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- To: "SEA International List" <seaint(--nospam--at)seaint.org>
- Subject: Tie Beams
- From: "Dennis S. Wish PE" <wish(--nospam--at)cwia.com>
- Date: Wed, 25 Nov 1998 12:02:01 -0800
I have a project that is being constructed on basically a level pad. It is a custom home and there are elevation changes that occur throughout the home based on the rise in the lot - however, the foundations are conventional.
I have designed a massive roof that overhangs the structure by some 8 feet. this is not a cantilever condition, but bears upon two columns. These are lateral columns (TS14x14x1/2) which resolve in a grade beam and construction pads below. The columns are about 18 feet in height and extend to the grade beam set at 4.5 feet below grade.
Although the building pad is level, the site is in the mountains and the slope occurs almost twenty feet in front of the columns. The geology is stable and there is nothing indicated in the soils report or geologist report to indicate fracture or slippage at the slope. However, I feel it is necessary to tie the columns back to the structure to prevent them from moving out of plumb in case of seismic activity.
1. Does anyone think this is a valid concern? If not specifically required by the Geotechnical or soils engineers should I be concerned? Is there a specific UBC code reference to tying foundations in two directions for this condition?
2. The tie beam at the high side needs to either be sloped to connect with the thickened slab edge of the home and the grade beam at 4.5 feet lower OR is it advisable to tie into the column - through the Sono-Tube enclosure at or slightly below grade level? If so is shall I wrap the steel around the column encased in the concrete tube? The moment occurs in the gradebeam and the Sono-tube is used to protect the steel from moisture. The Sono-tube will be around 30 inch diameter.
3. At the low end there is to be a 4.5 foot retaining wall for the elevation change. The footing for the retaining wall needs to "pass-through" the grade beam. The grade beam capacity is much more than the moment in the column and any moment induced in the stem - yet I question if if is advisable to tie the two together. The retaining all foundation can be used effectively as a tie beam as well.
There is one other thing that reduces the moment in the base of the stem of the retaining wall. The wall is not parallel but zigzags along the retained patio. This is an architectural design. I realize that the equivalent fluid pressure does not care about the direction of the soil, but the wall does become short segments (ea. no more than 30 or 36" wide) with 90 degree changes that would help braced the stems at the location of maximum moment (something like a buttress ). Therefore the stress added to the grade beam (which also occurs within 18" of the column) appears to be negligible.
Is it reasonable to allow the wall to pass through the grade beam (only some horizontal steel from the wall ftg and one vertical stem bar - possibly two will come up from the gradebeam.) - using it to retain 4.5 feet of soil as well as a tie beam for the columns?
Sorry if this one sounds complicated. I can email a DXF file or a HTML view of the condition if you wish.
Thanks in advance for any consideration and suggestions.
Dennis Wish PE
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