Portal Frame Column Base Connection Design 1

[Karlos80]

Hi,

I am designing a 30m span portal frame, 11m high. The frame has been analysed as a pinned frame. When designing the baseplate I found that due to horizontal forces I need to provide 30mm dim holding bolts. We also intend to cast the baseplates with concrete stubs.

My issue is that the plate cast in concrete will be very stiff and this will attract moments from the frame. I am concerned that due to these moments I will get a pull out failure (concrete cone failure) before steel yielding. As I have provided a large dim bolts for resiting shear they wont yield before concrete failure.

One option is to drop down the steel grade from 8.8 to 4.6 (195 N/mm2) to initiate a steel failure before concrete ,but I have always specified 8.8 grade to date so I am a bit slow to change our standard details.

Will appreciate all your advise.

Thank you
Karol

 

[jsha811]

First, you need to reanalyze your structure assuming a moment connection instead of pin at the base.
Second,based on what you said, you concrete stub is able to take the load. It's just because the concrete stub failure will happen before steel yielding. I am not sure if that is really something you need to worry about since you have already have enough capacity to take your design loads. But that is a good point though.
Bottom line is you can always reinforcing your concrete stub(if you don't want to resize the concrete stub) to prevent the pull out failure. That is what ACI Appendix D allows.

Moment curvature analysis,reinforced concrete column design and prestressed concrete pile design at

http://www.structurexpress.com

 

[Karlos80]

Hi Jsha811 , when checking the baseplates I calculated also moments at the base. (just to ensure that the holding bolts will fail prior concrete). The issue is a local concrete cone failure due to a pull out force before steel yielding of holding bolts.

I just want to be sure that wont get concrete pull out. The frame even designed as pin jointed it will still need to resist some nominal uplift forces due to wind etc. So if I will get a concrete pull put failure due to additional moments from the frame I assume this would effect my holding bolts and frame stability?

Maybe these local failure issues can be ignored as long as the frame and pad foundations are designed for pin bases??

 

[Karlos80]

Hi Jsha811 , when checking the baseplates I calculated also moments at the base. (just to ensure that the holding bolts will fail prior concrete). The issue is a local concrete cone failure due to a pull out force before steel yielding of holding bolts.

I just want to be sure that wont get concrete pull out. The frame even designed as pin jointed it will still need to resist some nominal uplift forces due to wind etc. So if I will get a concrete pull put failure due to additional moments from the frame I assume this would effect my holding bolts and frame stability?

Maybe these local failure issues can be ignored as long as the frame and pad foundations are designed for pin bases??

 

[jsha811]

Yes, if the your bolt or concrete stub fails, it will affect your frame. I think when you check your bolt and its anchorage in concrete, you need to model the frame as fixed at the base so that you get the correct loads(moment,shear or column uplift if there is any) for the base connection.If your bolt and concrete anchorage can take the load, I would not worry about too much about whether concrete pull out failure happens before steel yielding. Your base connection capacity is controlled by the smaller value of anchor bolt itself and concrete anchorage (pull out etc see aci appendix d). If you really want to do something to increase the concrete pull out capacity, you can add some vertical reinforcement in the concrete stub and make sure these bars fully developed through the pull out cone failure surface. I hope this helps.


Moment curvature analysis,reinforced concrete column design and prestressed concrete pile design at

http://www.structurexpress.com

 

[SAIL3]

typically a real pinned or fixed col base pl is difficult to achieve unless one goes the extra length to ensure this...most typical cases lie somewhere between pinned and fixed....I usually assume pinned for the steel design and depending on how sensitive the AB's are to overloading I may check them for a case of halfway between pinned and fixed.....or if really chicken, may check them for fixed...usually in cases of post-installed AB's where their integrity is contingent on the quality of installation....this way I bound the problem to address the above unknowns...a fully fixed condition requires a major effort to achieve.....

 

[structSU10]

see this thread:

http://www.eng-tips.com/viewthread.cfm?qid=351099

near the end I posted a paper that talks about using pedestal reinforcement to preclude concrete breakout failure. I would also recommend, as a check, to use the hilti profis program - I have verified its results against my own hand calcs many times and have never found an error for a rigid plate assumption. The nice thing is, you can tell it you are using anchor reinforcing, and then it will tell you the stress ratio, neglecting the failure modes the reinforcing takes care of.

 

[Karlos80]

Thanks Lads for all your responses.

Would you be able to give me some indication on baseplates/holding bolts sizes for 30m span portal frame and 10 meter high? Maybe a baseplate detail with a typical pedestal edge distances /rebar?

My columns are 686UBs. I assumed that the horizontal forces due to wind are taken by the holding bolts thus got 30mm dim bolts. I was going to provide 600mm embedment in concrete pedestals and keep 6xdim from the centre of the bolts to the edge of the stub.

Following structSU10 posted paper I understand that will need to provide sufficient rebar to avoid concrete failure in case my theoretical pinned bases will attract some high moments.