## Pushover analysis of a metallic structure

## Pushover analysis of a metallic structure

(OP)

I’m modeling a metallic structure whose roof collapsed under snow weight. I modeled this structure using beam elements and I specify that the roof elements are bolted together (no ball joint).

The calculation is made in static and the snow loading is applied using linear forces.

In small displacements, the code diverges while I have not reached the steel ultimate strength; I get the following error message: “The strain increment is so large that the program will not attempt the plasticity calculation at 171 points. The plasticity/creep/connector friction algorithm did not converge at 55 points.” I tried to increase the time period, to decrease the initial and minimum increment sizes, to increase AI or to reduce the size of the mesh… Nothing works! And in big displacements, the code diverges even earlier! As a result, I can’t perform risk analysis integrating imperfections.

I would appreciate a recommendation on how to improve convergence in small and big displacements?

The calculation is made in static and the snow loading is applied using linear forces.

In small displacements, the code diverges while I have not reached the steel ultimate strength; I get the following error message: “The strain increment is so large that the program will not attempt the plasticity calculation at 171 points. The plasticity/creep/connector friction algorithm did not converge at 55 points.” I tried to increase the time period, to decrease the initial and minimum increment sizes, to increase AI or to reduce the size of the mesh… Nothing works! And in big displacements, the code diverges even earlier! As a result, I can’t perform risk analysis integrating imperfections.

I would appreciate a recommendation on how to improve convergence in small and big displacements?

## RE: Pushover analysis of a metallic structure

What's the frame look like?

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

## RE: Pushover analysis of a metallic structure

I modelled an existing structure whose dimensions are fixed. Find attached a picture of the modelled structure.

## RE: Pushover analysis of a metallic structure

what material properties are you using? are they minimum properties, or realistic typical properties?

perhaps the joints in the model are more flexible than the real structure.(?)

if the roof collapsed, why do you expect your model to converge?

## RE: Pushover analysis of a metallic structure

I'm using Abaqus.

I'm using an elasto-plastic law with next properties : E=210MPa,fy=294MPa,fu=432MPa and epseu=0.2.

I don’t necessarily expect my model to converge but the error message is not clear. I can’t know if the code is diverging because the structure collapses or because there’s a numerical convergence problem.

## RE: Pushover analysis of a metallic structure

Depth of the trusses looks too small.

Some dimensions would help.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

## RE: Pushover analysis of a metallic structure

Software error messages that are 100% correct but totally useless are a fact of life.

= = = = = = = = = = = = = = = = = = = =

P.E. Metallurgy, consulting work welcomed

## RE: Pushover analysis of a metallic structure

The building is 54 m long, 45 m wide and 8.9 m high. You will find attached a description of one single roof frame element; the outside and inside diameters of the tubes are 48.30 mm and 42.5 mm respectively.

## RE: Pushover analysis of a metallic structure

## RE: Pushover analysis of a metallic structure

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

## RE: Pushover analysis of a metallic structure

I suspect they are failing with full load.

What loads are you using and

What is the material's yield strength?

1.9m depth of truss is probably half of what you will need to control deflection.

## RE: Pushover analysis of a metallic structure

## RE: Pushover analysis of a metallic structure

## RE: Pushover analysis of a metallic structure

Using a very minimal total load of 500 N/m2 (10 psf), trusses fail generating 2 x Yield Stress.

If the roof acted as a rigid plate, lets say you can carry load in both directions, roughly dividing the stress by 2. So that system most likely fails with about a 500 N/m2 load.

Hope it doesn't snow. Its barely carrying its own weight.

## RE: Pushover analysis of a metallic structure

## RE: Pushover analysis of a metallic structure

Too much stress in the fibres.

## RE: Pushover analysis of a metallic structure

## RE: Pushover analysis of a metallic structure

I use Pa for fluid pressure and stress.

The 500 N/m2 roof load produces 42ksi member stress.

## RE: Pushover analysis of a metallic structure

## RE: Pushover analysis of a metallic structure

## RE: Pushover analysis of a metallic structure

What do you mean by "using linear forces"? Are you referring to a force-controlled analysis? Are you applying the load as a force in several steps or in one step?

Usually, one must apply the load as a displacement (incrementally or in one step) or use arc length method to model collapse.

There is no "time" in a static analysis involving only material or geometric non-linearity. Are you referring to some type of numerical damping parameter?

You may start by reading the ABAQUS documentation explaining the non-linear solvers it offers and then choose a solver suitable for your problem. ABAQUS should have no issues solving a beam problem with elasto-plastic material behavior and geometric non-linearity.

If you use a proper solver, the collapse will not cause issues with numerical convergence, and the full load-displacement diagram will be produced.