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Pilaster Design in Residential Stemwall
2

Pilaster Design in Residential Stemwall

Pilaster Design in Residential Stemwall

(OP)
Has anyone ever done a pilaster at the end of a stemwall in a residential design? I have a large point load at the end of a 8" x 48" high stemwall and I am thinking this is the route to go. Are there any prescriptive resources or design guides/spreadsheets?

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

(OP)
Its seems that masonry pilasters are fairly common. The NCMA has a nice paper for ASD design:

http://www.ncma-br.org/pdfs/5/TEK%2017-04B.pdf

I'm not finding very much on reinforced concrete though.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

That's a pretty common occurrence. Basically, your pilaster is a short concrete column that's probably braced in both directions by your stem wall. It should be designed as a concrete column but the capacity will likely be vastly more than you require. Here's the procedure:

1) Normally, columns need to have 1% vertical reinforcement minimum. ACI lets your go as low as 0.5%, however, if you reduce your capacity somewhat based on some clause that I can't remember. In short, you'll want 0.5% vertical reinforcing. It's usually efficient to use as few bars as possible. That being the case, I try to keep bars to #6 or smaller in residential applications.

2) Provide ties according to the column provisions in ACI. In the absence of a high shear demand, these are generally based on preventing the vertical bars from buckling. 48 times the tie bar diameter or 16 x the vertical var diameter I believe.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Pilaster Design in Residential Stemwall

(OP)
This is kind of what I have in mind:



However, not ever having to design or engineer one of these before has me asking a lot of questions.

First and foremost is how to size the pilaster given the loads and the attached stemwall thickness and also that it is an endwall in the stemwall foundation. 30,000 lbs is not really much when it comes to concrete but when I thought about this much force at the end of an 8" stemwall it did give me some concern.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

The detailing appears to be headed in the right direction. As for evaluating the capacity:

1) If you go with the pier, you'll probably want to design it as a concrete column per ACI.

2) If you want to stick with just the wall, there are provisions in ACI for designing walls with concentrated loads that you can use.

When evaluating the wall on its own to resist the point load, it's kind of a tough call to evaluate the bracing situation. If one can count on the soil and slab on grade to brace the wall completely, then you can basically just use the bearing strength of the concrete which would probably work. Certainly if this were a wall corner condition, I'd considering going that route.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Pilaster Design in Residential Stemwall

(OP)
I've created a small spreadsheet to design the pilaster as a short column:



Based on these numbers the 12" x 12" column/pilaster is way over capacity given the loads, I'm wondering if I am too worried about this location and if the 8" stemwall might be fine without any further modification.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

(OP)
I am looking at Eqn. 14-1 in the ACI 318. This eqn. looks like the right one for concentrated loads on walls, now how to actually use it...

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

(OP)
Looking at the provisions of Section 14.3 I am wondering how often these minimums with regards to vert. and horz. reinforcement are actually followed in the design and construction of residential foundation walls and stemwalls.

In particular Section 14.3.5 states that vert. and horz. reinforcement should be not spaced more than three times the wall thickness, however I typically call out #4 vert. bars @ 24" o/c for most of my 6" stemwall foundations, technically this spacing should not exceed 18".

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

(OP)
Based on equation 14-1 with a wall thickness of 8" and a wall height of 4 ft. and assuming an Ag of 96 in2 (8in x 12in) with a comp. strength of 2500 psi I get a design axial strength of 82,783 lbs. This is more than twice the factored load of 37,743 lbs from my previous worksheet.

What I am a little unclear on is what to take as the appropriate Ag in the equation. Is this a unit area of the wall or is it the actual area of the bearing load (in this case it would be a 5.5" x 5.5" post.

To think that a 8" wall could take a point load of 82,000 lbs (given the appropriate footing/bearing) seems ludicrous but then again I am used to working with wood so my gut instinct on concrete strengths is probably all wrong.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

(OP)
On page 9 of this example problem the Ag is taken as the wall width times an effective length and not the bearing length, I am now thoroughly confused. What is even more surprising is the bearing strength of the concrete under the bearing plate has less capacity than a 15 ft. high wall which seems somehow counter intuitive to me.

http://www.assakkaf.com/courses/ence454/lectures/h...

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

a short 8"x12" column as you've designed it would easily hold that. The issue is when you start to account for bending as well that the capacity drops.

I would be checking for bearing capacity under the post but then again, the wood post would govern. and then I could see the load spreading to a 2' wide strip almost, I think only saying 12" is conservative.

are you planning on putting ties around additional bars?

I've put 38 kips on an 8" foundation wall before without even thinking about it. Although my 38 kips may have been more conservatively determined than yours.

RE: Pilaster Design in Residential Stemwall

(OP)
In reality what I have is a 6x6 LVL post with about 19000 lbs gravity loads coming from the glulam beam above and then a 6x6 chord of the shearwall next to the post with about 9000 lbs tension/compression from wind. So in actuality the the load will be spread over a 6"x12" area roughly, with the bulk of the load on the 6x6 end post.

I think a 8" wall could easily hold the load but if it was in the middle of the wall but I'm applying this point load at the endwall situation, my worry is the concrete spalling or shearing off in some weird failure mode that might be hard to account for.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

In my residential market (1/2" or 5/8") 40k rebar rod is all that is used

RE: Pilaster Design in Residential Stemwall

If you hooked your horizontal bars and maybe provided one or two extra near the top you'd likely be ok.

RE: Pilaster Design in Residential Stemwall

(OP)
Explain the hook, I'm assuming its a 180 degree and it extends back on itself for a foot or two to achieve further development.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

I was just thinking a 90 with a 6" long leg that you turn each bend towards the other wall face, essentially turning your horizontals into a stirrup of sorts at the end. I know you aren't getting enough embedment length but it's better that nothing. On second thought, I think I like providing hairpins at the end of the wall to confine the end two verticals.

I'm just suggesting things to try and mitigate your blowout concerns. I actually think your spalling/splitting concerns are reasonable, I can picture a wedge of concrete at the top of the wall trying to come off.

RE: Pilaster Design in Residential Stemwall

(OP)
Jayrod12 I think you've hit on the appropriate solution for this situation. I will provide a small detail with two vertical bars at the end wall and the horiz. rebar wrapping around them and then back at least 24". This should be a good compromise between a full column/pilaster design and no treatment at all. I just found a similar detail online that deals with an endwall in almost the same manner.

I think capacity was never as much of an issue here as was my concern of a "blowout" at the end/top of the stemwall. I appreciate everyone's suggestions and help. I now have two new spreadsheets for concrete columns and walls as well as a much better idea how to deal with concrete walls in the future.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

(OP)
Here is what I have so far:

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

I bet the contractor would rather just provide hairpin dowels with a 24" leg on each side as opposed to being the horizontal bars. Just for constructability.

And that would likely be ok in my eyes.

RE: Pilaster Design in Residential Stemwall

(OP)
I would tend to agree, here is draft #2:

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

I like that detail, might even consider making the hairpins 36" long each leg so you are sure you're getting embedment depth past your inboard pair of verticals.

But that's just a "feel" comment and they're likely ok as you've shown.

RE: Pilaster Design in Residential Stemwall

Hey medeek, sorry, hijacking your thread...but I have been meaning to compliment you on your CAD drafting, and most especially your color shading on wood members. Do you use AutoCAD to produce your drawings, and if so, do you use "hatch" or some other command to get the color rendering to wood members?

RE: Pilaster Design in Residential Stemwall

(OP)
Thank-you. I sometimes like to shade the wood since if it often confusing with some of these details as to what is empty space or framing. I use AutoCAD with a solid hatch.

I've got a few drawings posted here that you can open up and look at the shading and the colors I am using:

http://design.medeek.com/cad/cadlibrary.html

The only downside is the extra time and effort to apply hatching to the framed areas and then to make sure they are behind all of the other line and text objects.

I also like to use a bit of color in my drawings since I think it helps differentiate things like structural notes, dimensions and physical objects.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

Thanks for the link to your CAD library.

Your drawings have a very clean and concise look. Nice use of color without making it an 'architectural rendering'. Well done.

RE: Pilaster Design in Residential Stemwall

(OP)
I have played with the colors a bit over the last year and the goal is to use the effect minimally. If the wood shading is too dark it becomes distracting and overlaying dimensions are also harder to discern. Just enough color to indicate the wood framing.

For straps I've starting using a light gray shading with a blue outline, to help differentiate from wood or concrete objects. However, I'm not sure if I'm sold on the dark blue outline yet.

I'll admit I was influenced by different framing textbooks (Dewalt etc...) in my methods.

I'm not sure why I started using dark green as my dimension color but it somehow stuck and I am a creature of habit I guess.

For structural floor plans I use a light shade of gray for the walls, dark enough so that the walls pop out at you but light enough so the location of the holdown anchor bolts is clearly visible.

I've created a vast number of blocks so inserting windows, doors, posts etc... is fairly simple.

For structural notes and tables I like to use a dark blue to set them apart from my dimensions. I borrowed that idea from one of my mentors.

For shearwalls I use a deep red with a dashed heavy line weight.

I use the flux architect font by Clayton Cowan, mostly because it was free at the time I was looking for a suitable font, however it is missing a few important characters like the degree symbol but AutoCAD deals with this just fine.

The drawings really pop when you print them out full size at 24 x 36 (Arch D):



http://design.medeek.com/images/misc/WARD_RESIDENC...

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

(OP)


Meant to post as an image.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

(OP)
Yup, I've been fully derailed.

My philosophy on drafting is that with only four key variables some amazing drawings can be created. Those four elements are:

1.) Lineweight
2.) Line Type
3.) Color/Grayscale
4.) Hatch Type

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

I've never used actual colour on drawings. No one wants to pay the added charge of colour copies. And I guess I'm still stuck in the old ways of hand drafting where all you had was different shades of Grey.

Your stuff looks awesome but I wonder what it looks like once the contractor has printed it in black and white and then made a bunch of copies on the xerox.

RE: Pilaster Design in Residential Stemwall

(OP)
This is another reason I keep my solid hatches to a lighter shade, once they go to grayscale if they are too dark they will obscure arrows and other features.





The use of dark colors for text also is important so that the contrast is maintained once they go to grayscale.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Pilaster Design in Residential Stemwall

Quote (jayrod12)

I've never used actual colour on drawings. No one wants to pay the added charge of colour copies. And I guess I'm still stuck in the old ways of hand drafting where all you had was different shades of Grey.

Your stuff looks awesome but I wonder what it looks like once the contractor has printed it in black and white and then made a bunch of copies on the xerox.

Nor have I added color, but with PDF files and inexpensive 11x17 color printers even the small residential contractor has cheap access to color printing.

11x17 printing or the use of an iPad are my personal choices on site.

RE: Pilaster Design in Residential Stemwall

I have used shading like you show for a number of years now. It works very well when done correctly. Have gotten very good comments about clarity from other engineers and contractors/framers.

"It is imperative Cunth doesn't get his hands on those codes."

RE: Pilaster Design in Residential Stemwall

Medeek,

Do you have a copy of your Pilaster spreadsheet that you would be willing to share? I've really enjoyed your posts and the responding threads. I am getting into the residential market; just trying to figure out how to make money doing it.

Thanks,
PBW2

RE: Pilaster Design in Residential Stemwall

(OP)
Technically the spreadsheet shown above is for a short concrete column (axial loads only, no moments).

I'm not going to say it is a polished product since I've only used it once and it has not undergone multiple revs like some of my other spreadsheets, this is my first draft.

You can download a copy of it here:

http://design.medeek.com/resources/column/

There are more complete spreadsheet/calculators out there that do deal with axial + moment loading if that does become a design issue.

With regards to making money at residential I'm feeling the pain. I've recently updated my pricing to $0.40 per sqft for residential structural work, lets see where that gets me...

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

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