As the 3D Analysis determines a set of design forces which can then be used to design the members - why does Design (All) then carry on and do other "Chasedown" analyses?

The answer relates to expectation of results - in particular with regard to the following:

• Sway Effects under pure gravity loading
• Transfer beam designs
• Differential axial deformation

Consider this 8 storey model, when you click Design All, grillage and FE chasedown solver models are created in addition to the 3D Analysis model.

The Identified end frame below is resisting the sway - the design moments shown would be given by any analysis software.

It is an extreme example - but this result does not fit with traditional engineering expectation

The 3D Analysis results (below left) can be compared with those from Grillage Chasedown (below right).

The Grillage Chasedown results are more in line with expectation:

• -ve moment at LH end
• Similar moment profile at every level

Another example of why chasedown is wanted follows below....

Focus on the transfer beam in the highlighted frame.

At first glance the results look ok, but lets look more closely...

The 3D Analysis results (below left) can be compared with those from Grillage Chasedown (below right).

• 3D Analysis - frame deflects and loads are shared according to stiffness.
• Grillage chasedown - loads are collected floor by floor and then applied to the transfer beam - much higher moment in the transfer beam. Once again this is more in line with traditional expectation.

A third example of why chasedown is wanted follows below....

In this case a two span beam sits on walls at each end and a column in the center.

3D Analysis Results:

• Column is more highly stressed than walls and deflects downwards shedding load back to the walls.
• You end up with completely different moment profiles on beams that would traditionally be considered and designed as identical.

Grillage chasedown Results:

• the sub-models at each level are almost identical and you get very similar results over the height of the building.

• Grillage chasedown emulates a more traditional design style where continuous beams or sub-frames are considered in isolation.
• The 3 examples show where 3D Analysis gives results that are not liked (based on traditional design expectation).
• But once you start to think about it you may conclude that actually, the 3D Analysis result should not be ignored.
• By running Design All each analysis type is performed and members are simultaneously designed for each set of results.