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Welded Connections for Seismic Applications
Elastic and Plastic Analysis
and LSD
By Kevin Spacey & Johann
Fridriksson
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Elastic vs. Plastic
3 conditions at collapse
Elastic Analysis
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Element is assumed to obey
A.
Structure in equilibrium
Hooke’s law and recover to
B.
No M > Mp
original state on removal of load.
C.
Sufficient hinges to form collapse
Classic beam theory is used for
analysis.
3 theorems of plastic analysis
Plastic Analysis
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mechanism
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Stress remains constant while
the strain increases.
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The plastic hinge forms.
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Only Class 1 sections - can
develop the rotation needed for
A and B – solution is less than or
equal to or collapse load (LB)
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A and C – solution is greater than
or equal to collapse load (UB)
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All – solution equal to collapse
load (Uniqueness Theorem)
moment redistribution
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Limit State Design (LSD) & Seismic
Ultimate Limit States:
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Exceeding the load carrying capacity
Overturning
Sliding
Fracture
Serviceability Limit States:
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Deflection
Vibration
Permanent Deformation
FatigueFactored resistance ≥ Effect of factored loads
NBCC - Elastic Base Shear
V
Capacity design
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Ø*(Calc. Resistance) > Load Factors*(Calc. Loads)
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S Ta M v I EW
Rd Ro
Loads need not exceed values
corresponding to RdR0 = 1.3
Specific elements or
mechanisms designed to
dissipate energy
All other elements are sufficiently
strong to achieve energy
dissipation
Structural integrity is maintained
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Welding Techniques
Metal Inert Gas (MIG)
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Also called Gas Metal Arc
Welding (GMAW)
Semi – automatic
Continuous wire feed
Gas for contamination
protection
Flux – Cored Arc Welding
(FCAW)
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Related to MIG
Steel electrode surrounding
powder fill material
More expensive wire
Higher welding speed
Greater metal penetration
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Welding Techniques
Tungsten Inert Gas (TIG)
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Gas Tungsten Arc Welding
(GTAW)
Manual welding process
Separate filler material
Gas for contamination protection
Can be applied to all metals
Mostly used for stainless steel
and light metals
Plasma Arc Welding
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Related to TIG
Uses plasma gas to make arc
More stable current
Used on wider range of thickness
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Welding Techniques
Manual Metal Arc (MMA)
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Shielded metal Arc Welding
(SMAW)
Stick welding
Manual welding process
One of the most common
Separate filler material
Weld times slow
Slag must be chipped away
Generally limited to ferrous
materials
Special electrodes for other
metals
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Welding Techniques
Submerged Arc Welding
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High productivity
Manual welding process
Continuous wire feed
Commonly used in industry
 Large products
 Welded pressure vessels
Slag comes off by itself
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Introduction to Welding
Fillet Welds
What is welding?
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Full Penetration Groove Welds
Partial Penetration Groove Welds
Melting and fusing of two
pieces of metal with applied
heat
Produce truly “one-piece”
members.
Continuous connection between
two pieces of metal
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Degree of continuity not
achieved by bolting
Two Types of welds:
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Full and Partial Penetration
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Welding Continued…
Advantages
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Disadvantages
Continuous connections.
Economical way to connect
elements and fabricate
assemblies.
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Initiation of residual stresses in
connections
Warping due to weld shrinkage
Different weld types and
procedures
Tight tolerances can be
achieved during fabrication and
erection.
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Lamellar tearing.
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Design Strength of Welds – Fillet
Two Critical Dimensions of Fillet
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Nominal weld size: D
Effective throat: tw
Failure of Fillet Welds:
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tw
tension D
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shear through effective throat of
weld parallel to weld axis
shear through fusion face of the
weld parallel to weld axis
shear
D
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Design Strength of Welds – Fillet
Shear
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(lesser of)
Base Metal
 Vr = 0.67φwAwXu(1.00+0.50sin1.5θ)Mw
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Weld Metal
 Vr = 0.67φwAmFu
 (Clause 13.13.2.2)
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Design Strengths of Welds - Groove
Full penetration can be achieved
from one side
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Provide backing bars
Combine with ‘reinforcing’ fillet
weld to add strength
Failure of Groove Welds:
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shear strength does NOT
depend on orientation of load
Fail in tension when load is
normal to weld axis
Fail in shear similarly to fillet
welds
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Design Strength of Welds - Groove
Tension (lesser of)
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Base metal:
 Tr = φ Ag Fy
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Weld metal:
 Tr = φw An Fu
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Overall Ductile behavior is desired
 AnFu > AgFy (Clause13.13.3.1)
Shear (lesser of)
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Base metal:
 Vr = 0.67 φw Am Fu
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Weld metal:
 Vr = 0.67 φw Aw Xu
(Clause13.13.2.1)
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Design Strength of Welds - Groove
Groove welds may be
reinforced by fillet weld
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Increase strength
Tension
(Clause 13.13.3.3)
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CASE STUDY - Northridge Earthquake
Northridge, LA – 1994
Turning point for Welded MRF
connections
100+ structure survey
Welded-Flange-Bolted-Web (WFWB)
affected
Beam flange weld – single field
bevel CJPG
Beam web – field bolted shear
tab
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CASE STUDY – Connection Damage
Widespread Nature
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Connection Difficiences
Joint Failure
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Cracks develop through the
beam-column joint at weld
Occur at bottom flange of
groove weld
Damage In welds and steel
elements in or near steel beamto-column connections
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CASE STUDY – Connection Damage
Column Fracture
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‘Divot’ fracture
Flange/Column width
Weld Failure
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Fracture near face of column,
extending from root of weld
Fracture contained within metal
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Visual ID
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Ultrasonic detection
No local plasticity
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CASE STUDY – Cause of Damage
Low weld metal toughness
Leaving groove weld backing
bars on
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Act as artificial edge crack
Initiate brittle failure
High stress at beam flange
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Lack of web contribution
Workmanship/Quality control
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SUMMARY - Welded Connections
Mostly done in the factory
Quality of welds depend highly
on surrounding conditions
Most connections are a
combination of welds and bolts
2 phases of construction
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Factory (Welds)
Construction site (Erection,
bolting)
Strength is governed by:
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Base material
Weld material
CSA S16 and CSA W59
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SUMMARY - Welded Connections for Seismic
Seismic Design requires fillet
welds or full penetration groove
welds
Overall Ductile behavior is
desired
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AnFu > AgFy
All connections must have:
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Class 1 cross - sections
Strength > Mp
Full transfer of loads
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Stiffeners
Complete welds
Workmanship
Strength of materials
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