Masonry blocks max reinforcement ratio

[steelnz2003]

I am checking a masonry block supporting soil and lateral load the wall is 2.8-meter-high and span around 4 meter length, it is reinforced by 2HD16@400 centres.
That ratio is exceeding the balanced as I find a =44.7mm while a balanced =41.6 and according to the NZ standards which should be much same as other it is only allowed to be 0.75of the balanced section.

Is any max ratio for steel in walls or the ductile failure is not governing?

 

[dik]

Can you put compression reinforcing in it?

Dik

 

[steelnz2003]

Hi Dik
I did asked more experts here and they said it must have a ductile failure the NZ standards I find states max bar diamter as HD16 to be used with the 190 blocks this restriction due to the lapping requirments of bar as stated below

http://www.nzcma.org.nz/document/27...of_Reinforced_Concrete_Masonry_Structures.pdf

 

[EngineeringEric]

Use bigger blocks or pilasters. Laps and double bars in blocks is not a good idea and should be avoided when possible... it is difficult to impossible to construct as we intend, or so complicated we may as well just use concrete. I would always urge single bars in the center even when retaining, in the few cases you can go off center but do leave it more than 3" cover since wall thickness is not controlled (at least in the states where we sate minimum thickness not maximum).

 

[steelnz2003]

Sorry no it is an existing wall we thinking of demolishing it

 

[dik]

steelnz... adding compression reinforcing will improve the ductility. If an existing wall, then maybe possible to sawcut it vertically on the compression face and place reinforcing... tapcon a cover plywood/plank, leaving a sprue hole at the top for placing concrete/grout.

Dik

 

[KootK]

For an existing wall where the compression block is only 7% larger than the 0.75 pb size, I'd just leave it alone. No demo, no repair. The amount that you're off could easily get drowned out in the noise of construction tolerances and load/strength variability. Society's resources would surely be better deployed dealing with more pressing issues. The Brangelina debacle for example.

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.

 

[NorthCivil]

first off, steelnz I cant be sure of what you are trying to convey.

if what I & kootK are interpreting is correct, that you are 7% over allowable in some aspect, then I wholeheartedly agree with his statement. let sleeping dogs lie.

 

[steelnz2003]

Hi all
As here the council is very restrict in applying the standards specially the seismic requirements so it must fail in ductile manner the exiting design have 2HD20@400 spacing so we either have to add more compression steel on the other side or demolish in and rebuilt with greater size wall.

But to leave it no one can sign it off

 

[Ingenuity]

steelnz2003,

So you have 2 x 16mm dia deformed bars (f_y=500 MPa) at 400 c/c OR 2 x 20mm dia at 400 c/c?

What is your block unit size, 190mm?

f^'_m= 12 MPa?

 

[steelnz2003]

Hi
It is 2HD20 at 400 spacing
And the blocks is 190mm

 

[concretemasonry]

steelnz -

Since you have an existing situation, have you determined the f'm of the units in the wall?

The higher the f'm, the greater percentage of steel can be used in a wall section. - We are dealing with a designed based. This requires a determination of the components.

One problem you are facing is the rigorous restriction to the local codes (New Zealand and Australian) that have not recognized changes in materials and construction - For the last 30 or 40 years, there has been a constant local reluctance to recognize the other international codes (mainly the ACI/ASCE,TMS standard that is the most common standard used from a masonry design standpoint.

Upon reviewing the NZ codes, you will see the term "f'm" (strength of the masonry assembly in an assembly consisting of masonry units and mortar) to determine the composite properties and strength. Mortar effects are minimal (almost neglected), while the strength of the masonry units can be highly variable and pushed (by updated manufacturing methods) to many times the ASSUMED f'm in the NZ standards. The equipment and manufacturing practices used in NZ is virtually identical to what was been used in the rest of the world, but is not used unless the f'm is accurately determined. You may be basing your analysis on an f'm of 1350 psi, while the the actual materials (masonry units) may give you an f'm of as high as 4000 psi.

To get a balanced design, the actual material properties must be recognized. The strength of the masonry units allow the used greater percentages of reinforcement. I have seen engineers specify a maximum strength on mortar and grout to maintain a balanced, practical design.

Dick

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

 

[steelnz2003]

Hi concrete masonry
I check the grout it was 12 Mpa the blocks should be standard to be 17.5
I tested it and it is close to this valuable using the hammer

http://www.nzcma.org.nz/document/27...of_Reinforced_Concrete_Masonry_Structures.pdf

 

[steelnz2003]

Hi Again
Wants to join new these bars tot he existing wall it is connected in the end of the wall to another perpendicular wall so only shear load but I can not find any guidance for the lap length all shows ld which I thick is too long any idea I am using D20 bars