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Re: Tie Beams

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Dennis,
I am assuming that the slope drops 20 feet from the patio where the cols.
are located and that the grade beam runs perpendicular to the slope which
does not have enough set back from the daylight. Your concern about a tie
back is valid. The tie back beam doesn't need to be horizontal. It could
slope down to the grade beam. I wouldn't tie the col. It would involve
additional work, but the grade beam can be easily tied. I see no problem
with the retaining wall footing crossing the grade beam. The tributary
overturning load of a 4 ft high retaining wall on the grade beam would be
limited to grade beam width which shouldn't be very much, in fact it would
be in the opposite direction when the building's lateral load is reversed.
Another option would be to use caissons say 24" dia. and extend them 18 ft.
above the grade to replace the steel cols. if architecturally acceptable.
The caissons would be designed for vertical and lateral loads and this
would eliminate TS cols. pad ftg. and grade beam.
If the movement of col base is of concern, I would either tie the caissons
back to the building or design the caisson based on the desired
displacement of say 1/2 inch.  
I hope these comment would be helpful.
Karim Hosseinzadeh     

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From: Dennis S. Wish PE <wish(--nospam--at)cwia.com>
To: SEA International List <seaint(--nospam--at)seaint.org>
Subject: Tie Beams 
Date: Wednesday, November 25, 1998 12:02 PM

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.

Sincerely,
Dennis Wish PE