(Special moment frame) where to splice column if 2 floor levels requires different size of bars 2

[Penpendrum]

We have a plan that illustrates a special moment frame .. the structural notes states that column splices must be done at midheight also aci code states that splices must be 2D away from support if i am correct . .

if the column rebar required is 25 mm at the 1st level and the column rebar required is 20 mm at the 2nd level ... does this mean that I have to lap splice within the floor line and this is an exemption from the code???. . .

 

[wannabeSE]

The design intent probably assumes the larger bar will lap with the smaller bar at mid height between the 1st and 2nd floors. If the construction documents are not clear, submit an RFI for clarification.

Lap splices shall be permitted only within the center half of the member length . . .

 

[Penpendrum]

WannabeSE..what would you mean by probably? Do you mean i dont need to complete the 25 mm at 1st level...

 

[hokie66]

If your splice has to be at midheight, run the 25 bar up to splice with the 20 bar midway between the 1st and 2nd floors. What is hard to understand about that?

 

[Penpendrum]

Sorry hokie66, i must have mis read it. . . I was just confused because .. when we do our structural analysis we would usually design the column with the same bars from end to end of the column.. if there is an alteration of sizes at midheight , will this compromise the design?

 

[JoshPlumSE]

You want to have the bars fully developed at the location of maximum demand (which is at the floor level). Therefore, it's best to splice the bars closer to mid-height. Sometimes for ease of construction this will be done a bit lower than mid-height.

Remember your analysis is just a mathematical representation of what you're building. It doesn't have to match exactly. You are using that model to understand the demand on the structure. The model generally once understand rebar detailing, or development length or such. So, once you understand what sort of force and moment is demanded of your structure you are free to detail it out in a way that can be more easily constructed than what was idealized in your analytical model.

 

[KootK]

when we do our structural analysis we would usually design the column with the same bars from end to end of the column.. if there is an alteration of sizes at midheight , will this compromise the design

That's an interesting point. With the exception of the column to foundation connections and, perhaps, the column to roof connections, we generally design our columns with the hope that they will not form plastic hinges internally and, rather, will force plastic hinges to form in the connecting beams. Where that is the case, it seems reasonable and conservative to design the columns based on the smaller diameter rebar coming into the member from above or below.

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.

 

[Penpendrum]

by KootK "Where that is the case, it seems reasonable and conservative to design the columns based on the smaller diameter rebar coming into the member from above or below."

hello KootK. . thankyou for your time explaining to me.. sorry for the late replay.. but can you please elaborate what you mentioned above . . I cant seem to grasp it.. by what you said "coming into the member" do you mean to different bars meeting at midheight..

 

[Penpendrum]

thankyou JoshPlum for explaining to me.. now I know better

 

[KootK]

by what you said "coming into the member" do you mean to different bars meeting at midheight..

Exactly. You're generally designing your columns such that they have a minimum strength. Basing your column strength on the smaller of the bars coming in from the top or the bottom would accomplish 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.