Structural Bracings

Presentation by V. G. Abhyankar For Knowledge sharing sessions 26.03.2008

Objectives

• To understand various types of structures and bracing systems and their behavior • To explain the advantages of braced systems • To discuss the limitations of Braced frames • To study the method of Detailing bracing systems • To highlight advanced theories – in – bracing design

Types of Structural Framing systems

• Bare Frame (Moment Resistant / Rigid jointed frame) • In-Filled frame • Equivalent bracing system • Braced frame (Pin Jointed / Truss)

Types of Loads

• Vertical / Gravity Loads – Self-weight – Imposed Load – Live loads – Snow load • Lateral Loads – Wind – Earthquake – Wave / Current forces – Friction • Special Loads – Lack of fit – Sinking supports – Tsunami – Blast Load / Vibrations / Dynamic Loads – Fatigue – Shrinkage / Creep

Behavior of Structural Framing Systems under various types of loads VERTICALLOAD RIGID JOINT LATERAL LOAD BEAM SWAY BEAM DEFLECTED SHAPE

BARE FRAME UNDER GRAVITY LOAD

DEFLECTED SHAPE

BARE FRAME UNDER LATERAL LOAD

VERTICALLOAD PIN JOINT LATERAL LOAD BEAM SWAY DEFLECTED SHAPE DEFLECTED SHAPE

BRACED FRAME UNDER GRAVITY LOAD BRACED FRAME UNDER LATERAL LOAD

Comparison of Rigid Jointed / Pin Jointed Framing System

RIGID JOINTED FRAME BRACED FRAME

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Advantages More free space (allows openings) Less numbers of Members More sway as compared to braced frame 1.

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Disadvantages Full cross sectional area is utilized as members are mainly axially loaded Simple connections, simple behaviors (direct load transfer) Lesser sway Easy to analyze Capable of resisting accidental loads, twisting etc.

Easy to modify Aesthetically appealing Economical 1.

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The flanges only resist the moment. Stress diagram is triangular (less utilization of area) Connections are difficult to fabricate Bulky sections are required 1.

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More number of members Difficult to provided openings Heavy bracings are required for long spans Loads need to act essentially on joints Design load for the Bracing design is always debatable

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Types of Braced Frames

Single Diagonal Brace (Tension / Compression Both) 2.

X – Cross Brace (Tension / compression only) 3.

Chevron Brace (K-brace) 4.

A – Chevron brace 5.

Inverted chevron 6.

Braced Chevron Brace 7.

Braced Single Brace 8.

Knee brace

A few facts regarding Bracings 1) Force in Bracings – Compression only Bracings – Concrete – Tension only Bracings – Steel – Pre-stressed bracings are also used for industrial structures (wire-ropes) – Vertical Load carrying braces – Horizontal Load carrying braces • Seismic bracing • Wind bracing 2) Slenderness of bracing – Effective length for In-Plane action – Effective length for out-of Plane action – Wind load for out-of-plane bending need not be checked (but the wind acting on braces is transferred to the connected joints) – Stiff-bracings – Flexible bracings

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3) Torsion Control with bracings is effective 4) 2D-Plane - Portal frames when connected with braces demand wind braces 5) Design loads on braces are usually 2.5% to 3% Lateral loads of the respective vertical loads 6) Bracings are effective in resisting Seismic forces than bare frame. Plays major role in seismic retrofitting 7) It is difficult to provide opening at the location of bracing 8) In case of In-filled frame, the stiffness of Infill walls can be simulated in the structural model by means of equivalent brace. This philosophy was proposed by Mr. V. V. Bertero

Slotted connection • Slotted connections are provided if the bracing is expected to resist the load in a particular direction only.

RCC BEAM WP SLOTTED CONNECTION GUSSET PLATE STEEL BRACING

BRACING SYSTEMS • ELEVATIONAL BRACING • SECTIONAL BRACING • PLAN BRACINGS

Technical Reference

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“Structures” – by Daniel Schodeck AISC Presentations Tall steel Buildings – Steel Designers Manual Steel Detailers Manual SP-40 V. V. Bertor’s articles Standard detail drawings - Toyo Engineering

Structural Bracings

Presentation by V. G. Abhyankar For Knowledge sharing sessions 26.03.2008