Wind Pressures Acting on a Concrete Block Wall with Footings

[jefftsolar]

Keep in mind, I'm only an EIT with 1 year of experience trying to solve this problem.

I have a concrete block wall made nearly entirely of 4x16x12 solid block that is not attached to a building of any kind. The wall also have 8x16x12 Pilasters spaced 12'-0" on center typically. The wall spans 85'-0" and has 10" deep footings made with 4x16x12 solid block. I am to calculate a windward and leeward pressure on the wall and conduct a structural analysis to see if it's safe to install 60 solar panels onto this wall, each weighing 10 lbs each.

1) I'm not sure what design approach is recommended. I've been left to try to figure this out on my own. I'm planning on using MWFRS Method 2 for Free Standing Walls using the ASCE 7-05 Wind Code but not sure if it's correct.

2) If I had an example calculation to see if the concrete footing would be adequate, and to see if the added weight won't cause a greater moment on the wall with wind factored in, I could work my way through this calculation. Any recommendations on how to handle this scenario?

Appreciate any help with this matter, as I'm in a bind here.

 

[jdgengineer]

1) I think you are on the right track, I would use the method you mentioned for wind forces. Don't forget there are multiple wind cases to check. See Figure 6-20 of ASCE 7-05.

2) The footing will behave similiar to the footing for a retaining wall. You'll need to take the lateral load you determine from your wind loading, and apply that as the overturning force on your foundation along with the resisting dead load. You'll most likely have a trapezoidal force distribution from the soil's reaction. You'll then take the soil reaction and check the footing as a cantilever. I don't have a good example for CMU, but it's a similiar procedure to what is shown in MacGregor's Concrete Book for the design of a wall footing. Example 16-1 in my edition.

 

[steellion]

ASCE Free-standing wall is correct. Check the wall for C&C pressures and for MWFRS pressures with the eccentric cases provided. You can superimpose the moment from the wind with the moment caused by the weight of the panels (P x e) to determine if the wall is acceptable.

 

[DST148]

@jefftsolar: developing a standard hook in a 10" thick wall footing may be a bit of a stretch but it may be acceptable since the wall could span horizontally between the pilasters for wind loading. However, the depth of the footing at the pilaster will have to be deep enough so as to develop the rebar in tension.
Wider footings at a greater depth below the grade would help for stability. Although the passive resistance from the soil may be considered in stability and in reducing the moment on the pilaster, it is better not to count given the free standing nature of the wall.

 

[jefftsolar]

Jdgengineer - Thank you for your input. Is there a way you can by chance make a photocopy or screen shot of that example of a wall footing you have and send it over to me via private message or anything of that nature?

Steellion - Good input from you as well, thank you. When you say "Check the wall for C&C pressures and for MWFRS pressures with the eccentric cases provided", I'm not sure what you mean by that. Are you suggesting that I should calculate the wall pressure using C&C as well? Unfortunately, I am out of the office and don't have the code book with me, but where in ASCE 7-05 can I find the eccentric cases?

Again, thanks for everyone's advice, it definitely helps

 

[jefftsolar]

DST148 - I agree with you. Since the soil type is not known and more than likely won't be provided, I won't be able to know the passive resistance from the soil surrounding the 10" deep wall footing. Even though this is just a screen wall separating property in a residential neighborhood, do you think they would reinforce the concrete blocks on that wall with Rebar?

 

[jdgengineer]

Typically with a pilaster wall the lateral wind pressure is applied to your wall surface. The wall is assumed to span horizontally between the pilasters. The pilaster are then designed to resist the reactions from the wall spanning between. The pilaster then needs to have adequate reinforcing as well as adequate anchorage into the foundation to resist the overturning moment at the base. Often these foundations are deeper / wider at the pilaster so that the reaction from the pilaster is resisted locally. Alternatively, the footing of the wall can be designed to span this moment from the pilaster until the soil can handle the reaction. This puts the footing into torsion.

The foundation forces are then determined baed on statics. You have the overturning forces from the wind as well as the dead load of the wall. The soil pressure diagram can then be determined by statics assuming that the soil cannot handle tension loads. It will either be trapezoidal or triangular. Once the soil pressure is found the footing is checked for both moment and shear based on the soil bearing pressure. Depending on your pilaster conditon you may need to check torsion.

So to do these checks obviously you need to have a handle on the allowable soil pressure, the masonry strength, the steel strength, and the rebar layout. Do you have existing drawings? All of these items need some form of engineering judgement and it seems that there needs to be someone in your office that can help guide you. I'm assuming you are working under a PE who can guide you in the right direction? Otherwise, I think your office should not be completing this work.

One more thought - where I am, rarely does wind govern the design of these masonry walls. But if seismic is not an issue, I think maybe we are over thinking this. It seems to me (and maybe I'm thinking of something wrong) but the solar panels don't seem like they would effect the wind force that is attributed to the wall (unless it's an odd layout that s increasing wind surface area). From a wind perspective, the additional weight of the solar panels seems that it would be beneficial (ie it's a resisting moment). Therefore, it seems like you should be able to check the bearing pressure comsiderimg the weight of the wall and the additional solar panel weight (ie no wind) and ensure the foundation has adequate reinforcing and shear capacity for the additional dead weight and be done with it.

 

[DST148]

@jefftsolar - I missed the point that you want to propose solid blocks. What is your objective here? Do you want to use ordinary plain (unreinforced) masonry there by avoiding rebar trade altogether?
Ref ACI 530-05: Although wind will govern the design, but you still need to comply with the seismic provisions. Unreinforced masonry is acceptable in SDC A and B but not in C and above. Moreover, walls interfacing with pilasters can not be considered as flanges unless the provisions of section 1.9.4.2 are met.
What occupancy category you would assign to this screen wall? Since the wall is right on the property line, are you withing your legal limits with footing projections into the adjacent property?
Codal provisions apart, keep in mind (your OP) that cantilevers do not show any mercy.

 

[jefftsolar]

To make things more clear, the objective of this scenario is to check to see if the addition of solar panels mounted to one side of this concrete wall is safe. They require a set of calculations to be signed and sealed by a PE. It's required by a building safety specialist for the state of Arizona. As far as I know about Arizona, wind usually governs in that state versus seismic, but then again, I could be wrong.

We have been provided drawings, but only on an 11x17 sheet provided by a solar contractor. We have only one PE on staff, and I'm trying to help him out as best as I can with this scenario (and at the same time, he's "throwing me in the water to see if I'll sink or swim").

 

[Ron]

The location and orientation of the solar panels will be relevant and perhaps controlling. If, for instance, they will be attached to the top of the wall and oriented at an angle, that will induce a moment at the top of the wall. If they are to be installed in a plane parallel to the vertical wall plane, but not extend above the top of the wall, they will have little effect on the wall, other than connections.

Your ASCE approach is correct. As for the footings, and assuming you are in a reasonably high wind area, overturning will control, so be sure to check for partial loss of contact and an increase in the bearing pressure.

Even if you don't know the soil type (which you should) you'll then have to use the assumed bearing capacity allowed by your governing code. If you know the soil type and can call around to a local geotech, you will get some general guidelines as to allowable bearing capacity, keeping in mind it will likely be conservative; however, if you are not going to get any soils investigation (even hand auger borings with a visual classification would help), you are better off to be conservative.

 

[jdgengineer]

The location and orientation of the solar panels will be relevant and perhaps controlling.  If, for instance, they will be attached to the top of the wall and oriented at an angle, that will induce a moment at the top of the wall.  If they are to be installed in a plane parallel to the vertical wall plane, but not extend above the top of the wall, they will have little effect on the wall, other than connections.

Ron. You are correct I posted this in the middle of the night when I couldn't sleep. At the time I was envisioning this to be a "T-footing" with the solar panels distributed symmetrically on the wall essentially parallel to the wall. Obviously, this is too idealistic.

 

[jefftsolar]

Thank you guys for your advice. I actually ran into more information about this project that wasn't given before, and turns out the wall isn't exactly free-standing, but braced on both ends by walls that run like the sketch as shown.

http://tinypic.com/r/c6q2b/5

I know that Free-standing walls are calculated a certain way using MWFRS, but now since we know that they are braced, I'm not exactly sure how to calculate it. The PE in our department said that wind doesn't change horizontally, so I'm thinking of simply calculating qz / qh to determine the windward, leeward pressure on that wall. Just checking to see if it's the right approach.

Again, thanks for helping me out in this situation. I'm struggling like crazy to make sure it's right.

 

[BAretired]

A wall 85' long with pilasters at 12' centers is laterally braced at each end, so the ends are presumed to be braced, but the central portion of wall can be considered free standing.

How high is the wall?
How is it reinforced?
Where are the solar panels to be mounted?

BA

 

[BAretired]

A cross section would help.

BA

 

[shobroco]

Am I the only one who has never heard of masonry units of these dimensions? Our blocks are metric, but they are essentially the same size as imperial: 200X400 or 8"x16" face, 100,150,200,250,or 300 (4,6,8,10, or 12") in thickness. Why would one build a free-standing wall of this size of solid units? Even if you don't want to reinforce it, hollow units are cheaper to buy, cheaper to lay, and nearly as strong. Reinforcing it is of course impossible with solid units, and very desirable with a free-standing wall.

 

[ToadJones]

I was thinking the same thing. It's like the whole wall is made of cap stones or termite blocks.

Also, the OP sounds like the footing is made of block too???

 

[concretemasonry]

All around the world, the dimensions of a are block listed by thickness, height and length. A 4x8x16 sounds like a 4" wall made of 8" high units to most people using ASTM and associated code references.

Is this wall just a minimal 4" privacy wall between pilasters. As shobroco mentioned, if the block are laid flat, there is no way to reinforce them, but the wall is thicker and more stable to a height limit. It is just a wall that almost falls into the empirical design realm and is not an engineered wall.

Hollow units are obviously better since they can be reinforced (horizontally and vertically as needed). 6" thick block are also cheaper to lay than 4" block. In many areas, 6" block are the standard because of the cost and design flexibility, especially on high-rise buildings, but horizontal reinforcing can dramatically increase the wind resistance, although it is not done on high structures.

The block footings that cannot be reinforced are troubling problem, but they may be multiple layers with no appreciable true vertical load, but they can provide an amount of lateral resistance and some shear resistance.

I guess we just need some more masonry engineering classes in U.S. colleges. I say that because I helped to establish a 3 credit class about 25 years ago and now use international engineers as a benchmark in comparison to U.S. graduates.

Dick



Dick

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

 

[hokie66]

jefftsolar,
Your wall must be reinforced, and you can't reinforce a wall built of solid blocks.