Load Take Downs 1

[Bender888]

How do you guys like to do your load take downs?

Do you use FEM software, equivalent frame models or just do it by hand using tributary areas ?

I have been doing it by hand but i'm considering using SAFE. Just stripping down the architectuals and importing the floor plans in an assigning the relevant propeties.

 

[rapt]

Using FEM software makes the assumption that the whole structure is built in one operation. It does not allow for construction sequence.

If there are transfer members or large variations in framing stiffness, the FEM takedown will often be very wrong, especially for transfer members where it will put too much load in non-transferred columns and not enough in transferred columns. This leading to under-design of tranbsfer beams.

 

[msquared48]

By Hand... Using computer beam solutions where appropriate and integrating the solutions.

Mike McCann
MMC Engineering

 

[structuralhawaii]

We're doing it by hand here..

 

[Bender888]

Thanks for the responses.

 

[Grahammachin]

I do them by hand prior to running any computer simulations.

This allows for very basic checking of the model, making sure the model is basically correct, prior to a more in depth look at the members and stability.

 

[asixth]

I never do load run down's by hand these days. Mostly I use the reaction output from a computer package but I still have a speadsheet set out so I can quickly verify the reaction output if required.

 

[slickdeals]

I use SAFE or RAM Concept for computing tributary areas on complicated floor plates where columns are all over the place. I import the CAD model into RAM, and assign a load of 100 psf and get column reactions. Load / Reaction gives me tributary area for each column. I use this in a spreadsheet to calculate total building loads for all floors with appropriate load combinations and verify gravity loads from the lateral analysis model.

 

[csd72]

Why make things any more complicated than they need to be? Hand take downs can be easily checked by hand, FEM take downs are more complex to check.

I agree with rapts comment, sometimes you need to take worst case scenario in some areas.

 

[ron9876]

You can use ACAD to set up and measure trib areas. This is approximate but everything with concrete is somewhat approximate.

 

[inju]

I won't depend much on FEM output from SAFE, ETABS or any other unless I 'spot check' typical and non-typical conditions by hand.
We have a spreadsheet setup for Load Take Down but, We would like to compare the results of FEM load reaction by hand. Using customized spreadsheet may be a great time-saver.
CAUTION: you may not get a proper load reaction from FEM anaysis unless the floor is 'properly meshed'

 

[rapt]

Asixth/Slickdeals,

As I said above, there is no problem with this except cases where there are transfers. Then the real situation is somewhere between tributary areas and FEM, but FEM definitely underestimates the reactions in the transferring columns unless you allow fully for construction sequence in all cases and stressing sequence if prestressed!

 

[COEngineeer]

I use layer coloring system with autocad. Then I just copy and paste it next layer with an X mark so I know where the point load is and /\/\/\/\ mark if it is a distributed load from above. Works very well on residential project where things dont line up usually (crazy architect). Commercial projects are usually pretty straight forward and RAM does it all for me

Never, but never question engineer's judgement

 

[Einsteim]

I also use a colored system from framing plan to framing plan all the way down to the foundation. All by hand, as I am not 100% warm and fuzzy trusting the software. It is very satisfying however when the software results are within tolerance of the hand calcs.