Gantry truss - Column Base plate connection - Pin or Fix ??

[tngv752]

Normally , I have pin connection for portal frame or similar structure. However, I check the gantry truss say 16-30m span with the column pinned base connection, a large deflection occur compared to fixed based.

It seem to me that I have to use column base fixed connection.

 

[Lion06]

You will always get larger frame drifts with pinned bases compared to fixe bases.

What is your question?

 

[hokie66]

It may help if you explain what you mean by "gantry truss" and better define your structural system. The word gantry is used in several different ways in structures.

 

[frv]

As StrEIT wrote, what is your question?

 

[tngv752]

I model a 16m long truss supported by columns of 6m high. Pinned base columns give very high lateral deflection. I have to go for fixed based column to reduce to member size.
Then go ahead for rigid fixed base connection design.
Is it ok ?

 

[Lion06]

Sure, it's ok. Why wouldn't it be? Just be sure to design the baseplates, anchor bolts, pedestal/pier (if applicable), and footing for the required moment.

 

[apsix]

tngv752
It's not a particularly high building, do you know why the lateral deflection is high?
Maybe the columns are just too small?
There is nothing inherent in trussed rafters which would cause excessive deflection.

 

[tngv752]

It's not economic to increase the member size. Another solution is to put bracings but the client doesn't want it.

 

[parrot77]

Where a column is nominally pinned to a foundation that is designed assuming that the base moment is zero, the stiffness of the base may be assumed to be equal to 20% of the column stiffness when calculating deflections under serviceablity loads. This may be done by introducing dummy members or spring stiffness at the column base. In the experience of the author, this will significantly reduce eaves deflections

 

[Lion06]

parrot-

Can you post a copy of that paper or give the author and a ballpark date?

 

[parrot77]

S..EIT,
The above post is based on guidence offered in The Steel Construction Institute (Berkshire U.K.)publication SCI P252 titled 'Design of Single-Span Steel Portal Frames to BS 5950-1:2000'

 

[csd72]

Other option to design a partially fixed base portal frame.

Analyse whole structure as pinned based for strength.

Provide a separate load case with a 10k.ft moment applied at each column base. This will give you the amount that the deflection is reduced for each 10k.ft of base fixity.

Use superposition to calculate what the required base fixity moment is to keep deflections at an acceptable level.

Design base to act as fixed at the serviceability level.

 

[apsix]

StructuralEIT
Reference to the 20% base stiffness for defections can be found in Advisory Desk notes AD90, AD97 & AD194.
These can be accessed by doing a search on;

http://www.steelbiz.org