Biaxial-Bending on a Masonry Pier

[JNEnginr]

Hey Everyone,

Here's a new one.

I have a 16x16 masonry pier. Not a pier block, but (2) 8x16s, with the courses alternated as it goes up. The pier is 10ft tall. The (4) cells each have (2) #5 bars in each cell (probably just thrown in there), and they are fully grouted. No lateral ties installed in-between courses. This pier is already constructed.

Of course I have an eccentric point load coming down. 16k DL/25k LL (unfactored) The center of the column is offset in the +y direction by 4.5", and in the -x direction by 2.75". The column is sitting on a steel base plate as well. Also already erected.

Individual (not biaxial but x eccentric or y eccentric) give me stress ratios of 0.55 and 0.813. I'm worried about the combined (biaxial).

Option#1: It's all good and no reinforcement required. Any good way of calc'ing this? I know the neutral axis shifts...

Option#2: Reinforce the pier. How? Add another 8x16 on the side of the worst case eccentricity, reinforce the pier and grout solid, and dowel the new pier to the old with (2) #3 bars into the joint of the existing pier and into the joint of the new pier to tie the two together?

Any thoughts would be appreciated, as well as comments on are there any calc differences when using a pier block over alternating 8x16s.

I'm Stressing....Thanks!

 

[cal91]

Why don't you check biaxial bending?

Calc it as...

Max stress = P/A + P*ex/Sy + P*ey/Sx

 

[KootK]

No need to stress. Squarish concrete/masonry columns that work for uniaxial bending are almost never critical for biaxial bending. Your flexural lever arm across the diagonal increases for biaxial bending and that tends to trump everything else. Google Bressler's method for a quick and dirty.

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.

 

[JNEnginr]

Why don't you check biaxial bending?

I thought the calc was a little more complicated than that, no? Am i overthinking?

 

[KootK]

It is. By the book you'd do a strain compatibility analysis just like you'd do with a concrete column.

Any thoughts would be appreciated, as well as comments on are there any calc differences when using a pier block over alternating 8x16s.

I don't know that there is any difference. Your f'm is based on the weak link in the block/grout/mortar assembly so you should be good to go. With a pier block, you have the option of ignoring the block and mortar and just treating it as concrete column with some junk on the outside. That's another kettle of fish however.

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.

 

[mike20793]

QuickMasonry will check biaxial bending on masonry columns.

 

[cal91]

KootK,

I remember doing that method in my Masonry Class. The senior engineer I work with, however, once recommended the P/A + M/Sx + M/Sy method.

Not to challenge you, but out of curiousity, what is incorrect about the latter method? Doesn't take into account slenderness, or tension only resisted by reinforcing?

 

[KootK]

Not to challenge you, but out of curiousity, what is incorrect about the latter method?

Challenge away. I live for technical debate.

Doesn't take into account...tension only resisted by reinforcing?

Exactly this. At least in general, If the column doesn't actually crack, M/Sx starts to look a whole lot better.

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.

 

[concretemasonry]

If you are concerned about the stress of the masonry, most assumptions are far lower than the actual strength if a typical block prism was tested. - Most 8x8x16 block are about 50% to 100% over the ASTM minimums because of the manufacturing process and manufacturing economics.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[JNEnginr]

So it should work, and the calcs seem to show it does. I'll take that.

Just outta curiosity, if it didn't work and it was indeed over-stressed, how would you recommend reinforcing it? The method i mentioned above?

Thanks for everyone's input!