DID YOU KNOW?
Did You Know?
Ask PDCA: For isolated columns and stability issues, should the minimum
number of piles be one, two or three below a pile cap (the capacity required
warrants only one pile)? How about an exterior wall column supported by a
grade beam?
Response by Bill Marczewski, BSM Engineering, Inc.
Assumptions: The building columns are to be supported on deep foundations, subject to the criteria of the International
Building Code (IBC) and included referenced standards. I’m referencing IBC 2015 with ASCE 7-10 “Minimum Design Loads
for Building and Other Structures” – yes, you must use both documents.
You must classify your building and occupancy category
based on your actual building project. An essential
facility requires more than a non-essential facility,
where the Importance Factors are 1.5 and 1.0, respectively. You
must also identify your Soil Site Class and, eventually, your
Seismic Design Category (SDC) as either A, B, C, D, E or F.
Because you referenced a deep foundation that requires a geotechnical
depth exploration, I assume you will have this information
provided to you. In Seismic Design Categories A and
B, you have few requirements to meet other than Section 1.4
of ASCE 7 – General Structural Integrity. SDC’s A and B are
basically the non-seismic regions, where SDC C is where you
begin to transition into having some seismic design and detailing
requirements. In SDC D to F you are doing full-blown
seismic structural engineering, with site-specific geotechnical
investigations required for SDC F. You must know this information
or you won’t be able to use IBC/ASCE 7 properly for
design compliance, as the paragraphs are organized according
to Seismic Design Categories A to F.
Refer to IBC 2015, Chapter 18, Section 1810.2 – Analysis
through Section 1810.3 for the specific answers to your questions,
which will require engineering design judgement with
respect to structural system selection and methods used to provide
lateral stability (i.e., compacted grade; orthogonal grade
beams; structural slabs; etc.)
The following are the labels in the relevant part of Section
1810:
•• Section 1810.2 – Analysis
•• Section 1810.2.1 – Lateral Support
•• Section 1810.2.2 – Stability
•• See second paragraph for answers to pile supported
wall structures and pile/pile cap requirements; see
Exceptions for low-rise structures in this paragraph
•• Section 1810.2.3 – Settlement
•• Section 1810.2.4 – Lateral Loads
•• Section 1810.2.4.1 – Seismic Design Categories
D through F
•• Section 1810.2.5 – Group Effects
•• Section 1810.3 – Design and Detailing
•• Includes material specific requirements and you will
be required to know your SDC to navigate the chapter
After all of that, you will also need to refer to ASCE
7-10, Chapter 12.13 – Foundation Design; and Section
12.13.5.2/12.13.6.2 – Foundation Ties.
Quick summary of above
1. You are required to brace your piles in both orthogonal
directions. How you do this is optional, but should be
done according to standard practices (i.e., compacted soil
in SDC’s A and B; Grade Beams/Ties in SDC D to F is
my suggestion).
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