Evaluating Older Masonry Walls 1

[STR04]

I'm currently reviewing a load bearing masonry building in the state of Florida which was constructed in the 70's. I was wondering what compressive strength I should use when analyzing this structure. Some codes tell you to reduce the compressive strength by half if special inspection was not conducted but I couldn't find anything in the Florida Building Code for this criteria. Does anyone have any suggestions?

TIA

 

[hokie66]

That approach would mean most of the masonry buildings in the US could be technically overstressed because they were built before the term "special inspection" was invented. I doubt that is the intent.

 

[DaveAtkins]

Early in my career, it was common to use "half stresses" if masonry was not inspected. This requirement was in various Codes. But nowadays, we all expect masonry is inspected, and so we use full stresses.

For your case, I would not have a problem using full stresses. I always felt the "half stress" thing was way too conservative.

DaveAtkins

 

[ARLORD]

I would use f'm = 1500psi

 

[STR04]

So, no code requirement just engineering judgment. That's what I was thinking. Thanks.

 

[ARLORD]

Yes, and most likely it is unreinforced.

 

[STR04]

The original drawings actually do show reinforcing.

 

[mudflaps]

Any deterioration of the mortar or grout? As I remember a few years ago some nice multistory hotels near beaches had to be taken down because of salt in the beach sand that was used in the concrete mix. Should have some cores taken to check strength. You didn't say if this was CMU or brick masonry.

Old CA SE

 

[STR04]

Reinforced CMU.

 

[dik]

Earlier codes permitted partial reinforcing of masonry walls while others considered only unreinforced or reinforced.

Dik

 

[concretemasonry]

What type of building, wall heights and number of stories?

This may help get a picture of what the most crital stresses are so your investigation can first be aimed at these properties and what is important.

Are there any cracks that are apparent other than some old shrinkage cracks?

In much of masonry, a critical item is the prism or unit strength of the masonry, since mortar has a minor effect and the griut is intended to bond the reinforcement to the to the masonry units and relied on to add compressive strength. The block specifications (ASTM C90) have changed little for the past 50 years, with the exception of converting from a gross compressive strength to a nit unit compressive strength.

If the building is a loadbering or multi-story from the 1970's it may have been designed using the NCMA "green book" (something like tentative specifications for the design of multi-story builds). This was the forerunner of the ACI 530.

If you can saw cut an ungrouted (preferably several) block for testing to determine the units net compressive strength using ASTM C140. With this strength, you can assume a Type M or S mortar and use the current standards to determine a conservative f'm, which controls many properties. Cutting entire prisms or "small "coupon" samples is also possible, but these require very critical handling and testing and the meaning of results can be difficult to determine.

Very often, cores were reinforced and grouted for code requirements and not for strength putposes. This was very common in many areas on single story buildings and walls under 12-15'. A good lab could esternally determine the existance of the vertical steel for a comparison with the plans.

There is a remote possibility there may have been beach sand used, but that would be very rare. Beach sand can be used in concrete, but it would be very difficult and costly to use it in CMU manufacture. Block require tighter controls over gradation and a larger maximum size. Most Florida producers were vertically integrated and produced sand and rock, block and ready-mix concrete.

Much of the block used aggregates with a high calcium carbonate content and the dust added to the ultimate strength over time by acting like cement.

Dick

 

[kslee1000]

The building has been standing for almost 30 years, unless there was major damage, that was developed over long period of time and called into question of strength of the masonry, that brought up the need to perform an engineering study, the maximum estimated stress under current loading conditions shall be calculated and considered as the base/lower-bound allowable. Once the base is estabilished, you should be able to judge the comfortable/acceptable higher-bound alowable stress by interpretating earlier code philosophies (safety factors, allowable stress deviations..etc.) and conventional practices.