatigueIM = 33% others

3.6.1.4 Fatigue load shall be one design truck with 30 foot axle spacing.

CODE REQUIREMENTSSTANDARD / LRFD SPECIFICATIONS

Machinery ProvisionsStandard Specifications (1988) LRFD Specifications (2007)

2.1.11 The impact from live load shall be as specified in the current AASHTO Standard Specifications for Highway Bridges

6.8.1.5.1 Locking Devices

Center locks shall transfer live load and impact from one leaf to the other.

2.1.11 The end floorbeams of the moving span shall be proportioned for full live load plus twice the normal impact.

2.4.1.2.4 End Floorbeams

The end floorbeams of the moving span shall be proportioned for full factored live load plus twice the normal dynamic load allowance.

2.1.11 Allowance has been made for impact in trunnions, wire ropes, wire rope attachments, and machinery parts in the basic allowable unit stresses specified for such parts.

1.3.2 Limit States

For the service limit state, the load factors, Υi, will be taken as 1.0 unless specified otherwise and the resistance factor, Φ, will usually be taken equal to 1.0.Resistance should be based on allowable stresses

CODE REQUIREMENTSSTANDARD / LRFD SPECIFICATIONS

Machinery ProvisionsStandard Specifications (1988) LRFD Specifications (2007)

6.4.1.1 Unless otherwise stated, machinery design shall bebased on the service and fatigue limit states using the loads and resistances specified herein.

2.5.11 All of the unit stresses specified in this Article provide appropriate safety factors against static failure and against failure by fatigue with and without reversal of stresses. In the determination of the safety factor against fatigue failure, provision was made for stress-raisers which would produce local stress concentrations of 140 percent of the computed stress.

Machinery ProvisionsStandard Specifications (1988) LRFD Specifications (2007)2.5.11 Unit Stresses in Machinery Parts

In the absence of keyways or other stress-raisers in a shaft, the (allowable) unit stresses for torsion and flexure in a shaft may be increased 20 percent.

2.5.11 Unit Stresses in Machinery Parts (table)

Allowable unit stresses:Forged carbon steel:Flexure, Fb = 0.4 Fy or 0.2 Ft

Shear, Fv = Fb/2Forged Alloy SteelFlexure, Fb = Fy/3 or 0.2 Ft

Shear, Shear, Fv = Fb/2

Resistance for Machinery PartsForged carbon steelFlexure, Fy/3Shear, Fy/6

Analysis of Typical Florida Span Lock

Two lane bridge Double-Leaf Bascule 28’ – 0” clear roadway 122.5 foot span between trunnions 6” x 4” Forged Steel Lock Bar

ASTM A668, Class D Fy = 37.5 ksi Ft = 75 ksi

Section Modulus = 24 in3

Plastic Section Modulus = 36 in3

Analysis of Typical Florida Span LockStandard Specifications – Lock Bar as Structure

Shear = 60 kips HS-20 Truck Positioned for maximum shear W 2 x Impact per 2.1.11

Max. Moment in Bar = 540 k” fb = 22.5 ksi Fb = 0.55 Fy = 20.6 ksi

Table 10.32.1A Ratio of = 0.91

Service Check

Analysis of Typical Florida Span LockStandard Specifications – Lock Bar as Machinery

Shear = 36 kips HS-20 Truck Positioned for maximum shear w/o Impact per 2.1.11

Max. Moment in Bar = 324 k” fb = 13.5 ksi Fb = 15 ksi x 120% = 18 ksi

15 ksi from Table 2.5.11 20% increase due to lack of stress raisers

Ratio of = 1.33

Service Check

Analysis of Typical Florida Span LockStandard Specifications – Lock Bar as Structure

Shear = 60 kips (range) HS-20 Truck Positioned for max shear No Impact

Moment (range) in Bar = 540 k” fb = 22.5 ksi Fb = 24 ksi (allowable stress range) Ratio of = 1.07

Fatigue Check

Analysis of Typical Florida Span LockStandard Specifications – Lock Bar as Machinery

Shear = 36 kips (range) Single HS-20 Truck Positioned in Lane No Impact

Moment (range) in Bar = 324 k” fb = 13.5 ksi Fb = 15 ksi x 120% = 18 ksi

15 ksi from Table 2.5.11 20% increase due to lack of stress raisers

Ratio of = 1.33

Fatigue Check

Analysis of Typical Florida Span LockLRFD Specifications – Lock Bar as Structure

Shear = 68 kips HL93 (Truck + Lane) Positioned for maximum shear w/o 2 x Impact (33% impact)

Strength I Limit State γLL = 1.75

γQ = 1071 k” φRn = Fy x Z = 1350 k”

Φ = 1.0

Strength Check

Analysis of Typical Florida Span LockLRFD Specifications – Lock Bar as Structure

Shear = 68 kips HL93 (Truck + Lane) Positioned for maximum shear w/ 2 x Impact (33% Impact)

Service II Limit State γLL = 1.30

γQ = 796 k” φRn = 1.0 x Fy x S = 900 k”

Φ = 1.0

Service Check

Analysis of Typical Florida Span LockLRFD Specifications – Lock Bar as Machinery

Shear = 68 kips HL93 (Truck + Lane) Positioned for maximum shear w/ 2 x Impact

Service II Limit State γLL = 1.30

γQ = 796 k” φRn = Fy / Ns x S = 300 k”

Φ = 1.0 Ns = 3 Fb = Fy / ns = 12.5 ksi

Service Check

Analysis of Typical Florida Span LockLRFD Specifications – Lock Bar as Structure

Shear = 35 kips HL93 Fatigue (Single Truck, 30 foot

axle spacing) Positioned for maximum shear w/o 2 x Impact (15% fatigue impact)

Fatigue Limit State γLL = 1.50

γQ = 473 k” φRn = FTH x S = 900 k”

Φ = 1.0

FTH = 24 ksi (constant amplitude

fatigue threshold, Category A)

Fatigue Check

Analysis of Typical Florida Span LockLRFD Specifications – Lock Bar as Machinery

Shear = 35 kips HL93 Fatigue (Single Truck, 30 foot

axle spacing) Positioned for maximum shear w/o 2 x Impact (15% fatigue impact)

Fatigue Limit State γLL = 1.50

γQ = 473 k” φRn = δe x S = 768 k”

Φ = 1.0 δe = 32 ksi (endurance limit)

Fatigue Check

Analysis of Typical Florida Span Lock Summary of Code Checks

Analysis CaseAllowable / Actual or Resistance / Force Effect

Standard Spec LRFDStrength – Lock Bar as Structural Element NA 1.26

Service – Lock Bar as Structural Element

0.91 1.13

Service – Lock Bar as Machinery 1.33 0.38

Fatigue – Lock Bar as Structural Element 1.07 1.90

Fatigue – Lock Bar as Machinery 1.33* 1.62

Analysis of Typical Florida Span LockLRFD Specifications – Lock Bar as Machinery

Shear = 41 kips HL93 (Truck + Lane) Positioned for maximum shear No Impact

Service II Limit State γLL = 1.0

γQ = 396 k” φRn = Fy / Ns x S = 300 k”

Φ = 1.0 Ns = 3 Fb = Fy / ns = 12.5 ksi

Modified Service CheckγLL = 1.0No Impact

SR 31, Wilson Pigott Bridge

Real World Test Case Typical Florida Double-Leaf Bascule Heavy truck traffic Bridge instrumented and analyzed for effects of span

lock wear on main girder loads Lock bar is 6” OD x 4” ID tube Fy = 33 ksi, Ft = 45 ksi Standard Spec Evaluation

Bending stress Fb / fy = 0.73 Fatigue stress Fb / fe = 1.2

SR 31, Wilson Pigott Bridge

Lock Bar as Structure

Strength 1

Service II

Fatigue

Lock Bar as Machinery

Service IIService II Modified

Fatigue

LRFD Evaluation

Code EvaluationLRFD Progression from Standard Spec No clear guidance on use of machinery design requirements

for elements subject to vehicular live load Implication that locks are machinery Requirement to design center locks (and trunnions) for live

load impact Removal of notes that machinery unit stresses include

allowance for impact and stress raisers Removal of note to allow 20% increase in unit stress where

no stress raisers exist. Use of the same basic allowable stress as a method of

calculating resistance Increased vehicular loading (HL93)

Code Evaluation – Issues

Synchronizing LRFD specification for span locks is complicated

Lock bars may be better designed as a structure, but some span lock types have more complex elements best designed as machinery

Any changes to basic allowable stress (or F.S.) impacts other machinery elements

Some provision for impact seems appropriate for span locks, at least a commentary

Potential LRFD RevisionsShort Term Fix

Add to Commentary in 6.8.1.5.1, Locking Devices:“Lock bars which are of uniform cross section and free of stress raisers may be designed as structural elements. Such lock bars shall be designed for Strength I, Service II and Fatigue limit states per the requirements of the AASHTO LRFD Bridge Design Specifications. Twice normal impact shall be applied in the design of center locks designed as structural elements.”

Potential LRFD RevisionsLong Term Solution

Revisit the LRFD basic factors of safety and the allowable static stresses presented in Table 6.6.1-1 to determine if they already account for impact, stress raisers, and fatigue to some degree and clarify or adjust the values

Determine appropriate load, resistance and impact factors for design of machinery subject to vehicular live load

Determine if applying lane loading to span locks and other machinery is appropriate

Clarify if span locks and/or lock bars are to be designed as machinery and/or structural elements

AASHTO SPECIFICATIONS FOR SPAN LOCK DESIGN

Questions or Comments