H Piles - End Bearing 1

[LPPE]

I have H piles that are bearing on rock. Half the structure is founded directly on this rock, and the other half on H piles (rock slopes at one end too far to found on rock). The Geotech recommended an 8 ton per square foot bearing capacity for footings on rock, and then recommended 100 ton H piles.

Now, the area of a HP12x53 is about .1 ft^2. IF this pile bears on rock, my capacity is therefore only about 1600 pounds. What am I missing? How can I apply 200 kips bearing from H pile to a surface that can only handle 16 kips per sq. ft?

Also, do the tops of the H piles need a base plate (like an inverted column) at the pile cap? I'm concerned with the H pile punching through the cap.

 

[redhead]

Think of it this way. When a woman weighing 150 lbs. walks on a floor slab on stiletto heels, 1/4"x1/4" bearing area, she exerts 345,600 psf of pressure on the structure.Somehow, the floor you designed for 50 psf office loading is able to sustain this.

And yes, you do need the cap plate to prevent punching shear failure.

 

[LPPE]

Is the pile driven with the plate already on? It seems like the H without a plate would damage the pile driving hammer. However, after driving a pile with a plate on top, I would think this plate (and connection to H pile) is shot.

As for the heel analogy, she exerts a force of 2400 psi on the conc. which is less than 3000 psi. I wouldnt worry that much. Now, if she was 300# and the conc was 2000 psi, you do the math. Thats what I have with this pile situation.

 

[dlew]

An old Bethlehem Steel's H-Pile handbook indicates a steel pile cap 16" by 17" by 1" thick for a H12X53, 100-ton pile. The pile cap is welded in field after the pile is driven and cut to the right elevation.
The same Bethlehem handbook says that research done by the State of Ohio Department of Highways in 1947 concluded that if the top of the H-pile is embedded in a concrete foundation, the steel pile cap is not necessary.

 

[engcomp]

Dear Pylko

I have spent a few years as a pile driving contractor, after inventing a machine that could drive recycled railway lines. Forget your Geotech's 8 tonne per square foot - it is irrelevant to driven pile capacity. The H pile sockets into the rock until the driving resistance equals your target, about 300 tonnes for a 100 ton pile. Of course, you know how to assess the driving resistance by dynamic formula - if not, please email me.

Helmut
engcomp@ozemail.com.au

http://www.engs-comp.com/

 

[austim]

Hi, pylko.

Firstly, there is a fundamental difference between the Geotech assessment of the bearing capacity of the rock, and the capacity of a driven pile. The act of driving the pile provides a pretty good test of the local rock capacity at the specific pile toe location; if it is not high enough, the pile should continue to penetrate until the rock provides sufficient resistance. You get no such option with a spread footing.

For quite a long time (1966 - 1988) I was employed (in a structural design capacity) by one of Australia's leading civil engineering contractors. We did a pretty good 'trade' in driven piles, with a sideline in submitting alternative tender designs.

(a) We NEVER used cap plates at the top of H piles.
(b) Our rule of thumb was to limit long term working stresses to about 6 tons/sq.inch (this was before Australia metricated), so that we had the necessary margin of stress to take care of driving stresses.

The 1970 edition of the Australian Highway Bridge Design Specification included the following remarks re cap plates, which you may find helpful:

In general, cap plates are not required for steel piles embedded in concrete. In assessing the need for caps reference should be made to the following publication - Ohio Department of Highways. "Investigation of the strength of the connection between a concrete cap and the embedded end of a steel H-pile [Columbus] 1947. (Research Report No.1.)"

Now for my little historical tale.

Once we contracted to drive some test piles for one of our Federal Government authorities, who had specified the pile section and the ultimate resistance which we were to achieve. The piles were steel H piles, driven through some fairly soft sediments to bear on a basalt rock layer.

I did the obvious and simple calculation of dividing the required ultimate load by the pile area, only to find that we were being asked to generate a pile stress higher than yield stress.

We duly pointed that result out to the authorities engineers, only to have them scoff, along the lines of 'driven piles don't behave like that', (the clear inference being that we were just simple builders and would know nothing about these matters).

When the first test pile was driven, it reached the basalt layer, and just kept going, with no reduction in set per blow. Eventually we were paid to extract it, to see what had happened. There for everyone to see the flanges had been stripped from the web, and were curled round by about 500 degrees in spectacular fashion like ram's horns. The toe was cut off and mounted on a timber plaque for a place of honour in the Chief Engineer's office, but we never heard anyone suggest that perhaps our initial advice had been spot on all the time.

The lesson from that little tale is to check that your specified driving resistance (a la 300 tonnes mentioned by engcomp above) is below the squash load of the pile section by a reasonable margin.

 

[LPPE]

Thanks all for the help. Nice story, austim. I wish I could see that plaque! I have a collection of oddities around the office - strange cylinder breaks, concrete cores, nuts, bolts, rock cores, fractured crane treads, etc. That plaque would fit in nicely.

Of course, the geotech now is recommending socketed mini-piles. The site is basically 20-30 feet of fill (blasted boulders, voids, etc. from structure erected next door 25 years ago) on top of sloping gneiss rock at about a 4:1. The soil boring contractor had a heck of a time getting down to rock. Said be broke 4 bits and put 3 rigs out of comission for some time. I told the geotech I was concerned about driving piles through this debris fill (in a 7 pile cap, I could see one pile fetching up at 20 feet, and another at 40', another at 34', etc...) and then about the H piles skidding down the rock face once it gets there. Also, I would think the H piles would get mangled driving through boulders and such. Hence, socketed mini-piles.