Structural considerations for so cal seisimicity

Structural Considerations for Southern California Seismicity Ashi Dhalwala – CEG [email protected]

March 17, 2014

Seismic Risk Reduction in Steel Structures Steel Committee 1

Structural Considerations for SoCal Seismicity

Ashwani Dhalwala, M.S.,S.E.

California Engineering Group

(CEG)

Sources of Uncertainity

• Seismic Input-PGA,Amplification,Range

• Structural Model- Geometry, Soil, Stress/Strain Relationships, Buckling

• Dynamic Model-Mass,Damping,Integration Operators, Nonlinear Degradation

• Structural Response- Large Displacements, Connection Response


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California Fault Zones

San Andreas Fault


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FEMA 355D Research

• Numerous connection configurations tested and recommended

• High notch toughness electrodes specified

• Flaws introduced by backup bars eliminated

• Resulted in improve connection performance

FEMA 355D Research

• Resulted in AISC Seismic Supplement No. 1 Welded Unreinforced Flange – Welded Web (WUF-W) and Bolted Flange Plate (BFP) Moment Connections

• Adoption of RBS Moment Connection


March 17, 2014


Unresolved Issues

• Reduced performance due to Size Effects

• Effect of panel zone yielding

• Brittle behavior due to local triaxiality

• Triaxiality partially reduced by modifying the weld access hole

• Rational continuity plate design

• Lateral torsional buckling

Unresolved Issues

• Cover plated connection

• Strain rate effects

• Effects due to vertical accelerations

• Weld residual stresses

• Column/Beam moment capacity ratio


March 17, 2014


Unresolved Issues

• Unresolved issues apply to all steel connections – Braced Frames, EBFs, Steel Shear Walls, Base Plates

Suggested Cover Plate Design

Full Scale Tests

Circa 1998

Non Linear Continuum

Mechanics Analysis

by CEG 1999

Plastic Range in Red

Proposed and used in design by CEG

Simulation


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CBC 2013

• Use of updated USGS maps

• High Peak Ground Accelerations representing SoCal Seismicity (SCS) more accurately

NF and FF Structural Response

Ref: Mateescu et. Al.


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NF & FF Ground Motions

Ref: Mateescu et. Al.

Brittle cracking in steel

Column Fracture in 11 Story Building

Northridge Earthquake

Courtesy P.Maranian


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• Observed during the Northridge Earthquake

• Is brittle cracking a near field phenomena?


Ref: Fracture Mechanics Texts


March 17, 2014


What happens to a material with a small crack?

Yields then work hardens, absorb energy and redistribute stress. In other words, crack makes no significant difference!

Get high stress around crack, crack propogates and get sudden failure. Stress around crack is high due to Kt , but nominal stress is much lower than material yield strength!

What happens when you nick a brittle material??

Ref: Fracture Mechanics Texts


• Brittle Fractures are initiated at a the atomic scale due to severing of atomic bonds – Trans-granular Fracture

• Ductile Fractures – Inter-granular Fracture

• Determine which one governs

• Function of Connection Size, Geometry

• Function of Applied Loads


March 17, 2014


A plastic zone forms at the crack tip where the

stress would otherwise exceed the yield

strength σy.

Ductile Fracture:

Stages of ductile fracture:

b. Plastic def’m when stress exceeds yield.

c. Weaken and fail locally due to inclusions which act as stress concentrations – this creates tiny voids.

Voids continue to grow and coalesce to form larger voids.

d. Remaining area gets smaller increasing stress until tensile strength is exceeded then fracture.Ref: Fracture Mechanics Texts


• Most fracture analysis is performed using continuum mechanics principles and relates to stress fields at the crack tip where resistance of the material to crack extension “ fracture energy G” is formulated using material properties E and “v” and the Rayleigh wave speed Cr (speed of sound on a free surface)

• Precisely when new cracks emerge cannot be predicted – source of uncertainty


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Control of Brittle Fracture

• AISC Seismic Supplement No. 1

• WUF-W Connection

• Currently permitted for SMFs


Two main causes

1. Plane Strain Conditions

2. Ductile to Brittle Transition Temperature


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Plane Strain Conditions caused by

1. Triaxiality from external applied loads

2. Thickness effects

3. High Strain Rates


Triaxiality from external applied loads:

• Reduces shear deformation

• Restricts yielding of material

• Material is 100C% brittle if:

• Triaxiality >= Fy


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Triaxiality

Triaxiality and Strain Rates

Triaxiality = (sigma1+sigma2+sigma3)/3

Seppala, Belak, Rudd – Lawrence Livermore Labs

Control of Brittle Fracture Ductile to Brittle Transition Temperature


March 17, 2014



Ductile to Brittle Transition Temperature

• Occurs in Bcc Materials such as steel

• Restricts yielding of material

• Material is 100C% brittle if:

• Triaxiality >= Fy



• Occurs in Bcc Materials such as steel

• Lower shelf – cleavage failure

• Higher shelf – void coalescence

• Intermediate – mixture of the two above


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• Transition at higher temperature results in early brittle failure.



Transition temperature increased by:

• High stresses

• High strain rates

• Thick material

• Weld residual stresses and fast cooling rates

• Hydrogen entrapment (embrittlement)


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WUF-W Connection Solution #1

Maintain plane stress conditions by controlling thickness of material and beam depth to span ratio

Max material thickness for plane stress conditions:

t = 400 K^2 * (1-v^2)/E((Fy+Fu)/2)

This calculates to 0.645” for 50 ksi steel


This, for all practical purposes requires

beam flange thickness of 11/16” or less.

Limit maximum beam depth to 18”


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Control of Brittle Fracture WUF-W Connection Solution No. 2

Between 11/16” and 1” flange thickness,

Compute triaxiality at the beam column connection without vertical acceleration component.

Col/Beam moment capacity = 1.1

Add column tension force due to vertical acceleration + overturning forces + moment magnification.


Compute new triaxiality

Increase the column moment capacity by the ratio of :

New triaxiality/Original triaxiality.


March 17, 2014


The End

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Structural considerations for so cal seisimicity

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