collapse analysis basic

Collapse Basic 1

Collapse Analysis Basic• Collapse is a Large Deflection, Elasto-Plastic Non-

Linear finite element software tool for structural analysis.

• Fully Integrated into the SACS suite of programs. Same input as standard SACS analysis and does not require any additional modeling.

• Same input as standard SACS analysis and does not require any additional modeling file.

3/3/2011

Collapse Basic 2

Collapse Capacities - 1

Collapse program takes into account the following items:

• Geometric and material non-linear behavior.

• Non-linear elasto-plastic pile/soil foundation behavior.

• Both overall and local member buckling.

• Joint flexibility, joint plasticity and joint failure.

3/3/2011

Collapse Basic 3

Collapse Capacities - 2

• User defined non-linear spring elements (energy absorb devices in boat landings).

• User defined ductility limits to account for member fracture.

• Creates analysis results file that is read by Collapse View program which shows failure progression and the gradual plastification and collapse mechanism graphically.

3/3/2011

Collapse Basic 4

Beam Element: Hinge Formation - 1

• Collapse allows for hinge formation at any point along the member length by subdividing each member into sub-elements and monitoring the stress in each sub-element ( hinge formation not limited to member ends and center - this pre defines the failure mechanism).

• By default, each member is divided into 8 sub elements. Collapse allows for a maximum of 20 sub-elements per member.

3/3/2011

Collapse Basic 53/3/2011

Beam Element: Hinge Formation - 2

• Collapse allows the gradual development of plastic hinge through member cross section.

• Divide cross-section into sub areas and monitor the stress level in each sub-area.

• Tubular cross sections divided into 12 sub areas.


Plate Element

• Collapse allows plasticity to occur gradually through the plate thickness .

• Sub-divide the plate thickness into sub layers (5) and monitor stress levels in each sub-thickness .


Yield Criterion - 1

• Collapse uses Von Mises-Hencky yield Criterion to determine the onset of plasticity.


Yield Criterion - 2

• If the stress levels exceed the Von Mises yield surface at a particular sub area in a member, then the whole sub area (sub element volume) is assumed to be in a plastic state.

• If the stress levels in all sub-elements at a cross section exceed the Von Mises yield surface, a plastic hinge is formed at this cross section.


Yield Criterion - 3

Similarly for plates, if the stress exceeds the Von Mises yield surface at a particular sub thickness then the whole sub-thickness layer across the plate element is assumed to be plastic.


Material Properties

Material properties include bilinear stress strain profile including user defined strain hardening .

Strain hardening ratio = Ep/Ee.


Analysis Procedure -1Typical Collapse analysis procedure begins with the application a load increment to the structure


Analysis Procedure -2Calculate the internal load at each end of each sub-element for each member.


Analysis Procedure - 3

Calculate the axial and shear stress at each sub-area of each end of each sub-element.



strain

Calculate the plasticity by the amount of strain exceeding Von-Mises stress envelope. Retain this strain for each sub area throughout the load sequence for subsequent loading.

stress

Plastic strain


Use elastic stresses to compute self-equilibrating plastic forces on each sub-area.




Add plastic forces to the global load vector and iterate until the deflections and rotations have converged at each member end and also the sub-element ends.

Update the stiffness matrix, apply the next load increment and repeat the procedure.

[K] [x] = {F}


Load Increment Size

Collapse allows user to control load increment sizes through loading history.

Large increments in linear region.

Small increments in non-linear region.

d

P


Limit Point - 1Global Limit Point indicating overall structural failure.

Collapse solution will diverge and stop when load is increased beyond the global limit point indicating structural failure.

d

P

Global limit point

ΔP

Unstable

Stable


Limit Point - 2

Local Limit Point indicates local structural failure. Collapse solution will jump to the next stable configuration when load is increased beyond the local limit point.


BucklingGlobal BucklingCollapse can predict elastic buckling including full elasto-plastic behavior.

Local BucklingFour methods available to predict local buckling(1) API LRFD(2) Marshal Gates low limit of critical strain (3) API Bulletin 2U(4) ISO 19902 Section 13

A moment free hinge is inserted at the location of a local buckling point – axial capacity retained.


Joint Flexibility

Joint flexibility is the distortion of chord cross section due to forces in the brace and chord. It is particularly important for old structures where joint cans were not used.

The following three methods are used in collapse to predict joint flexibility.

(1) Fessler’s Approach(2) Single brace formula by Buitrigo, Healy, and Chang(3) MSL Approach


Ductility Limit

For any sub-element in the member if the strain exceeds the defined ductility limit, it is considered that the member is fractured.

P P


Ship Impact - 1

Design Criteria - Energy calculation

The total energy is absorbed by 1) vessel hull; 2) structure.


Ship Impact - 2

Energy absorbed by vessel hull: Collapse program has DNV curves for 5000 MT ship hull built in.

Energy absorbed by structure: Estructure = Etotal – Ehull


Ship Impact - 3

Energy absorbed by Structure:

• Local deformation of the impacted member due to denting and beam bending.

Denting member modeling: 1) isometric plate elements; 2) tubular members

• Global deformation of the entire structure.


Ship Impact - 4Features in Collapse Program

• The automatic unloading part can be calculated once the maximum impact energy has been absorbed.

• DnV ship indentation curves are inbuilt into Collapse.

• IMPACT line added to Collapse to define impact load case, point of impact, impact energy, ship indentation curve and automatic unloading.

• ENERGY line added to automatically calculate the kinetic energy of a moving object

• SHPIND line added for a user defined ship load-deflection indentation curve


General Considerations - 1

Iteration Tolerance Levels :

Deflection tolerance : 0.01 cm (Default), for large structures this can be increased to 0.1cm

Rotation tolerance : 0.001 RAD (Default)

Member Deflection Tolerance: 0.01 cm (Default)


General Considerations - 2Convergence

Use the continuation option to continue if the maximum number of iterations has been exceeded. (pass local limit point)

Avoid small elements to avoid member convergence problems.

Avoid very slender elements to reduce the number of iterations per load increment.

Avoid member end releases which may produce a mechanism.


General Considerations - 3Load Sequence

Collapse allows up to six load sequences to be defined – each load sequence is analyzed as an independent analysis.

Collapse only records the last load sequence for Collapse View.

Pile/Soil foundation

If the analysis includes pile soil interaction, the soil data used in the program are from standard T-Z and P-Y curves.


General Considerations - 4Collapse Runtime

Modeling parts of the structure which have little contribution to overall stiffness of the structure (boat landings, risers, I tubes) as dummy structures.

Parts of the structure (deck beams) whose elasto-plastic behavior is not important should be kept elastic.

Pre-combining loads where ever possible.

Including strain hardening to improve solution convergence.

collapse analysis basic

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