help me with end lenght offset plz! 1

[beamblanck]

Hi all,
I need some help about the command "end lenght offset"

PROBLEM 1

As you can see in the attachment i've a a problem witha column.
I've a column (a simple pipe) 4000mm high linked between two profile 500 mm high. I need to follow the distance between the neutral axis of the profile that is 4500 mm so when i model the wireframe structure i have to stretch my columnf of 500 mm (250 mm each part), altought the "free high" (i don't know in english, sorry) is always 4000mm.
This is a case where i've to use the end lenght offset command? and again...theoretically, modelling all the connection in the wireframe mode the "end lenght offset" command should be used everywhere?

PROBLEM2
my column is a pipe of several diameters, why if i divide the column in many parts and i check the columns with the eurocode 3 i obtain different ratios in every frame.
in practice: if a use an entire singular fram i obtain a ratio lesser than 1, if i divide the same frame in two, three different frame i obtain that some part had a ratio major than 1 so...not acceptable.


I hope you'll understand
many many thanks to all the people of the forum
Alberto.

 

[agostage]

For your first problem the answer is yes, you can use the end length offset in SAP. What it does is exports forces only to the face of the connecting section and puts a rigid link in as the offset.

I'm not sure what you're asking in your second question. If your section has different cross sections at different heights, then it's capacity will change at each different cross section yielding different ratios. That is what should happen. However I cannot speak for the eurocode. But I assume it's similar.

 

[beamblanck]

sorry agostage, in the second question i made a little bit of confusion. to undertand better immagine a simple column with an unique cross section from the start till the end.
(also forgot my problem about the end lenght offset).
In practice if i check the entire colum as a unique frame i obtain a ratios lesser than 1 so it's ok. if i try to divide this unique frame in two three etc.. frames (but mantaining the same sections and the same loads, just dividing a frame in more frames), then start the problems, because if i check again the structure, some frames have a ratios major than 1 so are not approved. why the program do this?
many thanks for the problem 1, i think i'll use the end lenght offset and i set the rigid zone factor to 1 (completely rigid)
Many thanks and sorry for my english

Alberto

 

[agostage]

I see what you mean. Off the top of my head I can think of two possibilities. Either your capacity is changing as you split the members, or it's attracting more load. If your capacity is changing it's likely that SAP is not assigning your length factors correctly to the segments. The default factor is 1, but if you split the member it should be changed to 2. Depending on your order of operations, SAP is known to mess this up. That will change the axial capacity of the segment and give you an incorrect answer. Another possibility is that you're attracting more load. This can occur in a nonlinear or buckling type analysis. By splitting the column your adding an equation at the mid span and allowing SAP to capture geometric non-linearity. This would likely cause the forces in the member to increase. In either case, your ratio would increase.

 

[beamblanck]

thanks again agostage,
the analysis is a simple linear so i think the problem must be searched in a non correct distribution of the lenght factor. Tomorrow i'll made some test and i ope to fix he problems.
many many thanks agostage and sorry for the time wasted

I'll be back with new information!

Alberto