Posted by Jonathan Maguire on Tue, May 11, 2010
In part one of this discussion, we talked about what a helical flight is. This post will hopefully add some insight into how torque equates to loading in helical piers. We will also discuss one of the HUGE blunders of a local manufacturer.
Helical piers are screwed in by a hydraulic gear motor. This motor generates a hydraulic pressure on the gear that equates to a torque (force) on the pier being installed. This torque is then multiplied by a soil factor (when soil testing is done there is an actual number defined for the soil and for the product, when soil testing is not performed a generic ‘average’ is used). For the purposes of this post lets use the following equation (used from a technical manual written by Don Clayton, PE) -
Ultimate Capacity = Torque x “k”
where “k” is equal to the soil multiplication factor.
This is a simple equation, and to give an example for a light duty pier – we generally install our helical piers to 4,500 ft/lbs and use a standard average soil factor of 8.
Capacity = 4,500 x 8 = 36,000lbs ultimate capacity
since the ultimate capacity on a single flight is between 40,000-50,000 lbs (steel and weld capacity) – a contractor could use a single flight configuration for this loading.
Now for the HUGE blunder by a local manufacturer. Let’s review some details of helical piers:
- Torque is one of the main factors of the ultimate capacity of piers
- Helical pier types can handle different torques (thicker pipe = stronger, etc)
- Flights have an ultimate capacity of 40,000 lbs for the flights – but the pier torque is what generates the ultimate capacity.
I have run across the situation where a standard residential pier (.203 wall) is sold as a high capacity pier (60,000-70,000 lb ultimate capacity) by the salesman telling the contractor that he’ll just add another flight to the pier. If you double the flights you do add additional capacity, but the problem is – You simply cannot install a standard residential pier to the torque required to generate 60,000-70,000 lbs. The pier will fail during installation – remember the equation
Capacity = Torque x “k” where “k” is the soil factor.
60,000 = Torque x 8
60,000/8 = Torque = 7,500 ft/lbs
Typical residential piers can be installed to around 5,500 ft/lbs – max. You cannot get there from here no matter how many flights you add to the pier shaft (in fact you will notice the equation doesn’t even factor in the number of flights). But I’ve seen this, and I’ve seen general contractors buy this lie from the local company “engineer”.
Posted by Jonathan Maguire on Fri, Mar 19, 2010
In 2003 there were 20 helical or resistance pier manufacturers, 4 of them were considered “Major”. The top two companies were AB Chance and Atlas Systems (where we received our name). In 2009 there were over 50 manufactures of piers, with AB Chance topping the list followed by Earth Contact Products then Dixie, RamJack, DriveRite, etc.
As you can see, there has been a LOT of manufacturers enter into the business in the last few years. Many of these manufacturers are making helical and resistance piers from their own shop or “basement”. The top companies – and ones with the longest history have licensed engineers behind all their designs, a full fledged manufacturing facility, and quality control. The smaller guys are making a few piers - and while some of these 2nd tier or 3rd tier manufacturers may have engineers that help design, there is no way to compete with the larger companies in quality control, manufacturing and oversight. When you are supplying multiple thousands of piers (sometimes tens of thousands of piers), your quality has to be top notch. You cannot afford product failures, you cannot afford for a 3rd party engineer or inspector to question your products. There is just too much riding on them.
Why does this matter? Atlas Piers of Atlanta only purchases from platinum manufacturers. We have too much at stake to trust a lesser manufacturer. So when you have someone look at making foundation repairs to your structure ask which helical pier or resistance pier manufacturer they install. Piers may seem like a commodity, but there are vast differences.