Reinforcement Lap Length at Column Base - Effect on Capacity Design of Foundations 1

[kellez]

Hi,

For the design of foundations such as tie beams in Reinforced Concrete structures EC8 suggests to force hinges to form in the columns base, above the grade tie-beams during a seismic load.
In order to achieve this you simply make the tie-beams stronger than the columns they support, but what about the lap length reinforcement. The area of reinforcement at the base of the column and where the plastic hinge is expected to form is doubled due to the additional bars required for the lap length (see pictures below).

what effect will this have on the capacity design of the foundations?

 

[KootK]

Agreed. That's one of the reasons that we keep splices away from beam ends in seismic moment frames. Technically, it would have been good to have your lap shifted upwards some. That does create something of a constructability nightmare of course.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kellez]

Any idea how much should the lap be shifted upwards?

 

[KootK]

1.5X - 2.0X the larger dimensions of the columns ought to do it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kellez]

So what happens if i have a column of 500x250mm ??? (500mm is the largest dimension of the column)

The lap should be shifted upwards by = 2x500mm = 1.0meters?

that doesnt look very reasonable to me, if you have to shift the lap upwards from the foundation by 1.0meteres then what shall remain for the lap length? and how high the staters bars need to be?

 

[KootK]

Yup. 750 - 1000 mm. Both the top and bottom of the lap would need to shift such that the lap length is unaltered.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kellez]

Interesting, I keep learning new things everyday, so what about shear walls, What happens if I have a shear wall of 1200x250mm?

 

[DETstru]

kellez, just for some more info:

In high-seismic areas in the US, lap splices in columns (regardless if they are part of the lateral load resisting system) are required to be located mid height of the column. Not at the ends. This is due to concrete spalling that occurs at the top and bottom of columns under significant building drift. The potential loss of concrete makes these splices vulnerable if they are present. Keeping them in the middle reduces this risk.

You can find this in Chapter 21 of ACI 318-11.

 

[kellez]

This is very interesting because all buildings that i have seen in my country are build with splices at about only 30cm off the ground.
It just makes me wonder, are the engineers really unaware of this or could there be some other alternative which they are aware of? for example what about if the height of starters bars is increased?

 

[KootK]

The splice offset at the foundation is a theoretical requirement but, truly, I don't know how much that is happening in practice. It's a hassle.

The shear wall example is interesting. Rather than a 4' shear wall, let's consider 20' shear wall. It's plastic hinge zone may occur over 30' and there's just no way that anybody's going to be using 30' starter bars. There it's okay though. It's okay to have the splice within the hinge zone because the 2' you might need for the splice is a small percentage of the 30' height of the plastic hinge zone and, therefore, doesn't have all that much impact.

A 4' shear wall presents a bit of a conundrum as it's in the in between zone where shifting the splice seem unreasonable but it's still hard to argue that the splice length is small relative to the hinge length. I've no easy answer for that.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.