what is this hair pin thing attached to anchor bolts

huangyhg

what is this "hair pin" thing attached to anchor bolts?
i have come across friends suggesting someone use hairpins in conjuction with anchor bolts in anchoring steel frames to foundations.
i have no single idea what this technique and its rationale are
i wont mind a little ancyclopaedic hint.
thanks,ijr
i suppose you intend to weld the airpins at intervals onto the outer surface of the bolt.
in this case their goal would to raise the shear that may be transmitted from the bolt to the concrete and finally to make the bolt shorter.
however it isn't a practice familiar to me: the diameter of the bolt does not allow for good inspectable welds and the anchor must be of weldable material (this is normally the case).
prex
hairpins are reinforcing bars bent into the shape of the letter "u".  the bars are used to anchor inserts, anchors, or post-tensioning tendons into concrete to prevent the concrete from "blowing out" at a free edge.  the name hairpin refers to the similarity in appearance and function to the pins used to hold one's hair in place.
be very careful if you are using rebar hairpins structurally.  the bending process will work harden the steel.  you may get microcracks in your bar which will reduce the steel bar's tensile/shear capacity.  in reinforced concrete this is less of a problem as the bent bar is surrounded by concrete, however, in conditions described above there is the work hardening problem and also the likelyhood that you will be pulling on a single point at the top of the "u" bend which will induce other unknown stresses.
regards
andy machon


hairpins are used structurally very commonly.  the 180 degree bend diameter should conform to aci 318 section 7.2 to avoid cracking the bars.  also, the hairpins are not typically welded to the inserts or tendon, they simply enclose and confine them.
i have come across extensive use of "hairpins" in the design of foundations for pre-engineered buildings. the structural frames used are usually pin ended portal frames. the horizontal shear loads at the base of the portal frames are transfered to the floor slab of the building using a "hairpin" connection. the "hairpin" consists of a v-shaped length of rebar which is cast into the floor slab and around the anchor bolts of the portal frame columns. the result of this procedure allows one to design the portal frame foundation pad and pedestal for axial load only. all horizontal loading is carried by the hairpin connection. a considerable reduction in foundation size and cost is achieved.
hairpins are most often seen and used in pre-engineered steel buildings where the exterior columns of the steel frame induce significant horizontal forces at the base of the column.  the column base plate and anchor bolts transfer this force into the supporting concrete foundation.
however, many times the foundation pedestals and footings cannot be designed economically to resist this lateral force.  if the footing is sunk deep for frost, or the base of the column is raised above the surrounding grade the high eccentricity would cause the footing to grow substantially.  in these situations, engineers use the hairpin bars to transfer the lateral force back into the interior floor slab, which in this case should be reinforced.
two problems with doing this:  1.  the slab may be altered, cut, or slotted for drains in the future which could compromise the integrity of the system.  2.  the slab-on-grade could settle, causing cracking and spalling around the interface between the foundation and the slab.  you could wrap the rebar, in the slab adjacent to the foundation, with a foam insulation for about 12 inches of length.  this would allow flex in the rebar if movement did occur.
there is more than one definition of the term 'hairpin' used in construction.
the reinforcing steel industry refers to supplemental cross ties as 'hairpins' which i think you are now familiar with based on the foregoing discussion.
there is another form of 'hairpin' used in my industry (guyed and self-supporting tower construction).   the term commonly refers to a solid steel bar bent in the shape of a u-bolt and threaded just like a u-bolt.   these tensioning devices are generally used for anchoring very large diameter guy wires.    it is used for applying pre-stress to guy wires and forms part of the guy wire assembly.   it attaches to the anchor steel by means of a shear pin.  a good description of this type of 'hairpin' is available in the ussteel wire rope catalog.
does nyone have a formula for the sizing of haripins for lateral transfer to slab?
rope
your question interests me too. but new visitors to this thread may miss your question and may end up answering the question at the top. you understand the time thing
lets wait. if we dont get response, lets start a new thread so that everyone can see your question
good luck
rope & ijr--
i had the same question a little while back.  i got hairpin design information from butler manufacturing co. (butler buildings) while we were planning on specifying their product for our project.  i'm not sure if i'm at liberty to copy this information and give to you, so i will give you the reference that the information came from:  "foundation design & construction manual", butler manufacturing co., computerized structural design, inc.  hope this helps.