Masonry Section Properites Horizontal Span

[Qrs]

Good afternoon! (for some of us)

Has anyone been able to recreate the section properties of a hollow masonry wall (spanning horizontally, no grout, face shell) as shown in many books/TEK publications? I am creating a spreadsheet and all of my section properties have matched these documents except the horizontal span average cross-sectional properties (see attached, values are highlighted.)

I must be accounting for the unmortared web incorrectly. For Aave, I keep getting 41in2 instead of the 40.5in2 listed. Any one worked through this and could throw me a bone? Not sure what I am doing wrong.

Thanks!

 

[CELinOttawa]

Could it be a rounding error? That screams rounding to me... If you are allowing Excel to carry more than 4-digits, it will often not match older publications. Engineers used to have discipline about that sort of thing.

 

[CELinOttawa]

Try using an excel function to reduce to four significant figures...

From one of my spreadsheets I use in forensic work: =ROUND(CELLNAME,4-LEN(INT(CELLNAME)))

 

[Qrs]

It shouldn't be a rounding error. I did these calcs by hand first and didn't carry past four sig figs. (Sorry for the confusion, I did them by hand first to verify results before putting them into excel...I am making a spreadsheet though.)

Thanks for the excel tip though! That will come in handy.

 

[concretemasonry]

Qrs -

Unless specified, concrete masonry is laid with face shell bedding, unless you want full mortar bedding, which is normally only used on the first course bearing on the footing/slab. - That is why the ASTM specification and ACI 530 code and specifications to determine the masonry strength (f'm) for a 2 block hollow prism.

Looking at the simplistic drawing from the NCMA TEK note closely, you will find that from a practical standpoint that is not realistic, but close. If the program writer looked at those drawings, he would assume full bedding mortar bedding, which really adds little structurally according to codes. - This is not common in the real world unless the specific masonry shapes are specified. Fortunately, the programs are reasonably accurate and there is always the question of the assumptions for load applications.

After 20 years in the concrete masonry in producing masonry units (up to 16,000,000 sf per year)and then working internationally with company that supplied molds as a minor product line, I found out they have over 1,000 different "standard" 8x8x16 CMU units available. The reason is the different preferences for specific dimensions and properties for different areas and in the U.S. The drawings in TEKs are reasonably accurate as far a global dimensions, but the configurations vary and the most common units can not be laid in full mortar bedding, unless units designed for the purpose are accurate specified. California has a myriad of different shapes available that were developed for the specific location and requirements. Internationally, the structural engineers often work for the large contractor/developers and design a product for the project and even get involved in certifying the units as produced, but this latitude is not available in the U.S. All of the variations are minor (especially on an idealized drawing made for international used materials).

Masonry is a very confusing in detail to design because the codes are are based on voluminous historic test over the past few decades. Don't worry about the strength of an individual masonry unit because it would cost more to make and deliver a low strength unit and a contractor could not afford to handle it and lay it. Continuity in the wall is the major item because you are designing a wall as a structural unit.

Dick

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

 

[AELLC]

I wouldn't worry about the 41 in^2 versus 40.5.

After all, you aren't calculating the trajectory and speed to place a spacecraft into lunar orbit.

 

[Qrs]

I understand that masonry blocks come in all different shapes and sizes, but if on our drawings we state that they must conform to ASTM C90, doesn’t that mean that the units have to have the minimum dimensions specified for face shell and web thickness? The section properties in this TEK use the minimum allowable dimensions, so the values are conservative, but I am ok with that. I would rather use the minimum dimensions than try to track down the actual dimensions.

I understand that most blocks (when laid) do not have a full mortar bed. In fact, I am not even considering this as an option in my spreadsheet.

I use the section properties of the blocks to determine the stresses in the wall and compare them to code prescribed allowable forces. Isn’t that what we are required to do?

I’ve got another masonry question that came up. I am looking at an existing masonry wall that is ungrouted and has (2) #3’s in each mortar bed. It was originally an interior wall but is becoming an exterior wall. I am looking at the wall as spanning horizontally between existing masonry pilasters. Can I count on the #3’s as horizontal tension reinforcement? The diameter of the bar is the same size as the mortar joint, and I know that is no longer allowed by code. They are also inset 1.5” from the face shell of the blocks. I would think it would work, but have never looked at anything like this before...

Thoughts?

 

[concretemasonry]

Qrs -

I would agree that the situation you have does not qualify as any reinforcement.

If the wall with #3 bars was built according to plans, there would not be any mortar to transfer the loads between the block and reinforcement. Even if it was a 12" CMU, the face shells could be 1-1/4", well below the amount necessary amount to make mortar bed for continuity between the structural "members" (total lack of bond and continuity). - Other specialized units could provide the necessary mortar beds and bonding. - Arbitrarily grouting of the entire wall later will achieve little or nothing.

Dick

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

 

[Qrs]

Thanks for your insight Dick.

 

[RFreund]

Just out of curiosity what is the maximum diameter that can be used in the mortar bed? Or how much thicker does the mortar bed need to be?

Thanks

EIT

http://www.HowToEngineer.com

 

[AELLC]

The mortar bed is 3/8" so even a #2 smooth bar would be too large - a more practical application would be the ladder-type joint reinforcing such as Dur-O-Wal 9 ga.

I don't see how it would be practical (or possible, workmanship-wise) to increase the mortar thickness.

 

[concretemasonry]

If it is a heavy duty galvanized ladder that could a problem on a wall with any architectural pattern unless it has butt welded cross ties.

The best solution if it is a true structural wall to design with an intermediate horizontal bond beam every 4' or then possibly decrease the thickness. Often, the computer programs do not treat a masonry wall as a structural plate and just spit out some numbers.

Dick

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