Steel gravity columns design

[karthik_m]

When we design steel gravity column design do we need to consider the drift induced moments in the steel gravity column? I'm modelling the steel in RAM Structural system and I don't think RAM does it since I don't see any moment in the gravity column. Please let me know. Thank you

 

[jayrod12]

Yes you do. I'm not familiar with RAM however is it possible you have P-delta effects turned off?

 

[Once20036]

This drift won't impact the design of your gravity column.
The load times the drift could impact your lateral system.

 

[karthik_m]

@jayrod12 RAM does lateral and gravity analysis separately. So yes P delta is turned on but it is in the lateral part of the model which designs only the lateral members I guess. But I wanted to know if that should be something we should be concerned about. Also, to get the moments we need to consider the interstory drift right?

 

[karthik_m]

@Once20036 If the building drifts in EQ then wouldn't the gravity columns also see the same drift and then the moment caused due to P delta ?

 

[KootK]

I agree with Once20036's comments with the following, subtle modification: for the moments in the gravity columns to be ignored, the lateral stiffness of the gravity framing must be relatively small compared to the lateral stiffness of the designated lateral force resisting system. For many conventional framing arrangements, this is true and ignoring the drift induced moments in the gravity framing is appropriate. Like most things though, it requires some degree of engineering judgments. Consider this scenario:

1) every beam in a building is W24x96
2) every column is W8x48
3) every beam-column joint is welded up for moment.

Obviously, in this case, it would not be appropriate to ignore drift induced moment in any of the columns just because they are designated as "gravity only".

Effectively, this is just an extension of the principle that stiffness attracts load.

 

[karthik_m]

@KootK Thank you, I agree with your explanation. But if that is the case then why do we check drift induced moments for gravity concrete columns in a Slab only system without beams where the main lateral force resisting system is Shear wall ?

 

[Celt83]

the cast-in place concrete by it's nature will create rigid joints where as the typical shear connections in the steel construction allow for some slip.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Once20036]

Karthik,
Think about your free body diagram.
It's a column with a pin at the base and a roller at the top. If you displace the top of the column, the column will rotate. You`ll need a horizontal reaction at the top to provide stability. Where does that moment that you're concerned with come into play? Can you provide a free body diagram to show that moment?

KootK - You`re right that I was assuming pin connections. I`m also assuming a very small angular rotation. I think that your moment connected scenario you describe is a different beast. I`m having a hard time imagining why you'd have all those moment connections if not for lateral loads, but maybe that question is best left for a different thread.

 

[KootK]

I think that your moment connected scenario you describe is a different beast. I`m having a hard time imagining why you'd have all those moment connections if not for lateral loads, but maybe that question is best left for a different thread.

Of course it's a different beast. The whole point was to create a hyperbolic example in which steel gravity columns would be appropriately designed for drift induced moments. Since such a thing is a rarity, there was really little choice but to dip into the absurd for source material. It sounds as though OP got the point of it though so I think that all is well with respect to the example.

But if that is the case then why do we check drift induced moments for gravity concrete columns in a Slab only system without beams where the main lateral force resisting system is Shear wall?

Touche... sort of. Here are some compelling reasons why this is checked in the seismic design of cast in place column/wall buildings:

1) The naturally occurring slab / column moment connections that Celt mentioned.

2) The primary issue of concern in CIP systems is punching shear failure at the slab to column joint. It's brittle and easily overtaxed under seismic drift. You don't commonly have anything analogous in steel.

3) If you're comparing the stiffness of a forest of large columns to a relatively small core wall assembly, there often is not a large relative difference in the stiffness-es of all the columns vs the shaft(s).

4) This one may be a bit difficult to grasp but is critical. In tall, multistory shear wall buildings the inter-story drift at the upper levels is roughly equivalent to the slope of the shear walls at those levels multiplied by the story height. And those slopes are the product of all of the flexural curvature occurring in those walls at all levels below. Long story short, upper level inter story drifts can be high even though floor by floor shear stiffness of the walls still grossly dwarfs that of the columns. The columns, in being forced to go along for the same inter-story drift ride, tend to attract meaningful moments at their ends.

 

[karthik_m]

http://www.iitk.ac.in/nicee/wcee/article/14_05-05-0112.PDF

Guys I'm referring to this paper! Please let me know what do you think Thanks!

 

[oopenhegan]

Would you still ignore drift effects on a single story steel column in a short 1 story w/ approx. 3½% drift? My structure is designed to MDE under non-linear time history analysis, so doesn't need to follow the drift limitations of ASCE 7 ch. 12.

 

[karthik_m]

@oopenhegan: I really didn't understand. Is the question towards me? I am analyzing my building just by modal analysis ( linear dynamic analysis).

 

[oopenhegan]

@karthic_m more of a question for the group along the same topic. When should we ignore the effects of story drift on gravity columns and when we should pay attention to them. Especially if drift is very large. When is a pinned connection going to rip apart under story drift?

 

[JoshPlumSE]

I'd like to separate this discussion into two questions.

1) Is it necessary to design the gravity columns to resist moments and shears due to the P-Delta effect of lateral drift?
Ans: Only if those P-Delta effects cause moments and / or shears in those columns. I think this is what most people have been referring to.

In that paper, the columns were assumed to have fixed bases. Therefore, they will experience some moment and shear due to lateral drift. So, they should be designed for that. If the columns were "pinned" at the base then they would not have to be designed for it. That being said, my tendency is to think of thim as fixed and make sure the base can resist a moment of P*Delta for the column.

2) Is it necessary to design your gravity only columns (and column connections) so that they are able to accept the lateral drift?
Ans: Yes, your connections should be capable of handling whatever drift you are expecting from design loads. Let's say you have a seated connection, if your drift is going to pull the beam off its seat, then it totally changes the load path assumed in your connection design.

If the gravity / shear connection is not sufficiently ductile to accept the lateral deflection, then you will have problems.