Calculating Load for Helical Piles
I’m building a deck (a 18’x31′ rectangle, with a 8’x12′ rectangle attached to it, in the shape of an L). It’s going to be between 22″ and 30″ above ground (there is a little slope). I think using helical piles would be environmentally better, not that more expensive, and indeed easier. Someone told me that calculating the load capacity for each pier would be easy; yet, I am having truly a hard time. All the calculators I find assume that I’ll be using concrete footings.
Does anyone have any tips?
Also, am I right in thinking that the helical piles company will install the piles for any load capacity I asked them to? (That’s what a representative for one of the installing companies seemed to say.) If so, how much do the calculations really matter? If, for instance, my calculations for the “tributary areas” give me results that are all lower than 2,000 Lb for each area, can’t just ask for each pile to have a 3,000 Lb load capacity?
Thanks for any input!
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Replies
-You might want to budget about $500 per pier. If using "Tecno Metal Post", they often suggest a P3 for better lateral resistance. I think there is a lot of truth in that (I think it is a 3" pipe vs a 2" pipe). Since your deck is "grade level" you may opt for a p2 as the lateral resistance is not as critical.
-This article is the best that i have found for sizing deck footing-
FHB- "complete guide to sizing deck footings"
-Yes, you can tell the pier installer what load you are looking for and they will provide that to you. But the load calculations are importantI so you know how much resistance you will need. If the soil is "not so good", they will have to add extensions to the pipe to get to the desired resistance, which can be a real bummer as it adds quite a bit to the cost.
I recently paid over $1000 per pier and the engineer I was working with said that was a fair price.
Was that for a deck? I've been quoted $300 and $325 from two different company. (In light of the comment above, I need to check on the diameter; I am not sure whether they were talking about 2" or 3", though the $325 quote I think was for 3" piles. I will know soon, as I am going to ask for an actual, written quote.)
It was for an addition to a house.
Thanks! (Likely a more demanding application -- I guess, but I don't know much about anything to be honest lol.). I'll definitely check what precisely I will be getting for the fees I was quoted.
Thanks! I am going to add a further comment below.
@Graham78
I just want to confirm that indeed the Techno Metal installer is recommending the P3 piles, precisely for the reason you mention, that is, lateral resistance.
I am also looking at the piles by Mascore, which a local installer has (and which would be a little cheaper, but that's partly because he would install a kind that, though 3" wide, has less load capacity, I think).
There is something that puzzles me about the Techno Metal quote I received: it comes with a bracket (to be chosen between 4x4 or 6x6). The installer cuts the pile approximately 6" from the ground, and the owner (or deck installer) installs the bracket. Those are not the adjustable brackets, however. I enquired about them, and he's going to quote me for the difference. Yet, he also mentioned that, in ten years, he has never installed the adjustable brackets. What confuses me is this: I thought the adjustable bracket was pretty much a must for any deck installation. I guess, with the fixed bracket, you just play with the length of the wooden post; yet, that won't work if trying (hoping?) to install the girder directly on top of the pile.
On the other hand, I wonder whether the adjustable brackets might be more wobbly. Could that be the case?
Clearly, I am still a bit confused about the matter, sadly.
I have used both the fixed and adjustable saddles-- both will work fine for your application. The adjustable base are wobbly until you tighten the nuts.
-With the "girder" set directly in the saddle- use a laser level and cut all the pile where they need to be. Then install the saddle (with shims to accommodate errors in cutting if need be) or install the adjustable saddle.
-You will just have to ensure that the piles stick out of the ground enough to get to "girder" hight (~ difference in elevation). This is a situation where knowing what load you need will be important. Tell the installer you need "X" of load and ~20" of pipe sticking out of the ground. The deck guy can cut the posts exactly where they need to be and install saddles. Setting the girder right on the saddle is a preferable way to do it (saves on the cost of post, fewer connections, galv. tube will last longer than wood posts).
Matt, can I ask where you're located? The reason I ask this is because I had an installer say exactly the same thing to me re: adjustable brackets, going so far as saying that there were some issues with them earlier on, and thus they've avoided them. When I asked about adjustability as far as beam layout goes, he told me the "regular" bracket does afford about 1" adjustability and he's never been off more than that.
I am in Central New Jersey. Both the Techno Metal and the Mascore installers only carry the fixed saddles. (They did not mention a 1" adjustability.)
In my case, it turns out, a fixed bracket will be just fine. I am going to have wooden posts (6x6's) into the bracket, not the girders. So, the height can be adjusted quite simply by cutting the wood at the right height.
Small world. ;-)
The way piers work is you tell the installer what load you need and they drive them in to that load. The theory is that the torque needed to drive them is directly proportional to the load the pier can support, they use a hydraulic driver that is set to release at specified torque. So they dial in the setting and drive until it releases. The piers themselves will have a maximum load rating.
Part of the appeal of helicals is that if your soil has poor bearing capacity you can just add extensions and keep driving until you hit the necessary resistance.
There are two main environmental benefits of helicals. First, you don't have to excavate, so if you are in an environmentally sensitive area that's a benefit. Second, you use less concrete, which is quite environmentally damaging in its manufacture.
In an article by a well-known deck installer, the following sentence appears:
"The smallest pile I install can support a 6,800-pound load."
If I calculated my tributary areas correctly, even when I multiply the area by 55 pounds per square foot (40 live + 15), the largest number I get is 3,341. So, it seems that I could ask the installer to go as high as 6,000 or so, and I will be abundantly covered. That's why I am puzzled by the need of any more precise calculations. Is there something I am missing?
-See attached screen shot of the different piers and there loads. (TMP)
-It looks like the #6800 refers to the P1 which my local installer does not install, he starts at P2.
-I'll guess that the $325 was for the P2- but there are additional cost added to that- (frost sleeve, saddle, (see cost attachment from 2020). I think the P2 would be an adequate selection for a deck with columns only 20" above grade.
-If you were to only want ~#4000--- once you get to frost depth and you have the required load you could cut off the pipe and install a saddle with either self tappers or welded for an increased cost. It is probably just as easy to drive the pier the whole depth (7') and be happy with the increased load.
-You do not need "precise" calculations- you need multiply the tributary area by the live/ dead load and tell that to the installer. If you ask the installer for #6800 and the piers are driven to the full depth of 7' but they are only getting a resistance of #4000. -- That is a situation where you want to know how much load you really need. Otherwise the installer would have to add an extension ($$) to get to the "excess" load that you requested.
-With such "small" loads you could reduce the number of footings and increase the beam size which often saves on time and materials.
Thank you SO much! This is very clear, detailed, and informative. I am going to get the quotes and ask for details on 1) the diameter of the pile and 2) the compression. I hope I can skip questions on matters that, in all honesty, I don't yet understand well: tension, allowable lateral capacity, etc.
If I go for helical piers -- and I really want to -- I think I will ask for the full 7' to be inserted, and get all the load capacity I can get. If I understand correctly, the final load capacity will depend on the quality of soil itself (here, it's quite clayish, but I don't know if closer to 2,000 psf, or to 1, 500 psf, or to other). Yet, if I start from a hypothetical maximum of maybe 11,200 lbs, if not more, I should be fine regardless of what the soil ends up being.
According to the design/plan I currently have, I would need 12 piers. I don't know if I will investigate reducing that number, and use stronger lumber, though it would be nice. I'll discuss matters with the carpenter/contractor who will eventually do the job (hoping that they will be acquainted with at least some of these notions).
(I got the heights wrong, as I was not considering 9 additional inches. The deck's flooring will be approximately 31"-38". Yet, that should make no difference to this conversation I imagine.)
Again, thanks!
One more thing:
- I know that at least one of the installers I had contacted would provide the saddle for their price. Should I also ask for the frost sleeve? (I confess that I don't even know what that is, but I am going to research it.)
I have received mixed messaging on the frost sleeves ,so I would recommend taking the advice of the installer. The TMP website advertises the frost sleeves as protection against frost heaves ,and the green frost sleeves have been an add on item in the quotations that I receive. More than once the installers have forgotten to install the frost sleeves and when I question this, this installers have told me that they don't usually use them...
Hello Graham,
are the loads in the tables already include a safety factor?
i.e. for simple math, for a TA = 100 sq.ft. and loads of (40LL+15DL=55p.s.f)
100 sq.ft. x 55 p.s.f. = 5,500 lbs
#5,500 lbs, does that mean we still need to add a safety factor? or if the installer says this pile was torqued enough and has a load rating of #9,000lbs. Does the #9,000 lbs have been reduced to include a safety factor?
for simplicity, doesn't the pile need 3x safety factor
Hi 2010,
Im not aware that applying a safety factor is required.
thanks Graham,
I literally just found the answer. lol
If the tables are listed as SLS then you don't need a safety factor on the load side.
Some engineers still specify the loads with a safety factor of 1.5LL and 1.25 DL but that is extra conservative and not required.
The SLS loads include a safety factor of 2.
$500 or $1000 per screw pile buys you a lot of day labor, I can't see it being worth it for any deck. At that cost go for concrete pads and call it a day.
I've used the Commercial Pylex 66 posts for small structures, they also make smaller adjustable ones specifically for decks, if your soil is sand or clay it is pretty easy to drive in. The only issue I've had is near large trees, hitting a big root can push it off location, sometimes it takes a couple of tries to get it placed.
Thanks, Akos! I intend to ask the people who would install the deck for me (I already have one quote) how much they would subtract if they don't have to worry about the piers. Maybe they will welcome the opportunity of not having to worry about digging and mixing concrete.
I was not aware of the Commercial Pylex 66 posts (or the adjustable version for decks). They are attractive no doubt! I am skeptical, though, that my township's inspector would not give me a hard time about using them (without the approval of some architect/structural engineer -- which of course would defy the purpose).
> $500 or $1000 per screw pile buys you a lot of day labor, I can't see it being worth it for any deck. At that cost go for concrete pads and call it a day.
I don't think it's as clear cut as that. For instance, I'm planning a deck adjacent to an addition. The deck will have a step down from the house and be two risers lower than the interior floor. As such, if I ledgered off the house, I might be in some hollow CMUs, so I would need epoxy sleeve anchors to carry the load. Then there's the lateral load, for which the prescriptive solution relies on attachment into framing, which is not feasible in this arrangement. These details push me to a freestanding design.
If I were to put a traditional concrete footing close to the house, I would have to dig down to footing depth of the addition's foundation. That would not be fun.
When you add up the complications, the helical piles look more and more attractive.
insaneirish,
It's dependent on so many things - site, climate, height above grade - I don't think there is a one size fits all solution that is clearly better than the rest.
Helical piers would be a real gamble here, where there is frequently bedrock close to the surface. We also have a shallow frost depth so footings can usually be hand-dug, and quickly formed up. If we do hit suitable rock they can be as simple as a threaded rod epoxied into place with a plate above.
Forming up pads with integral piers takes maybe 15 minutes each:
>Then there's the lateral load, for which the prescriptive solution relies on attachment into framing, which is not feasible in this arrangement.
FYI- if you want to nerd out on lateral load connections into concrete--Glenn Mathewson touches on this in the last section of the attached article and seems to think lateral load hardware into concrete is prescriptive.
graham78,
"For lateral load design, nothing about a dropped ledger precludes the use of either lateral-load-anchor method provided in the IRC. The point of these connections is to be independent of the ledger."
Isn't that only true if you are dropping the ledger, but not the deck? Don't both attachment methods pre-suppose the deck is close to the same height as the floor framing inside?
Malcolm,
Yes, of course you are right. I misunderstood the article. The Lateral Load Hardware would still be attached to the deck joists and into the mudsill as in a normal ledger situation and as "insaneirish" mentioned.
"The point of these connections is to be independent of the ledger. " This statement from the article drives your point home (for me).
graham78,
Maybe not a prescriptive solution, but I think all that happens when you connect the ledger to concrete is trigger a requirement to run it by your inspector. On new construction I prefer to pour the bolts into the stem-wall, rather than retrofit them.
Counterpoint, I had helical piers installed for a deck that will eventually have a roof. During install of the 6th and final pier the installer hit a large boulder (I'm in Maine). He dug it up with his excavator and plopped it in the garden.
If I had day labor had hand digging holes for concrete we would have been screwed.