CMU privacy wall with veneer 2

[COEngineeer]

Hey, I admit I have never taken any CMU design class. All I have is a privacy wall to act as a barrier from highway noise. So all I have is dead load and wind load. Can someone guide me to a link on basic on how to design cmu wall? I will make sure it wont tip over but I need to know how to design the steel at the stem. I dont want to just put #5 into each cell like the senior engineer suggested without calculation (probably be good enough but I want to learn). Plus I would like to be able to design basement wall with it in the future.

Never, but never question engineer's judgement

 

[civilperson]

CMU design is similar to reinforced concrete design. Calculate the moments at the base of the wall due to wind and design the beam/wall to meet this loading. If the steel is centered, d is half the thickness minus the radius of the bar. The usable strength of the concrete shell is dependent on the class of unit specified.

 

[COEngineeer]

The last sentence you mentioned is what I need to learn/know civilperson.

Never, but never question engineer's judgement

 

[haynewp]

"Hey, I admit I have never taken any CMU design class"

But I bet you had to take a bunch of world history classes, fine arts, great books, world culture, philosophy? The way the undergrad curriculums are set up now really pisses me off. I had to go back to grad school to take a cmu class. Sorry, I know way off topic...

 

[COEngineeer]

hayne.. i had to take physical education for my undergrad.. twice a week and it worths 1/2 credit.. I did go back to grad school but they didnt have that class :-(.

Never, but never question engineer's judgement

 

[Galambos]

you can find the free downloadable TEK manual at

http://www.ncma.org/map_pages/state_listings.cfm

this is a pretty well known manual for masonry design.

 

[UcfSE]

thread507-113569

thread507-194036

Keep in mind the majority of masonry in the US is designed using allowable stress. With the ACI 530-02, strength design was introduced.

 

[concretemasonry]

COengineer -

The last sentence regarding "class of unit" (correctly prism strength or masonry strength) is analagous to a concrete cylinder. It is measure of the structutal properties of the masonry portion of the wall. With that in hand, the amount and location of the reinforcement is determined.

There are two ways to determine f'm:

1. The first method is the conservative route consisting of determining the compressive strength of the masonry units according to ASTM C140. Using this strength and the type of mortar, you can select a prism strength (f'm) from a table in ACI 530. Looking at the table you will note that mortar strength does not play a large part in determining the masonry strength.

2. The second method is to construct a prism of the block used and the mortar. The prisms are constructed and cured on site for several days and then in a lab similar to concrete cylinders. Since block (which control the strength) are essentially cured before they are laid, the strength gain between 7 and 28 days is minor and many engineers prefer 7 day breaks since a highrise masonry building can go 1 or 2 floors a week.

The first method is easier because it eliminates prism construction and the associated handling, curing and testing problems. Most testing laboratories that are set up for concrete cylinder testing do not have the capacity to test higher strenght block prisms.

I have seen projects of 10 to 15 high rise loadbearing buildings constructed using the first method and never made a prism because of the monitorng of the units in advance of construction and the only site sampling was for mortar.

For a good description of masonry testing and the design and use of masonry go to the TEK notes produced by the National Concrete Masonry Association site at ncma.org.

Dick

 

[COEngineeer]

Thank you guys. That TEK notes is exactly what I need. It has design example and everything.

BTW.. this might be the stupidest question of the week (I wish they had something else other than a star to vote the stupidest question of the week ) What does TEK stand for?

Never, but never question engineer's judgement

 

[concretemasonry]

TEK was just a "gimmicky" replacement for technical as far as I recall.

At one time, I believe they were desribed as technical notes or technical reports, which took too much space and was not short enough. They have been around for over 30 or 40 years as far as I know. If you look at some of the old versions, the evolution is amazing. Like all technical information published for the public, there is sometimes a concession to not get them too technical, detailed and theoretical.

They are written by engineers that are active officers and voting members in ASTM, ACI, TMS, MSJC and other national organizations. The NCMA has an outstanding, unique lab where they can test very tall walls and other masonry assemblies for shear, compression, fexural. They also do some contract testing for various government organizations.

 

[concretemasonry]

COEngineer -

Why is it a wall with veneer? - Imposed to match/compliment existing?

It is very inefficient from an engineering and a cost standpoint.

A thicker CMU wiith an architectural face on one or both sides would allow a greater "d" dimesnion, be symetrical and more economical.

12" CMU walls using colored and split/racked/ribbed faces are common in many areas.

 

[COEngineeer]

It is up in the mountain area and they have strict rule about the look of a structure. It has to look like a natural stone. This is what I come up with. What do you think? I put it 4' deep for frost depth. The PE hasnt reviewed it yet. It is in 80 mph category B zone so I use 20 psf wind. Let me know what you think.

Never, but never question engineer's judgement

 

[UcfSE]

With a 12-inch thick footing, are you getting enough hook embedment to develop the no. 7 bar, l_dh? You don't call out the vertical bars. How did you get 20 psf?

 

[COEngineeer]

Yeah I realized I forgot the vertical. I dont think I will need the development length since the footing is 4' deep? Do you think otherwise?

As far as 20 psf, I just look it up on asce and find the maximum load in different zones and round it up. Is it too low? Where can i find a wind load for flat sign or something?

Never, but never question engineer's judgement

 

[Lion06]

If you designed the rebar for the wall and the footing size as the wall being fixed at the bottom, then I think you do need the development length of the #7 dowel.
I would just calc the windload from ASCE 7. It isn't a hard calc. I never trust the "simplified" method. I am sure it is fine, but when I sarted my job I was always asked to calc it out using the analytical method.
I also always apply a surcharge to the soil on the outside of the wall in the neighborhood of 100 psf.

 

[COEngineeer]

I made sure the overturning moment is 1.5 or more and the toe pressure doesnt exceed what was given. The maximum moment is at the grade and it is 3 ft from the footing. The footing will never see the maximum moment thus I only size the footing for OT moment not development length. I am about to show it to the senior engineer. Ill ask the guy what he thinks.

Never, but never question engineer's judgement

 

[Lion06]

The OT moment is dependent upon whether you assumed the connection from wall to ftg is fixed. The OT moment will be tiny if you assumed pinned, but if you did assume fixed you still need to make sure the No. 7's with the development length you have can take that moment into the footing (even if it isn't the maximum it still have some value to be checked).
9" embed on a #7 seems small - even with the hook.
For 4000psi concrete, I get 19" required. I guess if you have a little more than twice the required steel at that section, the 9" is fine.

 

[concretemasonry]

COEngineer -

I have a couple of suggestions/questions and some depend on the location and availability/cost of materials.

1. Since 1/4th of your wall is below grade, look into a larger (thicker) unit and eliminate the 4" block veneer. While 8" units are thought of as the common unit in many areas, there many other sizes that probably are available (10",12", 14" & 16" thick) that could reduce the cost and provide more continuity. Even 2-8" units grouted soild may be desireable. - Since you are in mountain country, you will have slopes and changes in footing elevations, which will increase the percentage of the wall below grade since you may be 4 or 5 feet deep for a panel adjacent to a footing step.

2. In addition to the usual tables and standards, find out what the local wind requirements are. Local conditions in mountains are know for radical wind and snow loadings.

3. Are you sure your application actually requires joint reinforcement in every course? What are the panel lengths? Extra heavy should allow greater vertical spacing. Trying to lap reinforcement with 3/16" cross rods every course is not good.

4. As an aside and not to step on an architect's toes, suggest or change the drawing to have a cap the is canted (in or out) to shed water and melting snow. A drip edge would certainly make him think. Collecting water in a freezing environment is never a good idea either for appearance or durability.

Keep asking for opinions - everyone has different backgrounds and experiences.

Dick

 

[COEngineeer]

I appreciate your feedback concretemasonry.. here are my answers:

1. You are right... actually my boss just told me the same thing and I was fixing the detail when you I received your reply.
2. They said 80mph exposure B.
3. I assume the coarse are 16" long? Anyway, I used concrete masonry cantilever highway noise barrier walls to design it. Go to this TEK link and pick TEK 14-17

http://www.ncma.org/etek/homefrm_map.cfm?spdm=oldcastle.com

I followed the general notes on the last page.
4. I will adjust the cap to show it sloping.

Never, but never question engineer's judgement

 

[UcfSE]

Wind pressure for solid free standing walls is found in 6.5.13 of ASCE 7-02 and the referenced tables. When you say you looked up the wind where did you look specifically, what table or figure? Any topographic effects? What K_d? What importance factor? These are things to consider. What made me ask is 20 psf seems a bit high.

You should read the ASCE 7 through when you get a chance, soon preferably.

How confident are you that you will always have 3'-0" of grade above the footing? How sensitive is your design to a change in this value?

You are showing three different sizes of rebar in that single detail. Are you sure you don't want to limit it to one or two? I would use a smaller bar than no. 7. Those are big, heavy, very difficult to field bend and cut, require longer lap splice lengths, etc. At splice locations you are getting two bars in one cell. Big bars will get pretty crowded. The footing cross bars aren't called out. Using more bars here is a significant labor increase versus using fewer no. 5 bars. Do you really need two bars in the bond beam? That's another good source of overcrowding and difficulty pouring.

If you have moment at the connection of the stem to the footing, you need to check the hook development length. If the footing is not thick enough to provide the hook length, considering cover and the other bars in the footing, then you have to increase the footing thickness until you satisfy all the requirements.