P big delta with pin base design assumption (brace frame, moment frame, tilt-up)

[engjg]

Interested in other's thoughts on P big delta affect when designing with a pin base design assumption.

With a pin base idealization, p big delta doesn't theoretically exist, and drift of structure would just result in a shear thrust at the base, no?

But since pin idealizations usually always have some fixity in reality, when/how does current standard practice consider these effects when modeling/designing a structure with a pin based idealization?

Three scenarios I am currently thinking about:
Steel braced frame
Steel moment frame with pin bases
Tilt-up concrete wall (for the sake of discussion consider a warehouse dock wall with slab support at 5 ft above footing).

 

[EZBuilding]

Big P-Delta does occur with considerations of a pinned based design assumption. The additional moments generated by the deformation of the structure under lateral loads will have to be resisted by the same mechanism that resists moments produced by the lateral loads in the first place!

 

[engjg]

My second example was bad; P big delta certainly exists there but would be underestimated with pin assumption.

EZBuilding - How about we talk about a pin pin gravity column subject to drift at top. I agree compression would go up due to p big delta because of angle but secondary moment effects due to unintended fixity at base would not be considered. What are your thoughts on that?

 

[KootK]

We did some pretty good work on similar subject matter recently here, including the sketch below: Link

 

[KootK]

I agree compression would go up due to p big delta because of angle but secondary moment effects due to unintended fixity at base would not be considered. What are your thoughts on that?

Yes, you would get P-Delta induced secondary moments in your columns. The hope in addressing that would typically be:

a) the flip side of ignoring those moments is that you're also ignoring the rotational restraint that would lower your effective length on the column.

b) a building's lateral system should generally be controlling drift tightly enough that large moments are not developing in gravity columns as a result of story translation.

The only time that I worry about inadvertent gravity column base fixity is when the lateral system is a high drift moment frame.

 

[KootK]

With a pin base idealization, p big delta doesn't theoretically exist, and drift of structure would just result in a shear thrust at the base, no?

I agree with that.

Three scenarios I am currently thinking about:
Steel braced frame
Steel moment frame with pin bases
Tilt-up concrete wall (for the sake of discussion consider a warehouse dock wall with slab support at 5 ft above footing).

Steel Braced Frame

With concentric braced frames, drift should be controlled tightly enough that gravity columns don't see significant moments as a result of story translations.

Tilt-Up Concrete Braced Frame

With normally proportioned shear walls, drift should be controlled tightly enough that gravity columns don't see significant moments as a result of story translations.

Steel Moment Frame with Pin Bases

I would give some measure of consideration to whether or not lateral system drift would adversely affect gravity column bases having incidental rotational restraint. I'd examine the column with the anticipated drift + diaphragm deflection considered and see what the base moments looked like in that scenario. It often doesn't add up to much for normal drift limits and a slender HSS post etc.