Analyygzing with Delta · 2015. 2. 7. · P‐Delta Deflection Little P‐Delta: fl i 226i 1 021...
Transcript of Analyygzing with Delta · 2015. 2. 7. · P‐Delta Deflection Little P‐Delta: fl i 226i 1 021...
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Analyzing with PAnalyzing with P‐‐DeltaDeltay gy g
Presenter:Presenter: Deborah Penko, P.E.Deborah Penko, P.E.
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What?What?
Wh ?Why?
When?
How?
PP‐‐Delta?Delta?
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Definition:
Destabilizing moment equal to the force of gravity multiplied by the g q g y p yhorizontal displacement a structure undergoes as a result of a lateral displacement.
P (Force of Gravity) x Delta (Horizontal Displacement)
What is PWhat is P‐‐Delta Effect?Delta Effect?
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Step 1: Model deflects Δ
Step 3: Model is re‐solved with V applied
P*∆=V*LV= P*∆
LStep 2: Secondary shear force (V) calculated
Step 3: Model is re solved with V applied
Step 4: Iterate until the model converges
What is PWhat is P‐‐Delta in RISA?Delta in RISA?
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P‐ d Little P‐Delta : curvature of the element
RISA Implementation:
Add Intermediate Joints to the element
What is little PWhat is little P‐‐Delta?Delta?
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PP Delta Required by CodeDelta Required by Code
AISC 13th & 14th Edition Design for Stability Direct Analysis Method
PP‐‐Delta Required by CodeDelta Required by Code
AISC 13th & 14th Edition‐ Design for Stability Direct Analysis Method
Second Order Analysis (P‐ Δ, P‐ δ)
ACI 2008 & ACI 2012
Nonlinear Second Order Analysis (10 10 3)Nonlinear Second Order Analysis (10.10.3)
Elastic Second Order Analysis (10.10.4)
Moment Magnification (10.10.5)
Foreign Codes (CSA, etc.)
Why?Why?
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1. Design Gravity Systems1. Design Gravity Systems No P‐Delta
2. Design Lateral SystemP D l ?P‐Delta?
3. Final DesignP‐Delta
When?When?
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P‐ Delta Deflection
Little P‐Delta:
fl i 226i
1 0211.166
1.795 1.807 1.809
Deflection: 1.226in3% Increase
1.021
1 2 3 4 5
Strong Axis Bending % Change
Deflection
1st Iteration: 1.021in
AISC 14th Edition:
Final Deflection: 2.342in2nd Iteration: 1.166in 14.20%
3rd Iteration: 1.186in 53.95%
4th Iteration: 1.189in 0.67%
5th Iteration: 1.19in 0.11%
96% Increase
PP‐‐Delta ExampleDelta Example
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Let’s Try the Weak Axis:
Deflection
Let s Try the Weak Axis:
41 22
65.432
99.439
9.84823.22
41.22
1 2 3 4 5
Weak Axis Bending % Change
Deflection
1st Iteration: 9.848in
2nd Iteration: 23.22in 135.78%
3rd Iteration: 41.22in 77.52%
4th Iteration: 65.432in 58.74%
5th Iteration: 99.439in 51.97%
PP‐‐Delta ExampleDelta Example
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The P‐Delta effect increases the flexural stiffness of members in tension
Compression OnlyCompression Only
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How do we get past a P‐Delta g pDivergence?
1. Turn off P‐Delta
2 Run the model2. Run the model
3. Review Deflection3. Review Deflection
4. Review Design Results
How?How?
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Instabilities
Inadequately sized members
Tension/Compression Only Members
Stiffness Adjustment (Direct Analysis method)
Model Errors
Common PCommon P‐‐Delta ProblemsDelta Problems
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Let’s review some examples!p
Common PCommon P‐‐Delta ProblemsDelta Problems
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Questions?
Please let us know if you have questions.
We will answer as many questions as time permits during the webinar.
Once the webinar is closed, we will post all Q&A’s to our website: www risa comwebsite: www.risa.com
f h f b hFor further information, contact us at: [email protected]
Presenter:Presenter: Deborah Penko, P.E.Deborah Penko, P.E.