Minimum f'm for ACI 530

[phamENG]

Masonry question - is there a minimum f'm value below which the equations of ACI 530 cease to apply?

Specific situation: I'm looking at a new connection to an old-ish clay brick masonry wall (double wythe). At this point I don't have good data on the existing material, and I'm betting I never will, so my approach was to assume a connection geometry that should work, and check the minimum f'm needed just to gut check myself. Well...f'm(min)=5.72psi to preclude crushing. On the one hand, I can't break the brick between my thumb and forefinger so it's probably strong enough. But...that's just so low I find it hard to believe the equation is telling me the correct story based on the context.

I searched google and the ACI 530 commentary but came up empty handed. Anyone have any insight? Thanks.

 

[BAretired]

I would be inclined to take equations involving f'm with a grain of salt for your situation.

BA

 

[phamENG]

Aha! Thanks, BA. I knew it had to be in there somewhere. I use ASD when looking at older load bearing brick, and chapter 8 doesn't appear to have a similar provision/guidance.

It's interesting, though, because the Brick Industry Association lists a unit strength assembly for f'm=1000psi (Technical Notes 3A, Table 5).

 

[haynewp]

For the unit strength method, the minimum f'm is derived by using the minimum strength of the unit that is dictated by the unit strength ASTM in combination with the mortar type. See Table 1 (for your clay masonry) and Table 2 (for concrete masonry) in TMS 2016 in the Specification chapter. Interestingly, it looks like the old f'm of 1,500 psi for concrete masonry is no longer permitted and the minimum f'm is 2,000 psi with type S and M mortars. This is all for new construction.

There is a requirement in 9.1.9.1.1 under strength design that requires a minimum of 1,500 psi but I don't see an equivalent provision in the ASD section as noted above.

Going back to older IBC versions the worst case for clay masonry is listed as 1,000 psi which still corresponds to Table 1 in the Specification in TMS 2016 for clay masonry. You could go back further and allowable stress for brick was being used in historic buildings. Regardless of how low f'm may have been used for design in the past, I would be inclined to use 1,000 psi as the minimum for clay masonry with ASD based on modern criteria.

 

[Celt83]

I've seen prism tests on existing multi-wythe brick come back in the range of 500-800 psi. Typically I'll usually limit new bearing plate connections to pockets in the wall or top of wall to about 100 psi which gives some leeway to be on the low end of the prism results. For any kind of face mounted connection I reach out to the local Hilti/Powers/Dewalt rep and ask them to do some anchor tests around the building.

My Personal Open Source Structural Applications:

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

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[phamENG]

Thanks, everyone. 500psi is the lower limit I usually try to stick to without additional testing when dealing with older structural brick, and is what I used here. The connections are through bolted, so my primary concern was local crushing due to shear.

haynewp- good to know about the change for 2016. It'll be a while before it becomes code here, but I might as well start getting folks accustomed to it. 1500 is still the norm, and people get nervous when you push to 2000...

 

[Boof19]

ASCE 41-13 has a list of lower bound values you can assume without testing - 1000 psi for clay brick I think. Similar to Celt83 I've seen compressive tests for existing brick assemblies come back around 800 psi - the actual brick tested upwards of 1100psi but the mortar failed around 800psi. Not sure how old your building is but keep in mind if its pre 1920's the mortar mix probably had a high lime concentration and no portland cement.

 

[masonrygeek]

Determining the compressive strength of existing masonry can be tough. Some masonry looks very solid and probably has a pretty high compressive strength, but without some testing you are just guessing. Some of the comments above are a good place to start. If you have something more substantial to design, then in-situ tests like a flat jack test method should be used. ASTM C1196 "Test Method for In Situ Compressive Stress Within Solid Unit Masonry Estimated Using Flatjack Measurements" has been used for many years and can provide a more realistic value to be used in design.

As for the minimum f'm for masonry, the 2013 ACI 530/TMS 402 bumped the minimum value from 1500 psi to 1900 psi (for Type M and S mortar and CMU) and then the 2016 TMS 402/602 made the minimum 2000 psi. And you don't have to wait to use the new values as these as values that the industry currently produces. In fact, most of the block manufacturers commonly make block above the minimum of 2000 psi for the unit and you could specify an f'm of 2500 pretty easily.

 

[phamENG]

Thanks, masonrygeek. Not sure if my question was completely clear. I wasn't looking for minimum to specify - I was looking for the minimum at which the published equations are valid. BA found the commentary section I was looking for in that regard.

The supporting research cited by the code was based on test specimens with f'm=1500psi to 4000psi. So we know that the equations are good for that range of strength. Below it is "uncharted" territory.

So I'm pretty confident that my solving of the masonry crushing value at the anchor bolt for minimum f'm provided a "false" result - I doubt a 5psi f'm would resist crushing in the way that the equation seems to "predict." I'm just curious where we cross back over - if I use the equation with a 500psi or 750psi f'm for older and softer brick, is the result reasonable or should I doubt it? Do you know of any additional research that has explored lower values?

I do a fair amount of historic work and plenty of the old stuff comes in below 1500psi. So this could a matter of some concern in terms of strength verifications on future projects.

 

[JLNJ]

It sticks in my mind that some of the old tables want down to 1200 psi. Obviously this is still well above 500.

 

[jayrod12]

In our firm we've done a fair bit of testing on historic multi-whythe brick and use an f'm=800 psi when analyzing based on our testing results. Many times it is significantly higher, but that is our relatively conservative design value barring anyhting else. Obviously the condition of the existing brick needs to be evaluated prior to using said number.

 

[phamENG]

Here's a quick sketch to outline my curiousity/concern.

If the testing has only been based on f'm between 1500psi and 4000psi, what do we know about the performance of masonry assemblies at failure that are weaker than f'm=1500psi? Does it follow the behavior of the tested material and follow the red dashed line? Is it somehow stronger (I doubt it, but stranger things have probably happened), or does it follow the orange dashed line and actually result in a weaker connection/wall/assembly than the equations predict? We assume that things like the elastic behavior of the wall follows the assembly's compressive strength. So as it gets weaker, it gets more brittle. Is that change substantive enough to alter failure modes and change the shape of the strength curve with respect to f'm?

 

[phamENG]

The answer may be "we don't know" and that it's a potential area of further research. It may never happen unless a conservation group sponsors it, but here's to hoping.

 

[masonrygeek]

All of my comments on minimum compressive strength were related to new construction, not existing walls. Determining or estimating the strength of an existing wall is tricky business as noted above by all. There are so many combinations of brick strengths, mortar types, workmanship, etc. that it is hard to pinpoint one number. That is why I recommended flat jack testing when you need more certainty.

There are a few sources of information on compressive strength of older brick walls. They include "Compressive Strength of Clay Brick Walls" by Stang et al - 1920's (

https://nvlpubs.nist.gov/nistpubs/jres/3/jresv3n4p507_A2b.pdf),

"Brick Engineering: Handbook of Design" by Plummer and Reardon - 1939 (

https://archive.org/details/PrinciplesOfBrickEngineering-HandbookOfDesign/page/n1/mode/2up),

and Building Stone and Brick by ICS (

https://babel.hathitrust.org/cgi/pt?id=uiug.30112047372476&view=1up&seq=133).

There are some other papers and articles that discuss compressive strength of masonry: "Evaluating Existing Masonry Construction" by Geister (

https://www.structuremag.org/?p=908)

and Antiquated Structural Systems by Stuart (

https://www.structuremag.org/wp-content/uploads/2014/08/C-EngineersNB-Stuart-Jan101.pdf).

But in the end, testing may be necessary.

 

[university_professor]

check buidling code appendix.

 

[Celt83]

Masonrygeek:

I think the question PhamENG is looking to get answered is can you enter into the TMS 402 (ACI 530) formulas for things like anchorage, axial stress, bending stress, etc. with an F'm lower than 1500 psi or are those code equation no longer valid.

For example equation 8-3: Bap = 1.25 Apt √ F'm , can F'm in this formula be below 1,500 psi?

PhamENG: Please correct me if I'm wrong.

My Personal Open Source Structural Applications:

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

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[phamENG]

Celt83 - you nailed it. I'm looking at the applicability of code equations and analysis methods, not determination of material properties.

I do appreciate your input masonrygeek - though not directly applicable to my specific question, it is unquestionably valuable knowledge to have all the same.

university professor - Did that and BA even posted a screen shot of it. Doesn't answer the question, unfortunately.

 

[masonrygeek]

OK, now I see what you are getting at and I don't think I have a good answer. I believe that the relationship would be rather linear as in Pham's middle red line, not the purple or yellow lines. Here are a couple of pages from "A Historical Survey and Analysis of the Compressive Strength of Brick Masonry" by C.B. Monk and the Structural Clay Products Institute. This paper pulled in data points from the 1890s to the 1950s so it has high and a lot of low values. Except for Type S mortar in Fig. 4-5, all of the data shows that f'm of the wall is linear to the strength of the brick unit - all the way down to around 1000 psi. I am sure that TMS excluded some of the oldest data points when they developed the Unit Strength tables in TMS 402/602 (previously ACI 530), but I don't know how many TMS equations are valid for anything below 1,000 psi.

 

[phamENG]

Thanks, masonrygeek. I'll be adding that to my library. A very handy resource, indeed.