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Presumptive Soils

Presumptive Soils

Presumptive Soils

(OP)
My firm does a lot of flagpoles and signs, and recently we were having some discussions with others that do this type of work as well on a few things in the code pertaining to the presumptive soil values.

Part 1:
It is my understanding that when you use the presumptive lateral values from Table 1806.2, and you have isolated poles, you can double the table value per the below code:

1806.3 Lateral load resistance. Where the presumptive values of Table 1806.2 are used to determine resistance to lateral loads, the calculations shall be in accordance with Sections 1806.3.1 through 1806.3.4.
1806.3.4 Increase for poles. Isolated poles for uses such as flagpoles or signs and poles used to support buildings that are not adversely affected by a 1/2 inch (12.7 mm) motion at the ground surface due to short-term lateral loads shall be permitted to be designed using lateral bearing pressures equal to two times the tabular values.

Question 1: If you receive a soil report that says, oh, let's say, you can take 150 psf lateral and you may increase it by 1/3 for wind or seismic, it is my opinion that you can't elect to swap out the 1/3 for the "double" per the above code since the code language seems to imply it is supposed to be reserved for the presumptive values, not those obtained from a soils report...and you certainly can't do both simultaneously. Thoughts?

Part 2:
In Section 1806.1, the code allows for a 1/3 increase in the same table values WHEN the alternative load combinations of 1605.3.2 (with wind or seismic) are used. These combinations required that you multiply your wind loads by an omega factor of 1.3, so even though your bearing capacity increases, you are only seeing a marginal benefit since the load increases almost as much.

Question 2: Can you apply the "double" mentioned in part 1 and this 1/3 increase simultaneously (pending you also use the omega = 1.3 on your LC)? I don't see why not, but another engineer I was speaking with mentioned that their firm doesn't not do for some reason like "it was left over from old codes and should be taken out" or something like that. I don't really care to dissect their reasoning as I might not have heard clearly, but rather just want fresh takes on the issue.


Thanks all!



RE: Presumptive Soils

1. I don't double values provided by geotech reports and agree that 1806.3.4 only applies to prescriptive values. I think that geotechs need to update their language if they intend to allow higher loads on transient loads. If they said something other than 1/3 increase we probably wouldn't need to invoke the omega of 1.3 that you mention in question 2.

2. I would use the 1/3 increase (with omega of 1.3) along with doubling - but what's the point? I'd rather not track the 1/3 and the omega for an extra 3%.

My biggest problem with the pole design formula (1807.3.2.1) is how 'd' is limited to 12' for the purpose of computing lateral pressure when using the prescriptive values. I've never been able to find any justification for this 12' limit and it's very punitive. When I have values from a geotech report I often ask for a maximum allowable lateral pressure or ask them to specifically note that this limit does not apply to their values.

RE: Presumptive Soils

(OP)
Concur with all and thanks for your input.

I also agree with the 12'...maybe someone has some insight there.

When I am doing signs and such, the soils report (if provided, rarely) is usually a few years old and was normally commissioned for a building on site, so I usually don't the luxury of going back and asking for more values, but I have seen the max allowable in some reports and when we are doing new buildings, I like to ask for it as well, especially since software like enercalc allows you to put it in there and opt out of the 12' limit.

Thanks again!

RE: Presumptive Soils

The doubling is in recognition that these types of items can accept a larger deflection. The old version of that table notes that it can be doubled for short term loading if displacements of half an inch are acceptable.

When a geotechnical report is prepared, the geotechnical engineer sets his values based on some allowable deflection. If you want to allow for an increased deflection level, you should be asking him to provide values for the amount of deflection you're looking for.

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