STRENGTH DESIGN OF REINFORCED 8" MASONRY WALL FOR DEFLECTION

[oengineer]

I have been asked to determine the minimum reinforcement (minimum spacing using #5 rebar) required for an 8" masonry wall based on strength design. I am new to masonry wall design. The wall heights I am checking to determine the minimum masonry wall reinforcement are 10 ft, 12 ft, 18 ft, 20 ft. My deflection criteria is L/600.

Based on TMS 402 Equ. 9-29 & 9-30, it appears that the deflection of the wall is not based on the amount of reinforcement. I am also using example 11.4-5 in the MDG2013 book as a reference,and at no point is the wall reinforcement considered to affect deflection.

It seems that you could not even have reinforcing in the wall and the L/600 criteria would still be satisfied.

Does the wall reinforcement affect the deflection of masonry walls based on strength design? Comments/suggestions are appreciated.

 

[oengineer]

I have obtained an axial-moment interaction diagram from TMS's website and when I enter a very large spacing for reinforcing it still passes. I am new to masonry design & comments/suggestions are appreciated.

 

[JLNJ]

I honestly have never checked the out-of-plane deflection of a masonry wall. Nor am I convinced that you must limit it to h/600 if you do check it.

I suppose if you push the limits of slenderness and have a significant axial load, it might be an issue (such as a tall skinny tilt-up CIP concrete panel).

 

[oldestguy]

Not my specialty, but I'd want to see all the loads on that wall, the restraints at the top, temperature differentials, etc. before even trying to solve it.

 

[oengineer]

@JLNJ

What I am discovering is that I would have to space the reinforcement by like 120" o.c. I am mostly basing this on Dead & Live Load and out-of-plane Wind Load. And even with this it just makes M[sub]cr[/sub] > M[sub]u[/sub], which only changes the deflection equation. I have not seen anything showing that the CMU wall fails for deflection with an increased reinforcement spacing.

 

[oengineer]

@oldestguy

I am using a DL = 700 plf, LL = 300 plf and WL = 30 plf. I using these loads at various heights (Max Ht = 20 ft) and #5 with various rebar spacing. My wall is being designed as a simply supported beam.

 

[oengineer]

Is there any CMU wall specialist out there that could point me in the right direction to determine the required reinforcement for an 8" masonry wall based on strength design (considering L/600 deflection criteria)? It would of great help.

 

[Celt83]

I am a bit confused as to what you are looking for:
Strength Design of the masonry is an ultimate limit state so you wouldn't normally be checking deflection under that loading, deflection is a serviceability check.

Masonry behaves similarly to concrete in that once it is cracked you cross from elastic design to plastic design, for deflection this means you can no longer simply use the gross moment of inertia, ie b*h^3/12 for a solid grouted wall (note you could use the transformed section here accounting for the reinf. but a single bar in the middle of the wall won't add anything), but must now use an effective moment of inertia or the cracked moment of inertia, this is the point where the reinforcement will impact the deflection.

Under strength design there is now the design process for slender masonry walls which is an iterative design process to account for P-Delta effects, essentially determine the deflection under the wind pressure and add in a moment equal to your gravity load X the eccentricity of the load point to the max deflected point, this new P*e moment will generate additional deflection so the process repeats until the additional deflection diminishes. If thru the iterations the deflection does not diminish then you need to up size the wall.

If you have a partially grouted wall the process is essentially the same but the cross sectional properties become a bit more of a pain to work out.


Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[Celt83]

So for a solid grouted wall with reinforcement at mid depth and a loading such that M < M,crack:

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[JAE]

Using NCMA - I get an 8" wall with #5 @ 40" o.c. required for the 20 ft. height and your loads.

The moment of inertia is around 350 in^4/ft.
f'm = 2,000 psi

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[UcfSE]

For deflection, reinforcement comes in to play when calculating cracked moment of inertia. If service moment is less than the cracking moment, reinforcement will not contribute much to changing deflection. At L/600 you may be in this situation.

 

[oengineer]

@Celt83,JAE, & UcfSE
Thank you all for your responses.

@JAE
What is "NCMA"? Are you referring to "National Contract Management Association"? Is this something I can obtain online?

 

[azcats]

[URL unfurl="true"]https://ncma.org/[/url]

 

[oengineer]

@JAE & azcats
Thank you for the link. I am not familiar with this site. Did you use the software to design this or did you use one of the TEK documents. I ask because I would need to contact my IT department in order to obtain the software.

 

[JAE]

Used the software at NCMA.

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[oengineer]

I thank all of you for your help!

 

[masonrygeek]

oengineer, I would recommend you consider becoming a member of The Masonry Society and purchase a copy of the Masonry Designers Guide (

http://www.masonrysociety.org).

That book, plus the information from the National Concrete Masonry Association (NCMA) can give you a lot of worked out examples and more explanation than is in the TMS 402 code. Good luck!

 

[JAE]

Fully agree with masonrygeek on the book.

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