One of the best video I found about the post tensioning in slabs created by Post Tension Services India.
ASCE 7-05 defined the base for seismic design as:
“The level at which the horizontal seismic ground motions are considered to be imparted to the structure”.
Most of the times this is misinterpreted or misunderstood that where the base is considered when determining the height of building for seismic calculations specially when the site is sloped or if the basement is open from one side.
Read the following article published in December 2009 issue of Structure magazine, this article will answer lots of questions.
When we use finite element software for analyzing deep foundations then it becomes important to model piles with accurate axial stiffness so that the load can distribute properly based on pile stiffnesses.
Some people suggests to calculate pile stiffness by AE/L formula. By doing this method the problem is that as you increase the length of the pile, the stiffness will reduce which is not accurate.
The best thing is to ask Geotech Engineer for the allowable loads and the allowable axial displacement on working loads. Most of the times this allowable displacement is ranged from ¼” to ½” which can be confirmed from geotech report.
Take your allowable load which will vary based on the depth of pile and divide it by the axial displacement, this will be the required pile stiffness.
k = Pile axial stiffness (kips/inch )
P = Allowable load (kips)
Δ = Allowable vertical displacement (inches)
Watch the video and admire the engineering:
Here is the brief description of project:
On April 10, 2014, the slide of the Milton-Madison Bridge was completed. At nearly a half-mile long, this is the longest bridge slide of its type in North America, perhaps the world. The slide occurred over a two day period with the actual slide time being approximately 12 1/2 hours. The total slide distance was 55 feet, going from temporary piers to refurbished, permanent piers.
The old bridge was where the new bridge is today – after the slide. The new bridge was built on temporary supports NEXT TO the old bridge. The old bridge still carried traffic. After the new bridge was built, traffic was switched to the NEW Bridge on the temporary alignment. The old bridge was demolished. The existing piers were rehabilitated. After that work was complete, the new bridge slides back over where the old bridge used to be. By reusing the foundations and piers the owner saved many tens of millions of dollars.