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Re: Pier ties per 1921.4.4.1 UBC

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Dear Mr. Duarte,
Your calculations appear correct.  Please see subitem 6 in 1921.4.4.1, you
need to determine the point of contraflexure for determining how much of the
pier shaft will require reinforcing ties.

If agreeable to the City plan review Engineer, you can use the provision for
120% of flexural length for minimum pier shaft length to be reinforced as
allowed by 1809.5.1 UBC.  

Consult with your Geotechnical Engineer for this project for soil restraint
conditions to properly model pier forces applicable.  You may have significant
lateral loading from structure mass above if there are unstable soils near
surface.  Loading will typically also include grade beam weight and floor
structure weight in addition to customary lateral building loads if near
surface soils do not provide sufficient constraint.  The pier/gradebeam system
may be considered a "moment resisting" frame requiring compliance to 1921 UBC
for the grade beams as well with attention needed to restrain moments in the
pier to grade beam connection (embedments, hooks, etc).  

Your firm Engineers should be able to lead you through the mechanics
necessary, I recommend you see if you have a "standard" for modelling/calcs
available, if not I recommend your firm develop one.

Good luck,
Jim Fruit, AIA, Branch Manager
Carl Mileff & Associates, Building Code Services.