Steel Column Base Plate Modelling In Sap

[Waqas Haider]

Respected seniors,

It is my 1st post. I  have got help from this forum many times but posting 1st time.. i am a junior structural engineer..

i am to model a steel column resting at 8 ft concrete pedestal of isolated foundation...

The base of steel column is attached resting at pedestal with the help of a hinged type base plate...

but i am confused what type of constraint i shud provide at the connection of steel column base at pedestal....

i even dont know exactly what a constraint is...

i am to design foundations of column. so kindly let me know how to model it properly and accurately... 

[Waqas Haider]

Thank you Waseem bhai for ur reply... i am infact confuse that i have two option to design isolated footing....

 

1st way is that i model steel column with hinged connection at base. the note down its reactions in SAP and do manual calculations for the design of footing and pedestal... the reactions, noted in SAP would be imagined at the top of pedestal of footing.. then i will have to transfer this reactions from top of pedestal at the top of footing along with the moments produced due to these reactions at the top of footing by multiplying these reactions with the height of pedestal. the i can use these Fx, Fy, Fz reactions along with moments Mx, My, produced due to this transfer because reactions are acting at top of pedestal and i have transfered this to top of footing... now i can design my footing for this Fx, Fy, Fz, Mx, My either manual or using safe........

 

2nd way i have is, I can model the very same pedestal at bottom of steel column and fix the base of pedestal... in this way i can get automatically, transferred moments (Which i was transferring myself by Fx*H or Fy*H) at the top of footing...

 

But the results of both of them are not matching. And I also cant understand what type of conenction i must provide at the connection of steel column to pedestal in SAP...... Which of my way is OK out of 1 and 2?

[WR1]

WAIT!

 

I dont understand why you are transferring your reactions from top of footing to bototm of footing? second thing is why you said the reactions on top of footing will produce moment at the bottom of footing?

 

Now I tell you what you do!

 

1-Apply pin supports to steel columns in SAP.

2-Note down the reactions for manual design or export to SAFE..(upto you).

3-If you do manual design then just design an isolated footing on this reaction.

 

Bearing pressure = reaction/area of footing

 

check shear 

 

and design for reinforcement

 

simple!

[Waqas Haider]

Ni bhai... At bottom of steel column,, there is not footing... there is a concrete pedestal of 8 ft height.. and footing is below this pedestal.. so the reactions obtained from SAP, will not be at top of footing.. it will be at top of pedestal... and due to this reason, i want to transfer my reactions.............. i know how to transfer loads along with accompanying moments manually i.e. by multiplying each reaction with height of pedestal... but can i model this pedestal into sap along with steel column?? if yes, then how to model it properly?

[Muhammad Imran Zafar]

If you are providing hinge support for column then release your moments for column by selecting the column member. However i would suggest you to make a fixed connection, in that case you don't need to release moment moreover it is easy to make on site, in Pakistan

[UmarMakhzumi]

A lot is going on this thread, and I would just point out 1 thing regarding "hinge type base plate".

Your base connections can be fixed or pin.

Depends where you place your bolts around column. If bolts are insider flanges, its pin, if outside, fixed or moment. You will have to design the baseplate accordingly.

Also the connection of column to baseplate matters and you can achieve a pinned connection by welding in a special way but to keep things simple, assuming fully welded column to baseplate, stick to bolt layout to determine if its pin or fixed.

Rest, you can transfer the loads like you said. Follow free body.

Thanks.

[WR1]

In addition to what Umar said about bolds inside or outside flanges...I would add that...

 

you can model 8 feet high concrete column just as you modelled the steel column. Add another line element made up of concrete (that is 8 feet high) directly connected to steel column. It will be a continuous (fixed) connection between 2 columns as mentioned in post#6. 

 

Then at the base of concrete column you dont really need to model pedestal in SAP. just apply a suitable support according to post #7.

[ahsun]

A lot is going on this thread, and I would just point out 1 thing regarding "hinge type base plate".

Your base connections can be fixed or pin.

Depends where you place your bolts around column. If bolts are insider flanges, its pin, if outside, fixed or moment. You will have to design the baseplate accordingly.

Also the connection of column to baseplate matters and you can achieve a pinned connection by welding in a special way but to keep things simple, assuming fully welded column to baseplate, stick to bolt layout to determine if its pin or fixed.

Rest, you can transfer the loads like you said. Follow free body.

Thanks.

 

Hello Umar,

 

Can you please send me any reference book or document for your quoted statement?

 

"If bolts are insider flanges, its pin, if outside, fixed or moment. You will have to design the baseplate accordingly."

 

Thanks.

 

[WR1]

There is no need for any reference for that statement. Its simple. Flanges will resist moment and and when there are no bolts around flanges, there will be no couple (tension, compression) to resist the moment. Only axial load will be resisted by bolts near the web.

[UmarMakhzumi]

You don't need a reference book for this. Do a simple thought experiment. When bolts are inside the flange, there is no rotational constraint. However, when bolts are outside the flange, load path is throught base plate bending and considering a reasonably thick baseplate(which is 99% of the times) bolts would create a couple offering rotational restraint. You need to ensure that your bolts can handle interaction of shear and tension. This is standard parctise.

Thanks.

[Juli]

Hello.. so after analyzing in SAP, for baseplate design and connection, can I do it via SAP? or manually? or should I then  use the values in another software and what? All I need is how can I easily and perfectly design steel connections and baseplate using ETABS or SAP? 

Edited April 2, 2016 by Juli

[UmarMakhzumi]

I do them manually. I take the reactions from SAP2000 or RISA 3D and rest all is manual. There are a number of 3rd party base plate design softwares available that you can also use. 

Thanks.

[EngrUzair]

I do the connections manually, using custom made Excel worksheets. However, I use ETABS for main frame analysis & design.  You may develop steel connection design worksheets in MS Excel, a very powerful spreadsheet software, yet not very difficult for developing structural worksheets for routine design office work. 

[Amir_hp]

Steel column base plate connections are very important in transferring the column loads to the foundation. Often design engineers model and analyze the structure precisely using available software such as SAP2000, STAAD, ETABS, MIDAS, ... but in the design of base plate connections , they use simple methods that have not the correct calculation in comparison with the real behavior of base plates.

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Edited November 22, 2017 by Amir_hp
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