Struggling with Basic Masonry Design Problem 2

[Usman3301]

Hi everyone,
I'm in a bit of mess currently, due to Masonry Design of Hollow blocks - fully grouted. Thing is I'll pursuing master's from abroad starting this fall and already have informed my boss that I'll be leaving office at the end of this month. While I have extensive experience in Reinforced Concrete Structures, masonry structure design is uncharted territory for me.

My boss has asked me to quickly perform calculations and prepare structural assessments to verify if the proposed masonry design is acceptable. The urgency of the situation has left me feeling overwhelmed, as I'm unsure where to even begin. Had it been a week or two, I would have brushed up Masonry Design basics and would have definitely come up with a solution.

Considering the ordeal I'm currently facing, I would be immensely grateful if someone could kindly lend me a helping hand with this task. Any guidance, resources, or expertise you can offer would be greatly appreciated.

I've shared the problem statement in PDF as well as image format.

BSc. Graduate

 

[RobertHale]

You need to look at the NCMA Tek Notes (

https://ncma.org/resource/tek-index/),

specifically 14-7 (of whatever code you're operating under). This note has the equations and enough diagrams to get you on the right path. Since you're new to masonry as a material, I would stay away from strength design and stick to allowable stress.

 

[JLNJ]

The reinforced masonry design is the easy part.

First, what's your load path? Is this a cantilevered wall? Is the wall supported by a continuous floor diaphragm at the top? Maybe fixed at the bottom and pinned at the top?

Second, I would expect the seismic force to be applied at the center of mass. Why the 2/3 number? What is the origin of the seismic mass? Just the wall?

Why the two layers of 6" CMU? Is this all existing construction? This seems odd to me. You can't use one 12" CMU?

The side cover on the rebar embedment from the outside wall centerline to the edge of concrete looks troublesome. Maybe you can ignore it and use just the rebar in the inside 6" CMU.

The footing looks pretty iffy. I'd take a hard look at that.

Your sketch doesn't indicate if there are gravity loads acting or how one would evaluate the application of wind load. This looks like a poorly-written exam question.

 

[LittleInch]

I'm no structural man but that embedment of the re-bar doesn't look like any where enough to do any good. What is the embedment length?

Would it make a difference if the blocks were turned 90 degrees?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Usman3301]

Thanks a lot for responding. Moreover, apologies for this poorly written thread. I agree that problem statement could have been explained a lot better.

It's a fence (boundary) wall and besides self-weight of the wall, wind and seismic forces are to be considered. The fence wall is located in high seismic zone (Ss = 1.79, S1 = 0.85).

Origin of the seismic mass is self-weight of wall only. As for the application of seismic load application, I've always opted for 2/3rd of wall height in cantilever cases.

Thing is that here hollow block sizes are only available in 4", 6", and 8" thicknesses and client wants CMU wall thickness to be at least 10". That's why I opted for 2 - 6" CMU blocks to fulfill requirement.

As for the keeping vertical reinforcement within inner CMU block, I'll definitely consider this option if there's a constructability issue. Though, thanks for pointing out.

And you are spot on with your assessment of foundation. Before proposed construction measures (CMU construction on existing R.C wall) it was barely sustainable against overturning but with addition of CMU wall, it's highly susceptible to overturning. Already have recommended to client that footing width needs to be increased to the left. It remains to be seen whether client will consider this option. For now, client is adamant on using helical piles under existing footing.

BSc. Graduate

 

[milkshakelake]

Since you're new to masonry as a material, I would stay away from strength design and stick to allowable stress.

Actually, I think if OP is very familiar with concrete design, LRFD might be the way to go because it's similar. You wouldn't have to get into things like seeing how much of the "flange" is used because it's fully grouted. In ACI 530, there are some differences like beta, phi, and max strain when compared to concrete. I'm not sure what code OP would use, but it's probably the same one based on the units. Check out these equations and diagrams, it almost looks like concrete:

https://ncma.org/resource/strength-design-provisions-for-concrete-masonry/

So that would be a shortcut way of doing it, if OP is very familiar with concrete design. That's how I did it because I couldn't be bothered to learn ASD, which would be a totally new thing for me.

You can make short work of deflection by calculating cracked moment of inertia using equations and applying load as a cantilever column. Or use an ACI 318 stiffness modifier (like 0.35) to get an approximate answer.

Edit: There's also a very, very quick way of doing it without learning anything at all, but it would also be very indefensible in court. But since there is a heavy time constraint, just design it as a cantilevered concrete beam with something like f'c=1000 psi.

 

[DaveAtkins]

Ultimate Strength Design of reinforced masonry is VERY similar to USD of reinforced concrete. The only difference is you use 0.8f'm instead of 0.85f'c in the equation for "a", and you need to ensure the compression block is not tee shaped, as milkshakelake mentioned above. As he mentioned, for your problem, that is not an issue because the masonry is fully grouted.

Oh, there is one other thing to check with reinforced masonry - rho max.

DaveAtkins

 

[structSU10]

These are 2 wythes of masonry that must be tied together in some way to promote horizontal shear transfer or they must be designed as separate walls that may or may not work together. I believe the TMS has provisions for what is needed to consider the 2 wythes composite.

 

[milkshakelake]

OP, another thing to keep in mind is that the reinforcement can't exceed a certain percentage of the 6" CMU cell. I think it was 0.04, but you'd have to check ACI 530. I wouldn't go too high anyway because you want the grout to flow around a splice. NCMA Tek has dimensions you can use to calculate the cell area. So this would probably mean that you use smaller sized rebar, but more of them. #5 @ 8" would satisfy the requirement, just don't go to something like #7 for a 6" block.

 

[Usman3301]

Thanks everyone for your input. I wasn't expecting this much help but you guys went above and beyond to assist me in this and I really appreciate it.

@LittleInch Thing is that client isn't interested in any proposal and just wants to make sure that whether the proposed layout will work out or not.

@milkshakelake I can't stress enough how helpful you've been always. Initially I opted for ASD approach but considering your advice, transitioned to LRFD since I was finding it hard to get things done right. Although I tried my best but still, don't know if these calculations will work or not. My flight is within a week, and I'm still entangled in this. If possible, please go through attached calculations if this work or not.

@structSU10 I was thinking of providing masonry wall ties to connect wythes together.

Note: I've attached Pdf file of calculations and it would be great if someone review these and let me know if it could work. Thanks!

BSc. Graduate

 

[Roukkia]

I would compare your calc with the flexure calcs in this textbook

https://whymasonry.org/2015dorms/?