HAIRPINS & TIE RODS 14

[sigma1]

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.

 

[Gumpmaster]

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.

 

[rrayda]

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|

 

[JLNJ]

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.

 

[DaveVikingPE]

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:

http://www.dywidag-systems.com/uploads/media/DSI-USA_DYWIDAG_Tie_Rods_us.pdf

 

[sigma1]

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?

 

[akastud]

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

http://www.TOREngineering.com

 

[sigma1]

I guess it all depends on how much can be carried by the hairpins and the reinforing rods or WWF.

 

[jike]

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.

 

[DaveAtkins]

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

 

[jike]

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.

 

[SperlingPE]

Note that after sizing a hairpin for the required thrust load, look at the concrete slab and reinforcing to see that it will take the load.

 

[Lutfi]

I have a Butler’s Foundation Design and Construction Manual. My copy is the second edition. It was written few years back. It deals with all design and construction aspects of PEMB foundations. It is over 200 pages long full of detailed calculations, formulas, tables and details.

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

 

[MattJM]

Perhaps a topic for another thread, but has anyone ever checked the Butler design methodology versus pure tensile stength of the concrete?

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?

 

[msquared48]

I would not rely on the tension of the concrete at all. To me, the use of a few extraq pieces of steel in the beginning is much cheaper than repairing the slab in the future.

Mike McCann
McCann Engineering

 

[DaveAtkins]

My only problem with assuming the concrete in the slab has tensile capacity is that it will crack, due to shrinkage, at control joints. If the thrust at the base of the column must be resisted by the thrust at the opposite column, the slab must be reinforced.

DaveAtkins

 

[sigma1]

Thanks you everyone.

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?

 

[haynewp]

I have the notes from the ASCE web seminar on metal bldg footings by Newman that have moment resisting ftg examples. I am also not completely comfortable using moment resisting footings to resist thrust.

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?

 

[JLNJ]

I looked at a fictitious typical bay which induced 100K of thrust resisted by a tie beam. The elongation of the beam caused a 15% increase in the center span moment of the girder. Not a killer, but not insignificant either. It looks like oversizing the tie beam to minimize the elongation is a good idea. The PEB manuf ought to take the possibility of some movement of the base into account.

 

[sigma1]

heynewp
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