Post on 23-Dec-2020
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