Block Piers Filled cells with mortar but no mortar in bed joints- what is their strength? 2

[ajk1]

I am reviewing some aspects of a cottage that has been built in the last year. I had nothing to do with its design or construction. To avoid putting in a pump for the septic tank, the cottage was built so it is raised up, sitting on concrete block piers. The land slopes, so the piers vary from 1 block high to 7 blocks high. Each pier block is 10" x 16".

All piers sit on rock according to the contractor. The piers were built with no mortar in the bed joints, but the contractor says that the cells were filled with mortar (although drilling a hole into each of 3 piers seemed to indicate void in one pier and solid in 2 other piers -- I have not been to the site to see -- the contractor refused to drill the 5 exploratory holes I requested, saying 3 was enough. There are 11 piers under the cottage).

Construction photos show that there are 2-15M dowels per pier, and that they extend about roughly 4 to 6" up into the bottom block from the concrete base below. The contractor says that there is no other rebar in the piers.


QUESTIONS

How to check the structural adequacy of the piers to resist the wind lod on the cottage and carry it down to the footing?

I have tried two methods, as follows:

Method 1: Check the maximum horizontal force that can be applied to each pier, based on a saftey factor of 1.65 against overturning. Sum this load for all piers, and compare it to the wind load acting on the cottage, in each direction. If it is less, then there is no chance of the piers having adequate resistance against the wind, and determining how much wind load goes to each pier (by computer modeling) is unnecessary.

Method 2: Similar to methiod 1, except base it on stresses in the grout, using only the grout in the cells and the Code allowable stress for grout (if I can find that). Perhaps I could take the face shell on the cmpressive side of the block as working with the grout in the cell, in carrying out this check.

Both of these methods seem to indicate the piers are inadequate (based on 30 psi allowable tensile stress in the grout in Method 2, although perhaps I could use a higher stress).

Any comment?

 

[CELinOttawa]

If there is a positive attachment from the cottage to the piers, I would apply your Method 1, but also add 0.6 or 0.9 of the dead load to help resist overturning. That's what the structure will really do in any case, and it is simply a matter of choosing what factor to apply in order to simulate the "true" beneficial force.

In terms of the pier, I would be checking it as a minor flexural element on the longer cases. 7 high is going to be close to 1.5m from tip to tail, and that may very well be enough to cause problems if poorly constructed and not well reinforced.

 

[ajk1]

Hi CELinOttawa:

The numbers I gave included the beneficial effect of the dead load on the pier from the cottage and the weight of the blocks less any wind uplift force, although I did not explicitly say this in my message. That is how I arrived at the 1.65 safety factor against overturning, namely a 1.4 load factor for wind load, and a 0.85 factor for dead load;then 1.4 / 0.85 = 1.65.

The Code used to require a safety factor of 2.0 against overturning, but I don't have the latest Code here at home (I will check in the office on Monday). I thought I could reduce it to 1.65 based on the Limit States Design load factors.

I am not understanding what you mean by a "minor" flexural element. How is that different than checking as a "major" flexural element?

As for reinforcing there are 2 -15M bars that extend from the concrete footing only 4" to 6" into the bottom block and no reinforcing above that. For this reason, I checked overturning at 1 block course above the footing. I assumed a point of rotation at the face of the block; this may be un-conservative.

I wonder if there is anything in the Code saying that it is not necessary to consider wind load on cottages? Even if there is, I would say that when they raised the cottage 7 feet above grade, then it is no longer a Part 9 structure, but should be designed as a Part 4 structure at least as far as wind load goes, although I suppose I might get some push-back on that from the contractor.

 

[ajk1]

Attached are some photos of photos that were taken by others on site.

 

[ajk1]

and another photo

 

[dhengr]

Ajk1:
You certainly should get some extra points for persistence. But, those points will be cancelled out quickly by negative points for your lack of understanding that you can’t turn a dog of a messed up cabin, from the construction standpoint, into a code compliant mess. Then when the contractor who didn’t need any engineering help in the first place, because he could mess it up all by himself, with his vast building knowledge, starts arguing with you about drilling a few extra exploratory holes in piers; piers which won’t check in any case, you ought to have sense enough to back out of this mess while you still have you’re a$$ attached. You’ve gotten lots of good advice on some of your other threads on these same cabin problems. You should reread them for their full engineering content. There isn’t a thing you’ve talked about or looked at that is done in any way approaching reasonable small residence building standards, or good structuring. Unless you have a contract with the cabin owner and are getting paid by the hour, in advance, you really ought to put this one out of your mind, and go on the something productive. You are going to end up being responsible for this whole mess, if you let some of this stuff stand as is, and don’t get good buy-in from the contractor and the owner, that they want to make it right. Some things you just can’t fix, and that doesn’t make you a bad engineer, particularly when the contractor won’t listen to any of your common sense, and doesn’t have any of his own. Put an exterior found. wall around that damn thing. It will solve several of their big problems, gain some dry storage space and/or a conditioned walk-out basement, and you can go on to the many other problems. You still have interior beam lines, shear walls and probably roof framing and hold down to resolve. All of this to gain a waste line to the septic tank which will freeze, but doesn’t need a holding tank and pump.

 

[msquared48]

Personally, I would blow it in place and start again, doing it right this time after firing the contractor.

Mike McCann, PE, SE (WA)

 

[BAretired]

The contractor has already demonstrated that the piers are not all filled with mortar despite his earlier assurances to the contrary. That, combined with his shoddy workmanship and his refusal to carry out the inspection you requested should tell you what to expect in your future relationship with him should you choose to carry on with this project.

Like dh, I admire your persistence but perhaps the time has come for you to simply back away from the Cottage from Hell.

BA

 

[ajk1]

Thanks for the advice. In my opinion I have a professional obligation to show definitively by engineering procedures and calculations, that it either does or does not work, and not be a quitter and throw my hands in the air because the as-built conditions appear to be unusual. Sometimes unusual construction will in fact check out, or not be so far off the mark as it at first may seem. After all, it has stood up for over a year with no signs of distress, so it is not a case of being so bad that it is a clear and present danger. And a local engineer up there has told the contractor that he sees nothing particularly wrong with the block piers (he does agree that the incised beams don't figure), although I think that he has not yet thought about the piers sufficiently.

If some remedial measures are required, then it is up to me as a professional engineer, to identify what they are. If all engineers heed the advice to not get involved, then what is the owner to do?

I guess I should re-read Part 9 of the Code and see if there are things I have missed, and see what is in the masonry design Standard.

Again, thanks for taking so much time to provide very wise counsel, which I have a feeling I may end up fully accepting if things go the way you believe they are going. Your advice is very much appreciated.

 

[CELinOttawa]

If the contractor is delivering a job to Part 9, then there is not a great deal for any Engineer to do. As has been discussed here many times, Part 9 is NOT a professional design code. It is a set of minimum design standards which have evolved through traditional practice. Many of those procedures are, frankly, an analytical and structural joke. They just work enough that they don't often fail, and those failures are not frequent or catastrophic enough to force change.

SO: It is entirely possible, and I have several times over my career, to find yourself unable to help a client simply because you MUST do it RIGHT, and Part 9 is not to a competent Professional standard. As such you cannot help someone to a Professional standard vis a vis a Part 9 structure if they are still wanting to play Part 9. A Part 9 repair to a Part 9 structure is fine, a Part 9 level of quality repair by a Professional Engineer is an incompetent repair and one just waiting for a lawyer to eat you alive. I have seen it, as have other members here.

Now take good advice and tell the client it needs to be done right, despite the fact that this means undoing what was done wrong. If they don't like it, and you help them do something lesser, I'll just say that I'm glad it won't be my license, insurance, house, family, etc, on the line.

You're a smart technical engineer; that is evident from your posts over the years. Be smart here and do something at least somewhat in line with all the good advice you've received here. Stop thinking with your heart and use your head. Sometimes the right answer is No.

 

[darthsoilsguy2]

The Contractor is right about 1 thing. 3 holes was enough to show they screwed up executing what was already a bad plan to start with.

 

[XR250]

Gawd, after seeing those photos, I would walk away or remove the piers and replace them with 8x8's. No calcs required to show that what is there is not gonna work.

 

[ztengguy]

"And a local engineer up there has told the contractor that he sees nothing particularly wrong with the block piers (he does agree that the incised beams don't figure), although I think that he has not yet thought about the piers sufficiently. "

Ask him if you can hire him and have him stamp the calcs, approval "ok" or whatever.

 

[RHTPE]

I have always believed that, given enough time & money one can design a solution for almost any structural problem.

That said, there is usually not enough of either.

A simple solution might be to wrap each masonry pier with a reinforced, cast-in-place concrete pier about 30" square. If the footing is smaller than 30" x 30", increase the size of the CIP pier, and continue it down to the bottom of the footing. It'll provide additional mass to help offset the wind forces, and if needed provide additional bearing on the soil (assuming it's properly compacted at bottom of footing level).

If you are under contract by the owner, go on the record with your recommendations as to what will work to meet the applicable code. You can't make him/her to actually do the work but at least your conscience will be clear.

If you are voluntarily assisting in a resolution, and no one likes your advice, I would run away.


Ralph
Structures Consulting
Northeast USA

 

[XR250]

" It'll provide additional mass to help offset the wind forces, and if needed provide additional bearing on the soil (assuming it's properly compacted at bottom of footing level)."

Mass to resist wind loads? How do you do that calculation? Intuitively, I see how that works for VERY short duration gusts, but what about sustained winds?
You guys are making this complicated. There is no way to get what is there to work. Even if the piers contained rebar, there is likely not enough footing to prevent overturning. Rip out the piers and the wood posts and replace with one cont. post down to the footing. X-braces would have to be modified to attach near the bottom of the posts.

 

[KootK]

Ahhh... AJK. Whatever your remuneration package is for this, I hope that it includes lots of virgins and a lifetime supply of Lipitor.

I have to say, knowing what I know of this building, I wouldn't let my kids near it in choppy weather. And I'm not being hyperbolic.

Those piers have to go. Jack the building one pier at time, use your wife's hair dryer to blow the piers out, and get something in there that you can live with. If the owner and contractor object, tell them that you've gone as far down this path as you're able to and it's time for you to recuse yourself.

It's easy to talk tough when it's not you who's being "impractical" or "not solutions focussed". I know it. As you'll see from a post of mine this afternoon, I've got similar problems of my own.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ajk1]

While I do appreciate the facetious comments (rather like reading about Rob Ford before he became ill, though I would never vote for any Ford again),I came on the site to receive thoughtful engineering comment on my questions, with a mind to assess what needs strengthening and what can be shown to work. Some of you have done that.

I am doing this gratis, as I am semi retired, it is someone I know whom I am trying to help. I have told him that the contractor's consulting engineer must take the responsibility, but I am just trying to identify what does not work, and guide him if possible to the items that I have found do not seem to work. I have made my friend aware in writing, why I cannot take this project on, as I would be attracting liability by repairing someone else's mistakes.

I particularly like RHTPE's comment, as I was thinking something along what he suggested. Another thought I had was to wrap the block piers in carbon fibre,to provide tensile strength, but I don't know that the contractor would do it right, or that carbon fibre has been used to strengthen block, although I suspect it has. It would not add mass, so would still have to connect the pier to the rock which may not be so easy, although may have to do that anyway because would have to add a lot of mass to make this work. Also, the higher piers are more flexible than the shorter piers, so they take less of the wind load than the shorter ones, although perhaps when add concrete around them I could make the taller piers wider and stiffer. Sorry to any Americans here, but I have used the Canadian i.e. British spelling of fibre.

I will think more about RHTPE's idea about wrapping the piers in reinforced concrete. There would still be issues though with the wood bracing, particularly in the north south direction where the masonry piers are not in line, so the wood braces are offset! They go to the floor joists, and are displaced from a straight line by at least one joist spacing.

I do not understand the comment about why "mass" should not be used to resist sustained wind. Irrespective of whether wind is sustained or gust, the mass provides a stabilizing moment to resist the overturning due to the wind, and is routinely used to resist wind.

 

[RHTPE]

ajk1 - I have a cabin very similar in construction to what you are dealing with, albeit much smaller in plan. Mine has 4x4 posts bearing on piers that extend to 36"+ below grade and perhaps 6" above grade. It has withstood wind forces from several hurricanes and blizzards over its now 30 year lifespan. I will admit that it is unlikely to meet any design code (e.g. ASCE 7) yet has performed well. Let me be clear - when constructed it was not intended to be a permanent structure, and will no doubt succumb to the forces of nature much sooner than a well constructed building would.

The foundation that you are dealing can be strengthened. While many comments made suggest starting from square one, that is a call that is easy to make from the outside looking in, but not so easy when the logistics are factored in.

My initial suggestion of wrapping the piers with a cast-in-place concrete "surround" will help. Perhaps something akin to a grade beam might be better and not so involved to construct. Encapsulate the 3 piers underneath the main beams with a grade beam running parallel. If the existing footing bears on bedrock, better still - anchor the grade beam to the bedrock with bars grouted into drilled holes.

Weathering of the bracing will be a concern. They are in a location where they are "out of sight, out of mind", so maintenance is likely to be forgotten. You might consider revising the bracing in the direction perpendicular to the main beams. A "K" brace connected to a new horizontal member running from top of post to top of post might serve better and be more substantial than what exists now, yet still provide "headroom" to access the space below the cabin.

I will not touch on plumbing issues as others have mentioned. I suspect this is a seasonal dwelling where those matters have alternate solutions.

Ralph
Structures Consulting
Northeast USA

 

[ajk1]

RHTPE - your comments about the impracticality of starting from square one are right on. Thanks. Yes, it is a "seasonal" dwelling. The idea of a grade beam is something I had not thought of, but sounds very interesting. Given that the highest block pier is 7 courses high (4'-8"), how high would you make the grade beam? Would you bring to say 2 courses below the top of the tallest block pier, and tie it in to the pier by means of grouted dowels?

The bracing may be pressure treated, but i have to verify that.

In the direction perpendicular to the main beam i.e. in the north-south direction, they have used "K" bracing, but the problem is that the piers do not align in this direction, so the K is displaced. Since the piers do not line up, it is a problem of how to put effective bracing in this direction.

 

[KootK]

Okay, no more tomfoolery.

Part of the reason that I made the recommendation that I did AJK is that we've come full circle with our solutions here. I was the original proponent of concrete wrapping / grade beams (shear walls) way back on September 1st: Link. What it has come down to, in my opinion, is that you're clearly not going to be able to get this building to calc out, by the book, from top to bottom. You can either make some local improvements and give the building a pass based on judgement, or you can wash your hands of it. And, of course, that decision is entirely up to you.

Masonry can be reinforced with carbon fibre. Generally, one of the goals of carbon fibre wrapping is to confine the material within however. In this instance, do we feel confident that there actually would be solid material within?

As for the braces, might it be possible to install horizontal bracing beneath the floor to move the vertical bracing loads to where you want them? Not pretty but maybe possible.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.