hairpins tie rods

huangyhg

hairpins & tie rods
lately i have been reading very interesting threads about "pemb foundation design". for small buildings the lateral loads can be handled but for large buildings it can be a dificult task for someone with little or no experience in this area. asce/sei offers a seminal sometime in january 2008, but how much can you learn at a
web-seminar?
is there a book or manual other than the "butler manual", which is not available, that provides some tenchical information and details on how to design and detail these foundations?
any information or help will be greately appreciated.
metal building systems: design and specifications by alexander newman.
it's not super technical, but it's better than nothing.  he has some good suggestions on the specifications part.
the mbma manual has some information on pemb foundations in the appendices.  not particularly helpful, but it's there.  they say something like "if the thrusts are small in magnitude, use hairpins, if they are large, use tie-rods".  
i try to use hairpins if at all possible because you avoid the common mec conflicts that tierods cause and they are much easier to connect to the pier/anchor bolts.  in 90% of the metal buildings you see, hairpins work fine.  
i recently designed a pemb foundation for a very large structure at a mining facility, ended up using 2- 1 1/8" rods along each mainframe line.  the thing you run into here is how the heck to get good load transfer to tierod group from the anchor bolts / pier when you are required to be below your pig of a 14" thick slab.  i welded a wt section onto the base plate and bolted the tierods through that.
|r|
one thing about tie rods is that if the building span is long then there could be some serious elongation of the rods. i don't know what effect the elongation has on the behavior of the steel frame of what (if anything) the metal building guys allow for.
i found some interesting info (and a drawing) in one of dywidag .pdf i downloaded a while ago. that is, there's a section view of a metal building system main frame with foundations using hairpins. ah! here it is:
for large lateral reactions some people recommend placing post tensioned bars on grade beams. however, i wonder what the differentail settlements if any will do to a long beam connected to the pier?   
any one have any idea what constitutes small or large thrusts?  i have run into the same thing, but what is the cutoff.  
just another thought, the aci discusses "tie elements" in chapter 21 for those of us that sometimes end up in the "high seismic risk" catergory.
akastud
david s. merrell, p.e.
tor engineering
i guess it all depends on how much can be carried by the hairpins and the reinforing rods or wwf.     
jlnj:
i thought about this very thing and do not want significant elongation to weaken the structural frame. also, when the columns sit above the floor on piers, any pier movement can also weaken the structural frame. the pemb will not account for any movement in his design, so i believe the engineer must look at this by modeling springs as horizontal foundation supports and see how much it may weaken the frame. if significant, i would suggest increasing the design load to account for the reduction in capacity.
according to the butler foundation design and construction manual, 27.4 kips is the maximum thrust a hairpin can take (and you need to use a #8 bar to get that!).  anything above this force needs to be resisted by tie rods, or by the footing itself.
i have actually done some projects where making the footing big enough to resist the thrust plus overturning due to thrust was the best solution.  it is not a big deal to make a rectangular footing for this situation.
daveatkins
cool dave! i have often thought about using the rectangular footing solution.
another thought was to offset the footing from the column line so that the vertical load x horiz. eccentricity = horizontal thrust x vert. eccentricity. or in other words, use the eccentric column load to offset the overturning due to the horizontal thrust. this would result in a uniform pressure under the footing.