HomeMy WebLinkAboutContinuous Footing Detail
Frank DeMarco
Harry Kim
Director
Mayor
Allan Simeon
Wil Okabe
Deputy Director
Managing Director
DEPARTMENT OF PUBLIC WORKS
Aupuni Center
101 Pauahi Street, Suite 7 · Hilo, Hawai‘i 96720-4224
(808) 961-8321 · Fax (808) 961-8630
www.co.hawaii.hi.us
Division Procedure: 2017-0602
Date: June 7, 2017
Code/ Edition: County of Hawaii Building Code Chapter 5 & IBC 104.11
Subject: R-3 (One & Two-Family) Minimum Footing Depth
I. PROCEDURE PURPOSE
a. To ensure fair and consistent application of footing depth when on solid rock.
II. RELEVANT LAW
a. Hawaii County Code Chapter 5 and International Building & Residential
Codes (IBC, IRC 2006 ed.)
III. SUMMARY OF LAWS
a.Sections 1805.2 (IBC) and R403.1.4 (IRC) state that footings shall be place
at least 12 inches below the undisturbed ground surface except when
erected on solid rock.
b.Section 104.11 (IBC) Alternative materials, design, and methods of
construction and equipment.
IV. FREQUENTLY ASKED QUESTIONS AND ANSWERS REGARDING COUNTY OF
HAWAII PRACTICE
1.Question: What depth of embedment into solid rock is required for slab-on-
grade footings?
Answer: None. When a one or two-story dwelling slab on grade is placed
directly on undisturbed or solid compacted rock over “in-situ” solid basalt
then no embedment is required per detail FTG-01 (6/17)
Commentary: In addition to the exception provided in the code, Section
104.11 provides alternatives and modifications to the practical
implementation of the code per Section 5-11(1) of the Hawaii County Code.
County of Hawai‘i is an Equal Opportunity Provider and Employer.
This alternative method has been demonstrated to be satisfactory and meet
the intent of the codes since their adoption.
2. Question: Can this method be used for footings other than slab-on-grade?
Answer: No. At this time, this prescriptive method has been established
only for slab-on-grade applications.
Commentary: Footings under braced walls / shear walls, where post and
pier construction is proposed, is under investigation for similar prescriptive
methods.
3. Question: Will this design and method be acceptable on soil other than
solid rock where fill material is used to provide a solid subgrade?
Answer: No. Soils other than solid rock, or crushed rock over native rock in
place (in-situ), require a soils engineer to establish adequate footing size
and fill material to be used.
Commentary: Soil types in Hawaii County vary considerably and have
significant variations in bearing pressure allowed. A design professional is
required to determine the soil type and bearing capacity for the structure
proposed.
Neil Erickson
Plans Examining Manager
nerickson@co.hawaii.hi.us
Approved
David Yamamoto, Division Chief
County of Hawai`i is an Equal Opportunity Provider and Employer.
V.APPENDIX
1. IBC Commentary on “Depth of footings”
“Footings are required to extend below the ground surface a minimum of 12”.
This is considered a minimum depth to protect the footing from the
movement of the soil caused by freezing and thawing in mild climate areas.”
Where no frost exists and erected on solid rock this requirement is waived.
Other soiltypes must be examined for lateral sliding value, expansion, or
limited bearing capacity.
2. IBC 104.11 Alternative materials, design, and methods of construction and
equipment.
The provisions of this code are not intended to prevent the installation of any
material or to prohibit any design or method of construction not specifically
prescribed by this code, provided that any such alternative has been
approved. An alternative material, design or method of construction shall be
approved where the building official finds that the proposed design is
satisfactory and complies with the intent of the provisions of this code, and
that the material, method or work offered is, for the purpose intended, at
least the equivalent of that prescribed in this code in quality, strength,
effectiveness, fire resistance, durability and safety.
County of Hawai`i is an Equal Opportunity Provider and Employer.