Slip-Critical Connection Design for Bridge Structures
High-strength bolted connections in steel bridges employ two distinct design philosophies depending on loading conditions and serviceability requirements. Bearing-type connections permit controlled slip between connected elements under service loads, transferring forces through bolt shear and bearing against hole walls after initial movement occurs. Slip-critical connections—required for fatigue-sensitive details, oversized or slotted holes, and connections where any slip movement proves unacceptable—depend entirely on friction forces generated by bolt clamping pressure preventing relative movement between faying surfaces throughout the structure's design life. The nominal slip resistance calculation for slip-critical connections incorporates a slip coefficient (Ks) representing the friction characteristics between clamped surfaces, with coefficient values determined empirically through standardized testing protocols measuring the load at which initial slip occurs under specified bolt tension.
The AASHTO LRFD Bridge Design Specifications, mandated by the Federal Highway Administration for all federally-funded bridge projects, establish design criteria including slip coefficient values for various faying surface conditions. The 8th Edition, published Fall 2017, revised Article 6.13.2.8 and Table 6.13.2.8-3 governing slip-critical connection design based on extensive industry research demonstrating that hot-dip galvanized faying surfaces require no surface preparation beyond standard galvanizing while achieving predictable, reliable friction performance. These revisions eliminate unnecessary wire brushing requirements, adjust slip coefficients reflecting research data, and expand coating system options including metallized coatings and zinc-rich paints over various substrates.
Slip-Critical Connection Fundamentals
Friction Force Mechanism
Basic Physics:
Slip resistance derives from friction between clamped surfaces:
Friction Force = μ × Normal Force
Where:
- μ = Coefficient of friction (slip coefficient, Ks)
- Normal Force = Bolt clamping force
High-Strength Bolting:
ASTM A325 or A490 bolts tightened to specified pretension:
- A325: 39-51 kips tension (depending on diameter)
- A490: 49-64 kips tension
- Creates substantial normal force across faying surfaces
Multiple Bolt Connection:
Total slip resistance = (Slip coefficient) × (Bolt tension) × (Number of friction interfaces) × (Number of bolts)
When Slip-Critical Design Required
AASHTO LRFD Mandates Slip-Critical for:
Fatigue Considerations:
- Connections subject to stress range reversals
- Details sensitive to fretting and wear
- Locations where slip could initiate fatigue cracks
Oversized or Slotted Holes:
- Standard holes: Bolt diameter + 1/16"
- Oversized holes: Bolt diameter + 5/16" to 1/4"
- Short or long slots accommodating fit-up tolerances
Serviceability Requirements:
- Connections where slip causes unacceptable deformations
- Architecturally exposed connections
- Locations affecting structural alignment
Previous AASHTO Requirements (7th Edition)
Class C Surface Condition (Hot-Dip Galvanized)
Old Definition: "Hot-dip galvanized surfaces roughened by means of wire brushing after galvanizing"
Slip Coefficient: Ks = 0.33
Practical Requirements:
Galvanizers or fabricators required to:
- Galvanize faying surfaces
- Wire brush galvanized coating after galvanizing
- Create roughened texture increasing friction
- Document wire brushing in quality records
Operational Challenges:
Labor Intensive:
- Manual wire brushing of every faying surface
- Time-consuming for large connection plates
- Difficult to achieve consistent roughness
Quality Control Issues:
- Subjective determination of "adequate" roughening
- No standardized brushing protocol
- Inspection verification challenges
Cost Impact:
- Substantial labor hours per connection
- Equipment and consumables (wire brushes)
- Quality documentation overhead
Limited Coating Options
Class A: Clean Mill Scale Ks = 0.33
Class B: Blast-Cleaned Ks = 0.50
Class C: Hot-Dip Galvanized (Wire Brushed) Ks = 0.33
Gap:
- No provision for metallized coatings (thermal spray)
- No option for zinc-rich paint over galvanizing
- Limited corrosion protection choices for slip-critical applications
Research Driving Specification Changes
Industry Studies on Galvanized Faying Surfaces
Multiple research investigations conducted by:
- Research Council on Structural Connections (RCSC)
- American Institute of Steel Construction (AISC)
- State Departments of Transportation
- University research programs
Key Findings:
Wire Brushing Provides No Benefit:
Controlled testing comparing:
- As-galvanized surfaces (no treatment)
- Wire-brushed galvanized surfaces
Result: No statistically significant difference in slip coefficient between treated and untreated galvanized surfaces
Rationale:
Hot-dip galvanized coatings exhibit inherent surface characteristics:
- Zinc intermetallic layers provide micro-rough texture
- Surface topography from zinc solidification
- Adequate friction without additional treatment
Wire brushing:
- Removes minimal zinc (0.1-0.3 mils typical)
- Does not fundamentally alter surface texture
- Adds labor without performance improvement
Slip Coefficient Data Analysis:
Historical test data compilation showed:
- Mean slip coefficient: ~0.30
- Previous 0.33 value: Conservative estimate from limited early testing
- Updated 0.30 value: Median of extensive dataset
Conservative Basis:
0.30 represents 50th percentile (median) ensuring:
- Half of all tests exceed this value
- Appropriate factor of safety in design
- Consistent with RCSC Specification methodology
Metallized Coating Research
Thermal Spray Coatings:
Arc-spray zinc or zinc-aluminum coatings:
- Alternative corrosion protection to hot-dip galvanizing
- Applied by electric arc melting zinc wire
- Coating thickness: Typically 8-16 mils
Testing Results:
Unsealed (pure zinc or 85/15 zinc-aluminum) thermal spray ≤16 mils thick:
- Slip coefficient: 0.50 (equivalent to Class B blast-cleaned)
- Rough as-sprayed texture provides excellent friction
- No sealer required for slip-critical applications
Sealed Coatings:
Thermal spray with sealers (acrylic, epoxy):
- Smooth sealed surface reduces friction
- Not suitable for slip-critical connections without testing
AASHTO LRFD 8th Edition Revised Requirements
Updated Class Definitions
Class A: Clean Mill Scale and Class A Coatings
Definition:
- Unpainted clean mill scale
- Blast-cleaned surfaces with Class A coatings
Slip Coefficient: Ks = 0.30 (reduced from 0.33)
Rationale: Aligns with RCSC Specification reflecting median of historical data
Class B: Blast-Cleaned and Metallized
Definition:
- Unpainted blast-cleaned surfaces (SSPC-SP 6 Commercial Blast or better)
- Blast-cleaned surfaces with Class B coatings
- NEW: Unsealed thermal-sprayed coatings (pure zinc or 85/15 zinc-aluminum) ≤16 mils thick
Slip Coefficient: Ks = 0.50
Impact: Metallized coatings now explicitly permitted for slip-critical connections without additional testing
Class C: Hot-Dip Galvanized (As-Galvanized)
Definition: "Hot-dip galvanized surfaces"
Critical Change: Wire brushing requirement eliminated
Slip Coefficient: Ks = 0.30 (reduced from 0.33)
Significance:
As-galvanized surfaces acceptable without surface treatment:
- Eliminates labor-intensive wire brushing
- Reduces cost and schedule
- Reflects research demonstrating wire brushing provides no benefit
Class D: Zinc-Rich Paint Over Blast-Cleaned (NEW)
Definition: Blast-cleaned surfaces with Class D coatings (organic zinc-rich paints)
Slip Coefficient: Ks = 0.45
Purpose:
Enables zinc-rich paint systems over any blast-cleaned surface including:
- Bare steel blast-cleaned then painted
- Hot-dip galvanized surfaces blast-cleaned then painted (duplex systems)
Application:
Duplex systems combining galvanizing + paint for maximum corrosion protection now explicitly accommodated in slip-critical connections.
Summary Table: Surface Conditions and Coefficients
Design Impact Analysis
Slip Resistance Calculation
Formula:
Rn = Ks × Pt × Ns × Nb
Where:
- Rn = Nominal slip resistance
- Ks = Slip coefficient
- Pt = Minimum bolt tension (per AASHTO Table 6.13.2.8-1)
- Ns = Number of slip planes (typically 1 or 2)
- Nb = Number of bolts
Design Slip Resistance:
Rr = φ × Rn
Where φ = 1.00 (resistance factor for slip)
Impact of Reduced Coefficient (0.33 to 0.30)
Change Magnitude:
0.30 / 0.33 = 0.91 (9% reduction in slip resistance)
Example Calculation:
Connection Requirements:
- Required slip resistance: 100 kips
- Bolt: 7/8" diameter A325
- Minimum bolt tension (Pt): 39 kips
- Faying surface: Hot-dip galvanized
- Single shear connection (Ns = 1)
Old Specification (Ks = 0.33):
Rn per bolt = 0.33 × 39 × 1 = 12.87 kips
Number of bolts required = 100 / 12.87 = 7.77 → 8 bolts
New Specification (Ks = 0.30):
Rn per bolt = 0.30 × 39 × 1 = 11.70 kips
Number of bolts required = 100 / 11.70 = 8.55 → 9 bolts
Result: One additional bolt required (12.5% increase in bolt count)
Practical Reality:
Most connections designed with:
- Geometric constraints dictating bolt patterns
- Standardized connection configurations
- Multiple load combinations including bearing and strength limit states
Typical Impact:
- Small connections (<10 bolts): 0-1 additional bolt
- Large connections (20+ bolts): 1-2 additional bolts
- Many connections: No change due to other governing criteria
Overall assessment: Minimal impact on typical bridge connections
Practical Implications
For Galvanizers
Eliminated Requirements:
No longer necessary to:
- Wire brush hot-dip galvanized faying surfaces
- Document wire brushing procedures
- Maintain wire brushing equipment and consumables
- Inspect for adequate roughness
Cost Savings:
Typical labor reduction:
- 0.5-2 hours per connection plate depending on size
- Labor cost savings: $25-100 per connection
- Schedule improvement: Faster throughput
Quality Assurance Simplification:
Standard galvanizing quality control adequate:
- No additional faying surface treatment verification
- Reduced documentation burden
- Clearer acceptance criteria
For Bridge Designers and Specifiers
Expanded Coating Options:
Greater flexibility selecting corrosion protection:
- Hot-dip galvanizing without additional treatment
- Metallized coatings (thermal spray) for slip-critical applications
- Duplex systems (galvanizing + zinc-rich paint) now accommodated
Design Considerations:
Coefficient Selection:
Match surface condition class to specified corrosion protection:
- Bare blast-cleaned: Class B (0.50)
- Hot-dip galvanized: Class C (0.30)
- Metallized ≤16 mils: Class B (0.50)
- Zinc-rich paint over blast-cleaned: Class D (0.45)
Bolt Count Adjustment:
For galvanized connections, slightly more bolts may be required compared to old 0.33 coefficient:
- Evaluate early in design process
- Verify connection geometry accommodates any additional bolts
- Consider bearing and strength limit states concurrently
Life-Cycle Cost Benefits:
Galvanized slip-critical connections offer:
- 75+ year corrosion protection with no maintenance
- Lower life-cycle costs versus painted alternatives requiring repainting
- Reduced coefficient offset by maintenance savings
For Fabricators
Faying Surface Handling:
As-galvanized surfaces suitable for slip-critical connections:
- No special handling or treatment required
- Standard shop practices adequate
- Avoid contamination (oils, greases) affecting friction
Bolt Installation:
Standard high-strength bolting procedures per RCSC Specification:
- Clean bolt threads and nuts
- Proper tightening to required pretension
- Turn-of-nut method or direct tension indicators
- Galvanized bolts acceptable with appropriate installation procedures
RCSC Specification Alignment
Research Council on Structural Connections
Role:
RCSC develops "Specification for Structural Joints Using High-Strength Bolts"—the definitive industry standard for bolted connection design and installation.
Coordination:
AASHTO revisions aligned with RCSC Specification ensuring:
- Consistency across bridge and building industries
- Common friction coefficient values
- Unified testing protocols
Future Harmonization:
Other specifications may adopt similar revisions:
- AISC Steel Construction Manual
- Building codes and state DOT specifications
- Industry consensus moving toward:
- Eliminating wire brushing requirements
- Accepting as-galvanized surfaces
- Recognizing metallized and zinc-rich paint options
Inspection and Quality Control
Faying Surface Acceptance
Visual Inspection:
Galvanized faying surfaces acceptable when:
- Free from oil, grease, dirt, or other contaminants
- No loose or exfoliating coating
- Uniform galvanized appearance
Not Required:
- Wire brushing
- Roughness measurement
- Special texture verification
Rejection Criteria:
Reject faying surfaces with:
- Heavy oil or grease contamination
- Thick wet storage stain (white rust)
- Paint, markings, or other foreign material
Remediation:
Contaminated surfaces:
- Solvent cleaning per SSPC-SP 1 for oil/grease
- Light wire brushing for white rust (if present)
- Zinc-rich paint touch-up for damage
Bolt Installation Verification
Critical Factors:
Slip resistance depends on proper bolt installation:
- Adequate bolt pretension
- Clean threads and bearing surfaces
- Proper washer installation (if required)
Inspection Methods:
- Turn-of-nut method verification
- Direct tension indicator (DTI) inspection
- Torque verification (calibrated wrench method)
Not Affected by Faying Surface Class:
Bolt installation procedures identical regardless of surface condition class.
The AASHTO LRFD Bridge Design Specification 8th Edition revisions to Article 6.13.2.8 eliminate wire brushing requirements for hot-dip galvanized faying surfaces in slip-critical connections while reducing the Class C slip coefficient from 0.33 to 0.30 based on extensive industry research demonstrating that as-galvanized surfaces provide reliable, predictable friction performance without additional treatment. The updated specification establishes four surface condition classes including new Class D (zinc-rich paint over blast-cleaned surfaces at Ks = 0.45) enabling duplex system applications and explicit Class B provisions for unsealed thermal-spray coatings up to 16 mils thickness, significantly expanding corrosion protection options for bridge designers while maintaining appropriate design safety factors. The modest 9% reduction in slip coefficient for galvanized surfaces translates to minimal practical design impact, typically requiring zero to two additional bolts in most connections, with this marginal increase substantially offset by eliminated wire brushing labor costs ($25-100 per connection), simplified quality assurance documentation, and faster fabrication schedules. Galvanizers benefit from eliminated wire brushing requirements removing time-consuming manual operations, equipment maintenance, and subjective roughness verification, while bridge owners gain access to broader coating system options including hot-dip galvanizing's 75+ year maintenance-free corrosion protection, metallized coatings for specialized applications, and duplex systems combining multiple protection mechanisms—all explicitly accommodated within slip-critical connection design criteria aligned with Research Council on Structural Connections specifications ensuring consistency across transportation and building industries. To read the original AGA resource article on this topic, click this link.
