Yeah, but I would like to be able to calculate and compare the factored resistance available from the rock to that of the factored structural limit available from the pile, so the design is well documented.
On rock with a relatively low compressive strength, it matters if you assume a plug or not (for instance, on 48"x1" cans).
I saw on one example that the designer assumed that a plug formed on the bottom of an HP 12 pile (pounded on rock) and therefore was able to use that entire area when calculating end bearing.
So i was just wondering if you assume a plug on the smaller pipes and H piles, but not on the larger ones, or if piles on rock were special and such you assumed the entire bottom area instead of just the area of steel for the bearing resistance.
When dealing with piles on rock, you should do the required investigation and tests on rock cores to get a good idea of the bearing capacity of the rock. Be conservative and use the section area of the pile in the analyses so there are no nasty surprises. Otherwise assume friction on the "box" perimeter.
When you say "assume friction on the 'box' perimeter", are you talking about calculating the skin friction component?
I pretty much have done as you suggested, and am waiting on data from the lab for the compressive strength of the rock, but in the meantime, used a presumptive value to get rolling.
Many people use the entire end area if the piles penetrate the rock in the case of the weathered portion of rock at the surface which may be about 1m thick. Others have advocated using half of the total end area for plug formation,yet others have recommended using the net steel area for H-piles. Calls for some imagination and experience gained from past designs in similar situations.
Assuming that you had a plate at the bottom of your pile. The effect would be to allow for aa assumed uniformity of stress on your rock layer. If no plate high concentrations of stress would prevail that may have an effect on the rock capacity. For driving onto/into rock one needs to evaluate the driving stresses as piles should not be subjected to excessive stress. Limiting values are provided. This can be done using the WEAP analysis.
On the other hand if the pile was to be plugged through the overburden material and came to rest on the rock, I would probably not use the full end area.
No matter how you lookm at the problem, I would not want to use a load that exceeds the allowable structural capacity of the pile.
hence the beauty of pile design usig static analysis is the conjuring up of all these possible capacities, throwing them in a pot, and finally deciding which would be best for the witches final brew - geomagic, mumbo jumbo etc.
If you are looking for high capacity piles you may wish to look at a drilled shaft. However, this would depend on the conditions of your overburden soil.
Finally, are you providing the allowable load for the pile or is a load that is required to be taken ptrovided by the structural engineer. It make a difference sometimes.
Generally the bearing capaity of moderately competent rock will cary H-Pile loads. Although in some soils you can et a plug to form, the effectiveness of the plug depends on the type of soil and the anticipated tip load. In rock, once the tip lads on the rock, there are a few blows to seat it. The tip of the pile is set into the rock and any plug action in the pile does not contribute. Sometimes, especially in sandstones and shales, the pile will be overdrive to "ensure good toe" into the rock. What happens is the rock below the pile is shattered and allows pentration of the pile into the fracure zone. I would not assume plug action, and use the cross section of the pile. Bearing stresses are typically limited to .33Fy. Unless the rock were very soft, I would not be concerned about the capacity of the rock. I would rather spnd time developing a resoable driving criteria for driving the piles
