Effective Height of Masonry Wall w/ Bond Beams

[Slugged]

Does anyone know of a resource for design explaining how to design a free standing masonry structure or designing masonry bearing walls to span vertically between evenly spaced bond beams? I'm designing to ACI 530. I have a tall CMU stair shaft that will be unbraced for 55ft. The shaft is 20ftx25ft in plan.

I want to design the wall spanning between bond beams spaced at 10-15ft on center, but I'm not sure what the stiffness and strength requirements would be for the bond beams in order to restrain the out of plane 'buckling' of the wall per ACI 530.

 

[OldBldgGuy]

Not sure where you're working but CSA S304.1 will cover you in Canada.

 

[Slugged]

Thanks OldBldgGuy. I'm looking for design aids for lintels bracing a wall similarly to what a floor would provide. For example, if a 30ft wall is braced a mid height, then the effective height for design is 15ft, because the wall would be forced into double curvature. What is this minimum stiffness required and could a lintel be designed to provide this stiffness out-of-plane in order to reduce the design wall height from 30ft to 15ft?

Is this even feasible with masonry or is this a fool's errand? Thanks!

 

[KootK]

Is this even feasible with masonry or is this a fool's errand? Thanks!

I think that the best approach would be to tighten up the spacing of your horizontal reinforcing and simply design your walls as horizontally spanning. At the end of the day, that's essentially what you'll end up with either way.

I want to design the wall spanning between bond beams spaced at 10-15ft on center, but I'm not sure what the stiffness and strength requirements would be for the bond beams in order to restrain the out of plane 'buckling' of the wall per ACI 530.

I get what you're saying and, for the method proposed, I think that you're right on track with your concerns. That said, I doubt that you'll find what you're looking for in terms of code guidance precisely because this is such an unconventional way to approach the problem. We could probably work our way through some kind of reasonable approach involving P-delta estimates and cracked masonry stiffnesses etc. It would be pretty labor intensive 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.

 

[Slugged]

I think that the best approach would be to tighten up the spacing of your horizontal reinforcing and simply design your walls as horizontally spanning. At the end of the day, that's essentially what you'll end up with either way.

Thanks for the response! For what would the horizontal bars be designed? Even if I space them at 16" o.c., how would I rationalize that it prevents out of plane buckling? I've seen some people used 2.5% of the vertical load applied horizontally to justify the wall being laterally supported. I'm not concerned with wind loads and out of plane loads, I'm only concerned with gravity loads and the stability of the wall. The allowable axial stress due to compression is 1/4*(f'm)*(70*r/h)^2

What is the industry norm for designing a freestanding masonry structure if it doesn't have intermediate floor diaphragms to brace a wall out of plane? How would out-of-plane stability be analyzed with lintels, horizontally spanning beam, reinforcing, etc.?

Is this even attempted or do designers simply use wider CMU or switch to steel framing? I can design a stable free standing tower out of steel in my sleep, but masonry just doesn't have the research backing it for me to rationalize any method of wall out of plane stiffness related to compression loading.

 

[KootK]

Ah, now I see what you're after. Many codes have unsupported span to depth ratio limits that can be applied vertically or horizonrally, whichever condition is more favourable. Honestly, I'm not sure that it gets any fancier than that. And yeah, walls like these are not uncommon.

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.