Concrete Detailing Question(s)

[Chosen One]

Two questions for some of you west coast guys who may be detailing concrete structures for seismic requirements:

Seismic Development: per ACI 21.7.5 bars should be developed in accordance with Equation 21-6 for a hooked condition and increased by 2.5 or 3.25 for a straight condition. I get this straight forward requirement, however in the example books I have this is nearly always ignored and the basic equations from ACI 12.2 are used...this includes splicing bars as well (Class B essentially)....am I missing a footnote or something else saying the seismic development can be waived for certain conditions? I can't find one example problem where 21-6 is used.

Resistance Factors (Phi): I understand the 0.6 factor from 9.3.4 should be used when shear capacity controls over flexural capacity (i.e. shear in a beam in special moment frame is based on flexural capacity of members so .75 can be used) and will normally control a shear wall (and doweling requirement at base). My question is in a intermediate or special moment frame would you ever use the 0.6 instead of 0.75 or 0.9? Seems like shear is always dependent on the nominal or probable flexural strength in these designs. I guess a shear wall could use a higher phi as well since flexure could control, although it is time consuming to calculate.

kevo? McEngr?

 

[Ble_PE]

I'm not sure if this will answer your questions or not, but here goes.

1. Section 21.7 is for joints of special moment frames so that could be the reason that the example problems calculate the development length differently.

2. I'm not completely sure about this, but I wouldn't imagine that you would see a beam in a special moment frame being controlled by shear capacity over flexural capacity, but I guess it could happen. It's just one of those things you have to check each time.

 

[Chosen One]

Thanks Ble.....makes sense, I guess most of the problems I was reviewing were for special shear walls, hence the chapter 12 standard developments were used...the NCEES Sample Exam Lateral essay problem calculating the splice length at transfer between the wall and footing was throwing me off.

As for the phi used in concrete moment frames, intermediate moment frames calculate the shear required in a beam based off of the nominal flexural capacity of the beams and special moment frames use the probable moment (1.25fy); columns use the flexural strength of the columns, but not to exceed the flexural strength of the beams.....so long story short the 0.6 doesn't apply in either scenario.

 

[McEngr]

I will think more (and investigate the shear phi factor question), but I would think phi = 0.75 without looking... I will let you know.

I assume you've checked whether Vu is controlled by 2Mpr/L???

 

[McEngr]

If you're considering special moment frames, usually these are dictated by LO for columns and 2h for beams. You have to essentially splice outside of 2h which should govern for interior beams. Exterior beams require at least a 90 degree hook unless it is spanning into a collector element within a shearwall.

Per ACI 21.5.2.3 states that no lap splices are allowed where flexural yielding occurs. Essentially it requires that splices are not allowed where anticipated yielding and probable failure (hinging) occurs. This significantly limits where straight bars (developing full yield) can be. This is probably why you aren't seeing an example problem - because there are so many other requirements that dictate.

Also, SRC shearwalls usually have no lap splices at all at the lower 1/3 of the wall because yielding occurs over such a large quantity of bars. The SEAOC Volume 3 has a good discussion of how to guestimate bars without the use of a spreadsheet.

 

[McEngr]

Another comment that might save your bacon: if you haven't gone through the development of bars in beam to column joints of chapter 4 of the 2008 pca notes, you might have a better understanding of all your seismic examples.