Moment Resisting Connections for Seismic Applications

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Agenda:

      Types of Connectors Moment Connections Steel Design for Seismic Applications Plastic Design Examples of Moment Resisting Connections Worked Example

A structure is only as strong as its weakest connection!

Moment Connections by Brook Robazza and Shilin Sun 4/7/2011

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Connectors

   Steel Connections are usually connected by three types of connectors.    Rivet Bolt Weld Prior to the 1950s’ rivets were the standard fasteners for all types of steel structures Nowadays, bolts and welds are the standard connectors Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Connectors

 Bolt Types (ASTM Standard)   Common Bolts (A307)     Low-carbon Steel Square-head Fu = 415 MPa Minor Connections (Seldom used) High Strength (A325 & A490)    Carbon Steel (Heat Treated)/High Strength Alloy Steel Fu = 830 MPa/1040 Mpa Mostly Used (All Moment Frame) Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Connectors

  Welding Technology   Forge Welding (Heating and Hammering) Shielded Metal Arc Welding (SMAW) Weld Types    Groove Welds (15%)  Full Penetration Required Fillet Welds (80%) Others (i.e. Plug Welds) (5%) Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Moment Connections

   Moment Connections   Connecting the flanges of the beam to the column (Mostly Welded to end plate or column) Web of the beam is also connected to the column (types of web connections) Moment transferred from beam to column through flanges of the beam Shear transfer through the web of the beam Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Moment Connections

Standard Design Practice: • Overdesign the Connections or joints in any structures • • • Make failure occur at structural members A Performance Factor ϕ =0.67 for all connections Weak Beam, Strong Column • Avoid complex stress conditions • Weld in the shop, bolt on site Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Bolting versus Welding

   Bolting  Friction and Interlocking Mechanism Advantages  Easy to Make and Inspect Disadvantages    More in-shop work (Punching and Drilling) Less Tension Cross-Sectional Area Weight of Bolts    Welding  Metallurgical Bonds

Adv.

   Direct Stress Transfer and Continuous load-bearing Joint Light Connections Deal with Complicated Shapes of Connections Disadv.

 Skilled Welders and Non destructive Inspection   Cracks under Fatigue Loadings Development of Residual Stresses and Distortion Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Steel Design with Seismic Applications

 Earthquake is a sudden movement of the earth’s crust which causes the ground to shake.

 Concerns • Lateral Load • Cycle Motion • • Reversals of Stresses Elastic Design vs. Economic Concerns   General Philosophy:  design efficient connection so that ductile energy dissipating plastic hinges can form at desired locations to maintain continuity in force flow path and avoid structural collapse

Cost Evaluation

 The repair cost after the earthquake Vs. Cost of designing elastic structure Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Steel Design with Seismic Applications

 Prior-Northridge Earthquake (1994) Construction        Size Effects of Steel Members Inelastic Behavior within the beam-column connections Low-notch-toughness weld metal Sub-standard welding practice  (inadequate fusion or forget to remove the back bars after welding Shear yielding of the panel zones instead of flexural hinging of beam  Weak panel zones instead of weak beams Variable yield strength of structural shape material Stress Concentrations Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Steel Design with Seismic Applications

 Post-Northridge Earthquake Construction     Remove the prequalified status of typical bolted-web welded-flange moment connections from the code Each connection design should be qualified by a program of prototype laboratory testing under FEMA (Federal Emergency Management Agency) . University of Texas –cover plate (1994-1996) FEMA 267 Rated Weld Metal and Standard of design and fabrication  Tests on haunched connections, reduced-beam-section connections, vertical rib plate connections, side plate connections and slotted web connections Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Plastic Design

Desirable plastic behaviour  

Why do we need to design structural members to perform inelastically?

Not economically feasible to ensure structure remains elastic during all seismic events.

Large seismic events induce substantial forces on structures but occur with only a small frequency.

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What are the implications of plastic design?

Sacrificial member, or members, is usually chosen to act as a “fuse” to allow dissipation of energy to occur such that undesirable failure mechanisms are prevented.

 Some parts of the structure will experience large amounts of structural and nonstructural damage.

 Energy dissipation lowers the stress levels in important members and prevents overall collapse of the structure.

Less desirable plastic behaviour Moment Connections by Brook Robazza, Shilin Sun 4/7/2011

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Plastic Hinges

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Probable Plastic Moment:

Probable plastic moment capacity at the location of the plastic hinge:

where

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Plastic Hinge Location:

Plastic hinge locations for prequalified connection types are suggested in CISC and FEMA-350. Suggested hinge lengths are only valid for frames with low gravity loads on the frame beams . 

Use of Plastic Moment and Plastic Hinge Location:

Required to obtain the factored moment and shear needed for connection design of bolts, end plates, and welds and general suitability of the connection.

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Design Objectives

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Two Main Objectives for Designing Ductile and Reliably Performing Beam-to-Column Connections: Reinforce Connection:

Provide a beam-to-column connection which is stronger than the beam section. Strong connection forces the plastic hinge away from column face. Causes large stresses and inelastic strains to be developed in the beam far enough away from column to prevent high stress and strain levels in the column.

Weakening the Beam:

Weakening the beam framing into the connection creates a “ductile fuse.” Limits the stress and strain developed near or at the column by concentrating it in the beam which can sustain large inelastic strains.

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Design Procedures

General Design Procedure for all Connections:

  Identify all undesirable failure modes as well as the primary yielding mechanism and any other yielding mechanisms.

Determine the probable peak capacity of the primary yielding mechanism and in some cases, the onset of yielding.

  Proportion the connection to ensure that the nominal resistances against all the undesirable failure modes at least equal the probable peak capacity of the primary yielding mechanism.

In order to achieve the most desirable sequence of yielding for connections in Type D and Type MD frames, proportion each connection to prevent yielding of any secondary yielding mechanism prior to the onset of yielding of the primary yielding mechanism.

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Bolted Unstiffened End Plate Connection

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Bolted Unstiffened End Plate Connection Construction Procedure:

Beam is shop-welded to an end plate which extends beyond the top and bottom flanges of the beam.

Flanges are welded to the end plate by means of CPJG welds.

Web is welded to the end plate using a fillet weld or CJPG weld. End plate is field-bolted to the column using 8 bolts.

Design Requirements:

May be used in Type D, Type MD, and Type LD systems within given member size limitations.

Must be checked for size of bolts, thickness of end plate, flange thickness of column, and web thickness of column according to limitations provided by CISC.

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Bolted Stiffened End Plate Connection

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Bolted Stiffened End Plate Connection Construction Procedure:

Beam shop-welded to an end plate which extends beyond the top and bottom flanges.

Flanges are welded to the end plate by means of CPJG welds.

Web is welded to the end plate using a fillet weld or CJPG weld. End plate is field-bolted to the column using 16 bolts.

End plate extensions at the top and bottom of beam stiffened by a vertical stiffener plate that extends outward from the beam flanges and conforms to size requirements prescribed in CISC.

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Design Requirements:

May be used in Type D, Type MD, and Type LD systems within given member size limitations.

Must be checked for size of bolts, thickness of end plate, flange thickness of column, and web thickness of column according to limitations provided by CISC.

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Reduced Beam Section Connection

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Reduced Beam Section Connection Construction Procedure:

Circular radius cuts are made in both the top and bottom flanges of the beam at locations near the ends of the beam or girder.

Beam flanges welded to the column using complete joint penetration groove welds.

Web connection utilizes a shear tab which may be bolted or welded in the field.

Design Requirements:

May be used in only Type D and Type MD systems within given member size limitations.

Must be checked to prevent flexural and shear failure using the plastic section modulus of the reduced beam section.

Must be checked for column flange thickness according to limitations prescribed by CISC.

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Worked Example

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Consider a Bolted Unstiffened End Plate connection for a beam-to-column connection with a uniformly distributed load of 12 kN/m and point load of 30 kN located at midspan across a beam spanning 8 m between column faces. Find V cf and M cf :

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Check Bolt Area:

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Find Minimum Plate Thickness:

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Therefore select a plate thickness of 100 mm.

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Worked Example continued…

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Check column flange thickness:

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Check column web thickness:

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Check thickness of panel zone: Assuming a storey height of 3 m:

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Connection adequate!

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The End

Questions?

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