concrete pier question

[Don_P]

Beavers,
That was flip, I was working on this post  
Thanks, I do appreciate the praise.
First off it is the law... I render unto but you can use your judgement. I have posted several options that I think an engineer would approve. The recent series of braced corner drawings from my end are outside of code, they are not full perimeter. They were the result of a conversation about a failure with someone I consider to be competent. If you're going to wing it those ideas are more stable by far then unbraced piers. Should I suggest something outside of what is known to be safe and will pass easily... probably not.

Rather than asking me, who is not competent to perform this work, pool your resources and have a set of basic guidelines drawn up. Engineering students need projects too...

One old credo did pop into my mind in the back and forth "An engineer's job is to do with one dollar what anyone else could do with two." But yes, I could tell some stories  

Don, you were posting while I was writing. I read back through that thread at the time, there was warning given, maybe not loudly enough but it was there. Notice all post bracing was assumed to be given by the soil and that unknown was not up to the task. If it is unquantified call it's bracing zero and brace adequately by another means.

This is from an earthquake. Obviously they didn't use any embedment in the ground to try to brace it but it was braced well enough for the posts to remain upright even when it slid off its footings. Notice how the braces oppose each other, are on every pier in both directions and connect low, like the drawing I posted above. I doubt those are just nailed. I've seen other pics from this quake with the houses on the ground and unbraced piers laying around. Those houses just dropped and broke.

[eddiescabin]

Don_P...those are great pics of the earthquake damaged home!  Wow, even tho the footing failed the simple 2x6 bracing held up!  Imagine the forces as it came off the footing!  To me the 2x6 braces to not look beefy enuf, I would have used the same dimension lumber as the pier itself, but look how well they did!

[John Raabe]

It looks like the Hawaiian foundation above had no anchorage to the pier footing other than gravity. With earthquake forces lifting up and then moving the ground (or building) laterally the bouncing house came back down someplace other than the center bearing of the foundation block.

A cast bracket in a poured tube pier could have helped hold the two together.

It could have been much worse without the bracing. Triangles RULE!

[umtallguy]

Don_P...those are great pics of the earthquake damaged home!  Wow, even tho the footing failed the simple 2x6 bracing held up!  Imagine the forces as it came off the footing!  To me the 2x6 braces to not look beefy enuf, I would have used the same dimension lumber as the pier itself, but look how well they did!

lumber is strong.... minimum specs for a 2X4 in vertical compression to pass grading is 7 tons I believe....

[MikeOnBike]

We are planning a 20x32.  The location is a rock shelf above the creek on our property.  The current plan is to sink pins into the rock and then place concrete piers w/rebar over the pins and extend the pier up to the support beam.  We do not plan to elevate the beams on posts.  The pier height will be just enough to make up for the slight slope of the rock shelf.  We do plan to cross brace from the concrete pier to the beams.

I see a lot of builders putting their cabins high up on the piers, often 2'-3' above grade.  Am I missing something here?  What would be the reason for elevating the cabin so high?  I only want the uphill piers just high enough to keep the beam off of the ground. probably 6" at the most.

[Don_P]

Terrain can be a biggie. I'm working with some owner builders on a pier and beam that runs from 8" to 6' pier height above grade.
Took some shots today for a job, 10' elevation change across the footprint

[MikeOnBike]

Wow, 10' feet, that's a lot!  I can understand the site grade dictating some tall piers.  What I don't understand is when all of the piers are at least 2'-3' feet tall.  An example is the tilting house in MountainDon's post.  I have seen enough of them that I wondered if I was missing something.  Sorry if I sound derogatory.  Unless I had a steep site, flooding issue or the extra couple of feet 'made' the view I would want the structure close to the ground to reduce the number of stairs and engineering complications.

[John Raabe]

Sometimes a higher building gets you a better view. Then there is the storage issue - kind of a half basement. In a hot humid climate the extra height can mean better ventilation and a house that is further away from moisture and the creepy crawlies.

Lots of things get factored into what we call "personal preferences". That's why every house turns out to be a custom house whether it started out that way or not.

[Woden]

I am building on exposed Canadian shield. It is quite flat. I would like to use the recommended concrete piers. Do you have any suggestions for such a situation (fastening, mortar?). I could find no such example in the plans.

[John Raabe]

Are you pouring piers on solid rock? Is that the question?

If so you will find that drilling and setting a pin that gets wired to the rebar of the pier will provide a solid anchorage in most situations. Ask a local foundation contractor who has experience with similar building sites.

Several forum members have built piers this way.

[Woden]

I was wondering if I could still use the pre cast piers by mortaring them to the rock?

[John Raabe]

The mortar would not make a very secure connection, but for a small building in an area without earthquakes or hurricane force winds something like this would likely work. It would be better to use concrete than mortar - it has better bearing strength.

Best would be a small concrete pier poured over pins epoxied into the rock. Then you have some real protection against a sidewards or uplifting force. Since the rock is dense you don't need much of an area - maybe one 8" or 10" dia. tube would do the trick. Just cut short sections for each pier and cast the bracket in the top.

[jfmann]

Design errors similar to the character of those noted in the previous post (Beavers).......relative to heavy construction........ can and do occur even for "engineered" plans of residential construction. I offer the following comments about this topic relative to residential design. I am a professional engineer with 33 years experience.......including a wide range of project types, from railroad bridge design to structural design of houses, including along the ocean.

Although design of basic elements..........such as floor joists, roof rafters, "simple" foundation walls..........can be performed adequately by builders and even homeowners using "standard" tables and other design assistance.........there are many other aspects of proper design that are all-too-often overlooked, ignored, neglected or just performed improperly.........especially when a house has geometry or conditions that are not typical.

Many builders and owners take the position that "good enough" is, well..........good enough for them.............meaning that adherence to minimum requirements of the building code is not necessary. They generally opine that design for the long term.......and conditions with low chance of occurrence....... is not important. This position neglects to consider that..........in all likelihood..........the building will be there long after they have gone.........either because they sell and move or because they move on to "higher ground". The next.......usually unsuspecting...........owner then has a house that is of a quality less than what should be to satisfy code requirements.

I try to break down this key issue to anyone who will listen as follows; ..........When you buy a car (whether new or used), you expect the brakes to meet some safety standard. If you knew.......before buying the car.........that design of the brakes did not meet minimum safety standards..........you would either not buy the car or you would expect a much lower price compared to the usual price for a car with up-to-standard brakes.

Design codes are based on the probability of events ........in the long run........that have low probability of occurrence in the short run. We try to ensure that what happened during Hurricane Andrew (widespread severe damage to buildings all at the same time) does not happen. Much (perhaps the vast majority) of damage that occurred to houses during Hurricane Andrew was the direct result of sub-standard design and construction..........due to those responsible taking the position that building should not have to conform with "overly conservative" requirements of the building code.

Houses are most generally designed by architects, without any input from a structural engineer. From my experience..........the vast majority of architects today do not have adequate training or experience to provide adequate structural design......except for perhaps a small rectangular house far away from hurricane zones.

It is also the case that even licensed engineers can.......and do....still make major design errors for residential construction. However........also based on my experience..........I have seen the aftermath of many major problems with houses that were supposedly "designed" by builders and (to lesser extent) owners themselves. More than a few of these have landed in legal mayhem.

For houses with relatively complex geometry..........proper structural engineering is essential.

The one design area that is least understood.........most neglected...........and most important (outside of serious earthquake territory)..........is design for lateral resistance to resist wind. Seismic is not so much of a problem because it is generally considered "important". Design for wind is very important in hurricane regions.........which includes the entire east coast and Gulf coast.........areas with extensive (and expensive) residential construction. Very often neglected is the understanding that hurricane zones extend inward from the coast line a fair distance.

When hiring an architect or engineer............pay attention to qualifications.........not just the fact of a license.