AASHTO 8th Edition - Equation 5.6.4.6-1 (spirals)

Brandon · Mar 6, 2019

In AASHTO 8th Edition, Equation 5.6.4.6-1:

rho=(4Asp)/(dc*s) >/= 0.45(Ag/Ac-1)(f'c/fyh)

does anyone know for certain if the term "Asp" is meant to be just the area of a single bar of the size being used for the spiral, or is it twice that (i.e. "2 legs" as for shear reinforcing)? I have a 42" drilled shaft with 6" clear cover to the spiral, f'c=4ksi, fyh=60ksi, and transverse reinforcing is not otherwise required for shear or seismic. It seems that this results in the core area (Ac) being small relative to the gross Area (Ag), which makes the second term of the above equation relatively large (0.0269), and I can't seem to get it to work for reasonable spiral bar size and pitch (I calculated that I would need #8 spiral with a 3.5-inch pitch which just seems absurd to me). If Asp is actually supposed to be 2x the bar area, this at least gets me to #6 spiral with a 4-inch pitch.

The reason for the 6" clear cover is that there is a smaller (36") rock socket below the portion of the drilled shaft in soil and the cage is just being extended up into the 42" shaft.

Thanks in advance.

BridgeSmith · Mar 6, 2019

Asp is the area of the bar. It isn't shear reinforcement; it's confinement. Along the face of the core at any point the area is just the bar on that face.

The reason it goes so high with that much cover is that the spiral is meant to provide 'replacement strength' for the concrete outside the core, in case it spalls off during a seismic event, etc. We ignore the requirement for drilled shafts, opting to design the drilled shafts to be adequate using only the core concrete. There's virtually no way to meet that spiral reinforcement requirement and the reinforcement spacing requirements in 5.12.9.5.2. For a drilled shaft, where vibration of the concrete deep in the shaft is generally not possible, the subsection specifies a minimum clear spacing of 5 times the max aggregate size or 5".

Section 10.8.3.9.3 refers you back to 5.6.4 for the laterally unsupported portions of drilled shafts where lateral load is insignificant only. Otherwise, it directs you to 5.11.

Brandon · Mar 7, 2019

Thanks for taking the time to reply HotRod10. When you say you ignore the requirement for drilled shafts, do you mean you ignore 5.6.4.6? If you design the shaft for the core concrete only, then what do you use as the basis for determining the required transverse reinforcement, since under this approach Ag=Ac and therefore equation 5.6.4.6-1 would indicate required transverse reinforcement of zero?

10.8.3.9.3 doesn't specifically address shafts that are laterally supported (which is the case I am looking at). It seems for that case you would design per the requirements for compression members in Chapter 5, which eventually gets you back to 5.6.4.6 for the required transverse reinforcement.

Where does this seemingly catch-22 situation end?? I apologize if I am being obtuse ;-)

BridgeSmith · Mar 7, 2019

"When you say you ignore the requirement for drilled shafts, do you mean you ignore 5.6.4.6?"

I suppose I shouldn't say we ignore it; I should say our executive staff (for the DOT Bridge Design section) made an executive decision (as the owner's representative) not to add an excessive amount of reinforcement in order to meet that provision. The directive from executive staff is to use no more than #5 spirals at 6" spacing.

"10.8.3.9.3 doesn't specifically address shafts that are laterally supported (which is the case I am looking at)."

We have taken it that the provisions apply to laterally supported shafts, except for the provision that is specific to shafts that are unsupported.

"I apologize if I am being obtuse"

Not at all. We've gone around in circles with this a few times with every new edition of the spec, and there's definitely some circular referencing going on. After the numerous iterations the spec has gone through, one would think it would be straightened out by now, but it sure doesn't seem so.

Brandon · Mar 8, 2019

Thanks again HotRod. I'm going to follow up with our state DOT. Their manual previously stipulated #4 spirals at 4.5-inch pitch, even if that didn't meet 5.6.4.6. A revision to the manual issued in January removes the #4x4.5 and simply requires compliance with 5.6.4.6.

BridgeSmith · Mar 8, 2019

You might ask your DOT people if they have a limit on the maximum cover they consider in the equation.

I'd be curious to know what feedback you get from them.

There's always the option of overlapping a larger cage above the socketed portion of the shaft, so that you have less cover. It would be somewhat of a PITA to fabricate, but likely preferable to using a #8 spiral, if you're backed into a corner.

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AASHTO 8th Edition - Equation 5.6.4.6-1 (spirals)

Brandon

Civil/Environmental

BridgeSmith

Structural

Brandon

Civil/Environmental

BridgeSmith

Structural

Brandon

Civil/Environmental

BridgeSmith

Structural

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