connection and bracing

By / Muhammad Azeem

Outline Introduction Steps for Design

• Geometry Modeling• Loading• Design Parameters• Analysis & Design• Connections & Bracing

Pipe Rack Design Flow Chart

From Previous Presentation

Outline Benefits of Steel Structure Load Resisting Systems Frame Types Concentric / Eccentric Braced Frame Steel Frame Connection Types

Simple ConnectionMoment ConnectionEM Connection Capacity Design Flow Chart

Benefits of Structural Steel

Reduced construction time & no seasonal effect. Light weight and reduced foundation cost. Durable , Long Lasting and Recyclable. Easier to modify and reinforce if required.

Benefits of Structural Steel

Fabrication off-site possible (right). On site erection is a time saving process (left). Schedule advantage.

Forces on Structure Forces from Gravity ,wind and

seismic is imposed on all

structures.

Applied vertical forces are

gravity loads.

Forces that act horizontally

such as wind, seismic require a

lateral load resisting system to

be built in structure.

When lateral loads are applied

to a structure, horizontal

diaphragm transfer the load to

the lateral load resisting

system.

Structural steel frame elevation

The type of lateral load (Seismic, Wind, Friction, Thermal ) resisting system to be used in a Structure Should be taken care in the Earlier Planning Stage.

Common lateral load resisting systems

Braced Frame Rigid Frame

Initial Stage System Planning

Rigid frame, utilize the moment connection and this type is preferred where diagonal or x- bracing should avoid due to access requirement.

Draw back of rigid frame is more cost than braced frame.

Rigid Frame

Braced Frame Braced frame is the economical method of resisting wind load in

multi storey structural frame. Bracing creates triangular configuration in the structures. Some structures like below picture take the advantage of both rigid

and braced at the same time.

Bracings Member or system that provides

stiffness and strength to limit the out of plan movement of another member at the braced point. (AISC Spec2010)

Two types of bracing is defined in AISC (1997) seismic provision in steel structural building.

A. Concentric BracingB. Eccentric Bracing

A- Concentric Brace

Concentrically braced frame are those in which the centerlines of members that meet at a joint intersect at a same work point to form a vertical truss system that resist lateral forces.

Commonly used bracing system in steel structure is X- bracing ( above left) Chevron (above right)

A- Concentric Brace

X-Bracing Connection for X- bracing

are located at beam to column joint.

Bracing connection may require large gusset plate at the beam to column joint.

Chevron Bracing The members used in chevron

bracing are designed for both compression and tension

Chevron bracings allows for doorways or corridors through the bracing lines in a structure.

“CHEVRON” “V” “DIAGONAL”

ELEVATION WITH SEVERAL BRACING CONFIGURATION

Chevron Bracing Chevron bracing members use two

types of connections. The Floor level connection may use a

gusset plate as like in x-braced frame.

The bracing members are connected to the beam/girder at the top and converge to a common point.

When gusset plate is used, it in important to consider their size when laying out any mechanical equipment pass through braced bays.

B- Eccentrically Braced Frame

Eccentric brace is commonly used in seismic regions and allow corridors and walkway through bay frame.

In an eccentrically braced frames, bracing connect to a separate work point on beam/girder.

The beam/girder segment or “link” between them absorbs energy from seismic activity through plastic deformation.

Link

E/B with typical brace to beam connection

Eccentric Brace

Gusset Plate

Beam or Girder

Stiffeners


Adopted AISC 327-05


Eccentrically braced frame looks very similar to chevron bracing.


Eccentric single diagonal brace can also be use to brace a frame.

Steel Frame Connection Type

The specification of structural steel building (AISC 2010) defines two types of frame connections.

Simple Connection (below left)Moment Connection (below right)Moment connection are of two types: FR & PR

Steel Frame Connection Type

Simple Connection: Connection that transmit negligible amount of bending moment between connected members.

Moment Connection: Connection that transmit bending moment between connected members.

• Fully restrained moment connection FR : transfer moments with a

negligible amount of rotation between connected members.

• Partially restrained moment connection PR : transfer moments, but

rotation between connected members is not negligible.

AISC Spec.-

2010

Steel Frame Connection Types

Simple connection (A) have rigidity but is assumed to rotate free. Connection (B & C) are examples of partially restrained moment

connections. Connection (D & E) are examples of fully restrained moment.

Adopted from Vol -II Connection .

Simple Connection

Connection is assumed to be rotation free. Vertical forces are primary forces transferred by the connection. For this Connection a separate resisting system is required to design.

Single plate Connection

( Shear Tab)A plate is welded to the supporting member and bolted to the web of the supported beam.

Double Angle Connection

The in plane pair of legs are attached to the web of the supported member and out of plane pair of legs attached to the supporting beam web or flange.

Commonly Used Simple Connection

Commonly Used Simple Connection

Shear end plate connectionA plate is welded perpendicular to the end of supported web and bolted to supporting beam.

Seated connectionAn angle is mounted with one leg vertical against the supporting column & the other leg provides a “seat” upon which the beam is mounted ,a stabilizer connection is provided at top of the web.

Moment Connection Designed as rigid connection with little or no rotation. Moment and vertical shear forces are transferred through the connection. Two types of moment connection are allowed:

1. Partially restrained moment connection.

2. Fully restrained moment connection.

Moment Connection

Fully restrained (FR)Connection Have sufficient strength to transfer

moment with negligible rotation between connected members.

Partially restrained (PR)Connection

Have sufficient strength to transfer moments with rotation between connected members.

Commonly Used FR Connection

Welded flange plate connection Bolted flange plate connection

*With column web

Top and bottom flange plates connect the flanges of supported member to the supporting column.

A single plate is used to transfer the vertical shear force.

Commonly Used FR Connection

Bolted extended end plate connection

A plate is welded to the end of supported beam and bolted to the supporting column flange.

Welded flange connection

Complete joint penetration groove welds directly top and bottom flanges of the supported member to supporting column.

A shear connection at web is used to transfer the vertical shear force.

*With column flange

Commonly Used PR Connection

PR moment connection

A double angle simple connection transfers vertical shear forces while top and bottom flange plates resist moment forces produced by wind.

Top and bottom angle with shear end plate connection

Angles are bolted or welded to the top and bottom flanges of the supported member and to the supporting column.

A shear end plate on the web is used to transfer vertical shear forces.

Calculations End Moment Connection

(1) MEMBER

Mm = 0.6Fy x Sx (kN-m)Vm = 0.4Fy x Awn (kN)Tm = 0.6Fy x 2Af (kN)Af = bfb x tfb (mm2)Awn = ( d1 - 2tfb ) x twb (mm2)

(2) HIGH STRENGTH BOLT

Mb = n1 x Qt x ( d1 - tfb ) (kN-m)Vb = n2 x Qs (kN)Tb = 2 x n1 x Qt (kN)n1= Number of tension boltsn2= Total Number of bolts

(4) END PLATE

Mp = Ff x ( d1 - tfb ) (kN-m)

Tp = 2 x Ff (kN)Ff = 4 x Me / ( αm x Pe ) (kN) Me = bp x 0.75Fy x tp2 / 6 (kN-m)αm = Ca x Cb x (Af / Aw)1/3 x (Pe / db)1/4Pe = Pf - (db / 4) - 0.707S1 (mm)Ca = 1.13Cb = ( bfb / bp )1/2db = (mm)Aw = twb x (d1 - 2tfb) (mm2)

(5) STIFFENER DESIGN

(5-1) COLUMN FLANGE BENDING STRENGTH Fcap = 4 x Me /( αm x Pe ) Me = bs x 0.75Fy x tfc2 / 6 (kN)bs= 2.5 x ( Pf + tfb + Pf ) (kN-m)Pe = g / 2 - db / 4 - k1 (mm)αm = Ca x Cb x ( Af / Aw )1/3 x ( Pe / db )1/4 (mm)Af / Aw = 1.0min Mst = Ff x ( d1 - tfb ) (kN-m)Ff = Fcap + Fst (kN)min Tst = 2Ff (kN)(5-2) TENSION CAPACITY OF STIFFENER PLATEFst = 0.6Fy x Ast (kN)Ast = ts x ( bfc - twc - 2 x 5 - 2 x 25 ) (mm2)(5-3) SHEAR CAPACITY OF STIFFENER PLATEFst = 0.4Fy x Ast (kN)(5-4) STIFFENER WELD CAPACITY OF COLUMN FLANGEFst = fw x Ast (kN)Ast = 0.707 x lw x Sflg (mm(5-5) STIFFENER WELD CAPACITY OF COLUMN WEBFst = fw x Ast (kN)Ast = 0.707 x lw x Sweb (mm2)

lw = 4 x (dc/2 - tfc - 25 - Sweb) : HALFlw = 4 x (dc - 2tfc - 2x25 - 2xSweb) : FULL

(3) WELD

Mw = Ff x ( d1 - tfb ) (kN-m)

Ff = Fw x Awf (kN)

Vw = Fw x Aww(kN)Tw = Fw x 2AwfAwf = 0.707 x S1 x (2bfb+2tfb-twb ) (mm2) Aww = 0.707 S2 x ( d1 / 2 - tfb ) x 2 (mm2)

SAFE LOAD OF CONNECTION

Ma =min.(Mm,Mw,Mb,Mp,Mst) (kN-m)

Ta = min.(Tm,Tw,Tb,Tp,Tst) (kN)

Va = min. (Vm,Vw,Vb) (kN)

References AISC VOLUME-II CONNECTION CONNECTION TEACHING TOOLKIT (AISC) AISC 327-05 SEISMIC DESIGN MANUAL AISC 2010 SPECIFICATION FOR STRUCTURAL

STEEL BLDGS

Thank You

connection and bracing

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