RE: base plates subject to uplift[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: base plates subject to uplift
- From: Charlie Carter <carter(--nospam--at)aiscmail.com>
- Date: Mon, 19 Jun 2000 09:22:32 -0500
>Table J3.1 of my ASD specs lists the diameter of an oversize hole for 3/4"
>bolts as 15/16 inch. If your book has 1-5/16 inch, you need to get an errata.
Hole sizes for steel-to-steel structural connections are not the same as hole sizes for steel-to-concrete anchorage applications. In the case of steel-to-steel connections, the parts are made in a shop under good quality control, so standard holes (bolt diameter plus 1/16 in.), oversized holes (bolt diameter plus 3/16 in.), and short and long-slotted holes can be used quite successfully. However, the field placement of anchorage has long been subject to more permissive tolerances (and often, inaccuracies that exceed those tolerances anyway and may require consideration by the structural engineer of record).
AISC published Design Guide No. 1 Column Base Plates back in the early 90s.
At that time, it was recognized that the quality of foundation work was getting worse and worse. To allow the erector (and designer) greater latitude when possible, the permissible hole sizes in base plates were increased. These same larger hole sizes are given in the 2nd Edition LRFD Manual. The values there are maximums, not a required size. Smaller holes can be used if desired. Plate washers are generally required with them because ASTM F436 washers can just collapse into the hole, even under erection loads.
The larger hole sizes are mostly intended for the usual application of a base plate, which is to transfer axial compression from the column into the foundation. The anchor rods don't usually do much after erection in that case. For other applications, such as base plates with moment or uplift, the hole size is more of a concern for load transfer. Harold Sprague already offered a detail that he called a bolt box and I've heard others call a boot. Whatever you call it, it gets the moment/uplift load transfer to occur from the column to the rods directly, not through the base plate. It's a more efficient and direct method to get the load out of the column, but if you choose to use the anchor rods and thick washers over the base plate holes, that can be done too in many cases. As Roger Davis already posted, you can find washer sizing guidance in AISC Design Guide No. 10 Erection Bracing of Low-Rise Structural Steel Buildings
Column base design, erection considerations, etc. are also covered in the steel tips here:
Rick Drake at Fluor Danial has also done work in this area, including an article in the AISC Engineering journal "Beam-Column Base-Plate Design -- LRFD Method". 1st Quarter 1999.
Hope this helps.
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