HAIRPINS & TIE RODS
HAIRPINS & TIE RODS
(OP)
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.
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.






RE: HAIRPINS & TIE RODS
It's not super technical, but it's better than nothing. He has some good suggestions on the specifications part.
RE: HAIRPINS & 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|
RE: HAIRPINS & TIE RODS
RE: HAIRPINS & TIE RODS
RE: HAIRPINS & TIE RODS
RE: HAIRPINS & TIE RODS
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
www.TOREngineering.com
RE: HAIRPINS & TIE RODS
RE: HAIRPINS & TIE RODS
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.
RE: HAIRPINS & TIE RODS
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
RE: HAIRPINS & TIE RODS
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.
RE: HAIRPINS & TIE RODS
RE: HAIRPINS & TIE RODS
It is an excellent book. Dr. Jim Fisher authored it along with Butler manufacturing company.
I often wonder if they ever updated this book.
Regards,
Lutfi
RE: HAIRPINS & TIE RODS
This goes to the same point as many threads here discussing the location of slab control joints when using hairpins. My thought is this - if you create a long enough perimeter (by extending hairpins), sometimes the concrete tensile capacity itself (usually zero for reinforced, but not zero for plain) is greater than the capacity of whatever slab reinforcing is provided.
This would open up the possibility of using long hairpins in fiber reinforced slabs.
Thoughts?
RE: HAIRPINS & TIE RODS
Mike McCann
McCann Engineering
RE: HAIRPINS & TIE RODS
DaveAtkins
RE: HAIRPINS & TIE RODS
I have a case where a 5 ft x 10 ft footing with a 2 ft eccentricity can do the job for overturning. However, to resist sliding I need a key or go much deeper than what is required around here. In addition, I don't feel confortable relying of the footing alone.
What about if we provide both. Momemnt resisting footings per DaveAtkins/jike and as a back up dowels (bent in the field) extending from the foundation walls to the slab that is reinforced with WWF or minimum re-bar? The construction joints can be located parallel to the thrust forces while the control joints cut perpenticular to the construction joints. Does anyone sees a problem with this redundant approach?
RE: HAIRPINS & TIE RODS
I would also tie into the slab with hairpins with the moment resisting footing to give some extra, but that is just my preference. I worry there would be settlement under one of the moment resisting footings, and the pinned assumption by the metal bldg. mfr would not be maintained and a problem with the frame may occur. This may be a result of my couple of years in designing prefab steel bldgs and knowing how bare minimum they tried to get these in order to save their steel. But it should work fine in theory with a moment resisting footing only for smaller thrust loads.
A 10ft footing is pretty big, what is the thrust and uplift?
RE: HAIRPINS & TIE RODS
RE: HAIRPINS & TIE RODS
The factored loads are: Horizontal thrust approx. 50 kips and the net uplift about 30 kip. OT is fine. Sliding is a problem but with a key or deeper foundation will be OK.
Please keep in mind that these are preliminary calc's.
Can properly sized dowels act as uniformly distributed hairpins?
JLNJ. The maufacturer will not want to hear about added movements.
TY
RE: HAIRPINS & TIE RODS
Are you saying you want dowels from the foundation wall into the slab on grade instead of a hairpin? It could work, but you would have to transfer the thrust force horizontally along the top of the foundation wall, which would result in a lot of horizontal rebar in the top of the wall.
DaveAtkins
RE: HAIRPINS & TIE RODS
I see that your horizontal thrust is in fact factored. If I am not missing the point, to handle this amount of thrust you need about 5-#4, 60 ksi rebars in the active region of the footing. But, as you start to move away from the column towards the center of the slab, this requirement will gradually decrease. This is due to the fact that more and more area of conrete from the slab will take part in resisting the thrust. If you are concerned, you probably can allow only 3*sqrt(f'c) tension in the concrete. If you consider this then, you need only about 300(+) sq in of floor area to resist this thrust, considering 3000 psi concrete.
If you are considering the weight of the footing to resist the uplift then, you may need to use service uplift to size the footing.
RE: HAIRPINS & TIE RODS
What happens to the grade beam when the thrust force is not there, or it is very low, and the beam in reality is a long concrete column under compression.
Should the beam/column be designed for the full length or can someone assume the soil and the slab above are providing lateral stability?
RE: HAIRPINS & TIE RODS