Steel Connections - Concept Map
Steel Connection Design & Failure Modes
Related concepts
Bolted Connections
Sub concepts
- Bolt Shear Strength
- Bearing Strength
- Slip-Critical Connections
- Bolt Tension
- Block Shear Rupture
- Double Shear vs Single Shear
Relationship to central
Primary connection method; uses discrete fasteners to transfer force
Welded Connections
Sub concepts
- Fillet Welds
- Groove Welds
- Weld Throat Area
- Electrode Classification
- Effective Weld Length
- Full Penetration Design
Relationship to central
Alternative connection method; creates continuous force transfer
Limit States & Failure Modes
Sub concepts
- Shear Rupture
- Bearing Deformation
- Tensile Rupture
- Block Shear Rupture
- Weld Throat Rupture
- Slip at Service Load
Relationship to central
Governs design strength calculations; determines governing failure
Design Parameters
Sub concepts
- Material Properties (Fy, Fu)
- Bolt Diameter & Area
- Plate Thickness
- Edge & Pitch Distances
- Electrode Strength (FEXX)
- Resistance Factor (phi = 0.75)
Relationship to central
Input values required for strength calculations
LRFD Methodology
Sub concepts
- Nominal Strength Calculation
- Resistance Factor Application
- Design Strength (phi*Rn)
- Service Load Checks
- Safety Margins
Relationship to central
Load and Resistance Factor Design framework for connection design
Code Standards & References
Sub concepts
- AISC 360-16 Specification
- NSCP 2015 Provisions
- ASTM A325 Bolt Standards
- Electrode Classifications
- RA 544 Building Code Requirements
Relationship to central
Regulatory framework governing connection design in Philippines
Concept connections
- Bolt Shear StrengthDouble Shear vs Single ShearNominal shear strength doubles when there are two shear planes instead of one; design strength = 0.75 × Fnv × Ab × (number of shear planes)
- Bearing StrengthPlate Thickness & Edge DistanceBearing capacity is directly proportional to both plate thickness (t) and edge/pitch distance (lc); larger distances increase bearing strength up to the 2.4db limit
- Bearing StrengthHole Diameter CapThe 2.4×db×t×Fu term caps bearing strength; even large edge distances cannot exceed this limit, controlling over-design
- Fillet Weld Throat AreaLeg SizeThroat area = 0.707 × leg size (a); throat, not leg size, is used in strength calculations; common error is using leg size directly
- Bolt Shear StrengthBearing StrengthBoth limit states must be checked; connection design strength is the minimum of bolt shear and bearing, whichever is smaller
- Block Shear RuptureAngle & Gusset ConnectionsBlock shear failure is particularly critical in angles bolted on one leg and gusset plates; must always be checked for these configurations
- Design StrengthLRFD Resistance Factor phi = 0.75All connection limit states use φ = 0.75 in LRFD; design strength = φ × nominal strength; safety factor embedded in resistance factor
- Nominal StrengthMaterial Properties (Fy, Fu)Nominal strength calculations depend on yield (Fy) for gross-section limits and ultimate (Fu) for net-section/rupture limits
- Bolt Shear StrengthA325-N vs A325-XA325-N (threads in shear) has Fnv = 372 MPa; A325-X (threads excluded) has Fnv = 469 MPa; choice affects bolt shear capacity significantly
- Fillet Weld StrengthElectrode Type (E70, E80)Weld nominal strength proportional to electrode strength FEXX; E70 (482 MPa) is common, E80 (550 MPa) provides higher strength
- Connection Design ProcessGoverning Limit StateAll limit states must be calculated; the minimum controls connection capacity; common exam error is reporting bolt shear without checking bearing
- Block Shear RuptureTension Path & Shear PathBlock shear combines a shear failure plane and a tension failure plane; the sum of these two rupture paths determines capacity
- Weld Effective LengthFillet Weld CapacityDesign strength proportional to effective length L; longer welds develop higher total strength; per-unit-length strength = 0.75 × 0.60 × FEXX × 0.707a
- Slip-Critical ConnectionsFriction & Clamping ForceSlip-critical bolts resist load via friction (μ) between clamped surfaces; required when slip cannot be tolerated (fatigue, dynamic loading)
- Plate ThicknessBearing & Block Shear StrengthThicker plates improve both bearing strength (proportional to t) and block shear net area; minimum practical thickness often set by bearing or shear limits
- Bolt DiameterBolt Cross-Sectional AreaBolt shear strength proportional to Ab = π×db²/4; larger bolts significantly increase shear capacity but also affect bearing and hole dimensions
- AISC 360 SpecificationNSCP 2015 AdoptionNSCP 2015 in Philippines is based on AISC 360-16; formulas, resistance factors, and procedures are identical; RA 544 requires compliance
- Edge Distance LimitBearing Strength CapEdge distance affects bearing strength via the 1.2×lc×t×Fu term, but is capped by 2.4×db×t×Fu; very large edge distances do not increase bearing beyond the cap
- Member RuptureNet Section AreaMember tensile capacity at the bolt-hole section; net area = gross area minus area of holes; often checked separately from connection but must be verified
- Groove WeldFull PenetrationFull-penetration groove welds develop the full strength of the base metal; no strength reduction from weld geometry; controls member rupture at net section instead
- Bolt TensionInteraction with ShearWhen bolt carries both shear and tension, combined stress reduces capacity; interaction formula adjusts shear strength when tension is present
- Minimum Weld SizePlate ThicknessAISC J2.2 specifies minimum fillet weld size based on plate thickness; thicker plates require larger welds; common sizes 1/4" to 1/2"
- Clear Distance lcBolt Hole SpacingClear distance is the gap between the edge of one hole and the edge of the next (or plate edge); controls bearing strength; minimum pitch typically 2.67×db
- Design Strength DeterminationGoverning Failure ModeProcess: (1) calculate all limit states, (2) identify minimum, (3) that minimum is connection design strength; board exams heavily test this concept
Diagrams
Steel Connections: Complete Concept Hierarchy
Comprehensive mind map showing the complete hierarchy of steel connection concepts, from bolted and welded types through failure modes, design parameters, and code standards. Central node represents the overall steel connections design system, with six primary branches expanding to sub-concepts and detailed elements.
Bolted Connection Design Process: Strength Determination
Step-by-step flowchart showing the bolted connection design procedure. Starts with bolt shear determination (accounting for single or double shear), then checks bearing strength, block shear if applicable, and finally compares against required strength. The governing limit state is the minimum of all checks—a critical concept for board exams.
Welded Connection Strength Calculation
Flowchart illustrating fillet and groove weld design procedures. For fillet welds, shows the calculation of throat area (0.707×leg size), effective length, electrode selection, nominal strength, and design strength. Includes minimum size checks and verification against required load. For groove welds, emphasizes full penetration and member rupture checks.
Connection Design: Governing Limit State Selection
Critical flowchart showing how to determine the governing limit state in connection design. All four major limit states (bolt shear, bearing, block shear, member rupture) are calculated, then the minimum—which is the connection design strength—is identified. This is essential: the connection capacity is controlled by whichever mode fails first, not by bolt shear alone. Common board exam error is ignoring the controlling failure mode.
Load Transfer Through a Bolted Connection
Sequence diagram illustrating how force flows through a bolted connection from one member to another. Shows the load path: from applied force → bolt shear plane → bearing at hole → internal shear flow (block shear) → remaining net section → second member. Identifies each failure mode that could occur if that component is overstressed, reinforcing the concept that connection strength is limited by the weakest link.
Material & Bolt Grade Selection Impact on Connection Strength
Flowchart showing how bolt grade and plate material selection impact connection design. A325-N (standard threads-in-shear) and A325-X (threads excluded) have different shear strengths. Plate material (Grade 250/345/415) affects bearing strength. The interaction between these choices determines which limit state governs—a critical design consideration for optimizing cost and safety.
Connection Failure Progression Under Increasing Load
State diagram showing the progression of connection failure as load increases. From service load through elastic behavior, into yield zone, then through progressive yielding and deformation of bolts and bearing surfaces. The diagram shows how either bolt shear rupture or bearing rupture will dominate depending on geometric and material properties. Understanding this progression helps predict which failure mode governs under design conditions.
Block Shear Rupture Analysis Procedure
Detailed procedure for analyzing block shear rupture, which is critical in angle and gusset connections but often overlooked. Shows identification of the failure block geometry, measurement of net shear and tension areas, calculation of both rupture and yield paths, selection of the smaller, and application of the 0.75 resistance factor. Emphasizes that block shear must be compared against all other limit states to confirm if it governs.