Slender Reinforced Masonry Column Design

[Eng16080]

For a slender, reinforced masonry column designed per the strength design requirements of TMS 402-16, how are column slenderness and P-delta effects being fully accounted for in the design and why are the requirements for a column different from those for a wall?

I understand that the nominal axial compressive strength, Pn, from Equations 9-15 and 9-16 of the code includes the slenderness term h/r (effective length divided by radius of gyration). However, there are no further requirements for a P-delta analysis for a column. On the other hand, for a wall designed with out-of-plane loads, P-delta effects are required per Section 9.3.5.4 in addition to using the Pn value mentioned above. Those wall requirements are similar to the requirements for reinforced concrete whereby options to account for P-delta include a 2nd order analysis or moment magnification factor.

Section 5.3.1.1 of TMS 402-16 provides limits both on the maximum distance between lateral supports for a column (99/r) and a limit on least side dimension (8 in.). Is it perhaps that these provisions are a safeguard against any adverse P-delta effects?

While there are a lot of good references for the design of slender reinforced concrete columns, with examples showing different end restraint conditions for columns both in sway and non-sway frames, I'm unable to find anything comparable for reinforced masonry. It seems that both materials would act similarly, so I'd expect the requirements for masonry to be similar to concrete. (I would even expect you could provide a safe masonry design by just following the concrete code using the appropriate material strengths for masonry instead.)

Maybe I'm digging too deep here, but any guidance is appreciated. I want to be sure I'm not overlooking something.

 

[Eng16080]

To answer my own question:

Based on some further research, the design guide "Strength Design of Masonry" indicates that the requirements for P-delta (2nd order) effects for columns aren't explicitly stated in TMS 402 as they are for a wall. On page 6-46 of that guide:

TMS 402 Section 9.1.2 requires that all masonry members be designed for a strength level moment, Mu, that includes the moment induced by relative lateral displacement, or the second-order moment. However, TMS 402 provides no guidance for determining the second-order moment except for walls subjected to out-of-plane loads. Fortunately, the methods given for walls subjected to out-of-plane loads are equally applicable for other masonry members, including columns and pilasters. There are no code deflection limits, though, for columns and pilasters as there are for walls.

Based on the last sentence, it still seems that the requirements for a wall per TMS 402 are more stringent than for a column, which seems a little backwards.