1970's unreinforced masonry 1

[slickdeals]

Folks,
I am looking at a set of drawings for a building built in 1970 in South Florida.

The Building code of the time (South Fl. Building Code) required masonry compressive stress to be calculated based on gross area of masonry and the stress not to exceed 150 psi for a type M mortar for concrete blocks.

Shear stress are limited to 0.1 * compressive stress (15 psi).

Was the shear stress to be calculated based on gross area of masonry or net area? Can anyone point me in the right direction?

 

[ron9876]

You will have to find an old copy of the code. Sorry but I don't have a copy of the old code. The values depend on whether it is a tie beam/tie column system or reinforced masonry.

That code would have allowed 1/3 stress increase. Be sure you follow the Existing Building Code also.

 

[slickdeals]

Yes, it appears to be a tie beam/tie column system with UNREINFORCED masonry.

My confusion stems from the fact that the shear stress is a tenth of compressive stress. The compressive stress is based on gross area.

Should the shear stress calculations be based on gross area rather than net area?

 

[ron9876]

Not sure.

 

[concretemasonry]

slickdeals -

The figures you cited look like figures from an older prescriptive/empirical code (not really engineered). Do you have a code of the original code?

Are your trying to determine compliance with the original or compliance with the current codes and revised loads?

Since that time, the concept of strength and terminology of masonry was changed, so modern codes, beginning with the original ACI 530 (written about 1998) which is the basis for most modern codes, may give some clues, but the measurement of unit strengths in ASTM were changed from a gross area basis to a net area to recognize the potential of manufacturing possibilities for radically higher normal strengths. More modern masonry strength is now based on the net compressive strength of an actual hollow, face shell bedded prism(f'm), instead of a minimum ASTM required unit strength. - The old required gross unit strength was 1000 psi, while more modern prisms can range from a bare minimum of 1500 psi to over 5000 psi if tests are conducted.

Keep in mind that if it was a prescriptive/empirical code, the only requirements for lateral wall resistance were determined by either a l/t or h/t (18 for hollow walls). The newer ACI 530 includes the concept of shear walls perpedicular to a face of the wall in question, the shear of the external wall due to lateral loads only requires an l/T or h/t check for lateral support. Empirical and prescriptive code sections are very conservative and difficult for a modern engineer to grasp.

If you are concerned with analyzing the structure for modern adequacy and current loads, the section of of ACI 530 for Allowable Stress Design (unreinforced walls) may be of some help.

Much of this is from memory, and I always had a hard time with the Florida building/design methods in the 1970's after looking at other buildings (engineered unreinforced or very lightly reinforced) built to other accepted codes. The empirical design assumes everything is minimum quality with a minimum of design and no testing or inspections.

Good luck unraveling the the history and changing codes and standards and applying it to your structure.

Dick

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

 

[slickdeals]

Dick
Thanks for the historical information.

This is a bizarre review I am doing. A owner wants to determine if the building (built in 1970) was built per the code of the time (South Florida Building Code & ACI 318-63).

The code I am looking at seems to be empirical. There are code prescribed panel areas (not to exceed 256 SF) that have to be bounded by tie columns/tie beams and if built per those provisions, it works. No questions asked.

However, I am modeling the building in a 3D FEA program to figure out forces/moments and such. I wanted to see if an unreinforced masonry wall has a capacity to carry a certain shear and I am not sure what the capacity should be.

 

[slickdeals]

And yes, I have the 1970 South Florida building code and ACI 318-63.

 

[concretemasonry]

Obviously, a 40 year old code provision based on earlier proven building performance and was really not intended to be an engineered code similar to those based on newer construction methods and assumption.

If you can warp the program enough and make some assumptions on the continuity you may be able to get something out of your program.

Empirical/prescriptive standards are very conservative.

If the owner is interested in the big (40-60%) insurance savings the standards are much more stringent than the current Florida codes in the critical coastal areas because the additional requirements based on past performances and learned by recent problems and claims and not codes.

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

 

[a2mfk]

Slick-
Why does the owner care if it was built to a 40 year old code? As I am sure you are scratching your head too, but maybe they at least gave you a half-baked reason. You know its not even close to meeting 2007 FBC requirements...

I've been in Florida my whole career and the type of structure you are describing is very common, especially in old shopping centers, grocery stores and retail box. I think you work in S FL and live there so I am not preaching to the choir. Reinforced concrete tie beams and columns with unreinforced CMU infill are everywhere from the 60s and 70s. What we have always done is try and talk the owners into slot cutting the CMU and doweling into the tie beams if the upgrade is not-mandatory. Unreinforced CMU to me is just a blow-in failure waiting to happen in South Florida, and I'd make sure the owner is aware of this. If they are doing extensive renovations this may not be that expensive, and when you are all done you re-stucco the wall..

But to answer your question I would say this:
1. Have you done a thorough exterior inspection? How many step and other types of cracks in the mortar joints did you find? I would safely assume quite a few, in which case, I would not count on cracked sections of unreinforced CMU to have much shear capacity.
2. Uncracked mortar joints I would use the equivalent solid thickness, not the gross area. This is what I do for new masonry construction, and I assume this is right. You would only use gross area for fully grouted walls.
3. If you have good drawings, I suppose you could do a frame analysis of the tie beam-column system. You may not have proper development length (or know what it is), and also I would think as soon as the frame deflects it will load the CMU anyway. But it would be a failure check.

But like I said before, even if your shear analysis checks you would likely have a lateral failure way before shear is an issue... And no owner wants a partially enclosed building during a hurricane! But they rarel want to pay a little extra to prevent it.

 

[a2mfk]

To clarify- I meant you would likely have a local lateral CMU failure (blow-in) prior to the lateral force system failing globally...

 

[msquared48]

Ron is RRREEEAAALLL old and should have a copy of the code you need.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ron9876]

The tie beam/tie column system worked in Miami when Andrew came thru. There were problems with the tie column connections to the diaphragm but the wall panels didn't blow out. I was surprised.

Don't think the ACI code of that generation will help because it sounds like the building was designed by the prescriptive method.

Not sure why you would need a 3D analysis. If you stay within the height/width/area requirements and if you design the tie columns/tie beams for their tributary loads then the masonry works for lateral loads on the walls. Only need for allowable shear stress is to check shearwall loading. The masonry wall panels from that coe didn't meet the requirements of ACI.

 

[a2mfk]

"The tie beam/tie column system worked in Miami when Andrew came thru. There were problems with the tie column connections to the diaphragm but the wall panels didn't blow out. I was surprised." (How do you do the cool quotes?)

Interesting... Having been through some lesser hurricanes in 2004, I am thoroughly convinced of shielding (ie Exposures Categories) and similar effects have saved a lot of structures from wind damage that had no business not failing. In Orlando some of the only real wind pressure damage were signs, privacy walls, gas station canopies, and residential roofs (mostly shingles). Tree damage was horrific, it looked like bombs went off, but houses built in the 30s (I build better forts as a kid) made it through. I wonder if this is the reason so much unreinforced CMU has made it through hurricanes...

But it seems URM is the first to fail in an earthquake, where its mass cannot hide it from the forces.

 

[a2mfk]

ron98- "then the masonry works for lateral loads on the walls."

What do you mean the masonry works for lateral loads? It meets the prescriptive code from the 1970s? Even if its unreinforced?

I'm wondering what I would rather be in during a hurricane, a single story CMU house build pre-1980 or a wood frame house build pre-Andrew? They did usually put a tie beam or bond beam at the top course and maybe some vertical pours at the corners...

 

[slickdeals]

I am not sure of the whole prescriptive method. The current as well as previous Florida Building codes have the following statement.

SECTION 2118 HIGH-VELOCITY HURRICANE ZONES-DESIGN
2118.1 Masonry shall be designed by a method admitting of rational analysis based on established principles of mechanics.

The whole tie beam-tie column method appears under
SECTION 2121 HIGH-VELOCITY HURRICANE ZONES-CONSTRUCTION DETAILS.

I don't think anything supersedes the requirement that the design be per accepted engineering methods (Section 2118).

If masonry in a tie beam-tie column system constructed exactly per the requirements of the building code fails under wind loads, does it mean that there is no blame to go around? I don't think so.

 

[a2mfk]

Slick- I am still confused. What is it the owner wants from you? Whats the goal?

Any building I have ever worked on in Florida with URM I assume has no lateral capacity. I don't trust an intact and reliable mortar bond to even go down that route checking it. Then if I am required by the Existing Building section of the code, by distress or damage, or if I can talk the owner into it, we try to upgrade by slot cutting and reinforcing the cells...

If we modify the walls in almost any way I use it as a chance to upgrade. I look at it like I have bought the building by modifying it, and so would an opposing attorney. Especially on any building over ten years old because the statutes have run out for the EOR (I believe this is how it works...)

 

[slickdeals]

Our goal was to check if the building was designed per the building code at the time and verify if members met their strength requirements per the design codes of that time (SFBC 1970 and ACI 318-63).

I don't know what the owner wants to do, but that's the directive we got.

We were not asked if the building performs to today's standards or if we have suggestions for improving the as-built building. Strange, but it's their money. Interestingly, the 40 year old building did not show any step cracks at bed joints or any other signs of distress.

 

[a2mfk]

Weird, I'd love to know where the are going with that. I work with insurance companies all of the time and never have been asked that one, not on a building so old. We sometimes comment on structures not meeting current code...

Also surprised you have no normal hairline settlement cracks, but if this is Miami the foundations could be right on good limestone.

 

[slickdeals]

@a2mfk,
Is there a way to get in touch with you other than the forums? Especially since you live and work in Florida.

 

[ron9876]

a2mfk yes I did mean that it did meet the prescriptive requirements of the code. In Dade, Broward and to a lesser degree Palm Beach counties in south Florida almost all older buildings were built with that procedure. I believe it is still in the High Velocity Hurricane Zone portin of the Florida Building Code.

At that time there were some different perscriptive requirements for houses--I think something like 1#5 at corners, edges of openings and 6' c/c.

Andrew was an entirely different animal. Some buildings were shielded but the ones that were exposed to full load failed unless designed and constructed correctly. The wind load provisions of the code that was in effect at that time had a mistake in the determination of the pressures to use for design. The design was supposed to be for 120 mph sustained wind and 110 mph in the rest of the state but that wasn't really the case. The storm was a 160-165 mph 3 second gust storm. It was amazing to me because buildings that were designed and built correctly withstood the storm. Windows blew out, roofing came off but the structures held. Says something about how large the safety factors are. There were plenty of failures of the tie beam/tie column buildings but that was typically because the tie columns concentrate wind loads at their supports and if there wasn't a proper connection detail they came apart. Entire walls laying on the ground. Very sad but also very educational.