Residential Structural Inspector (CAS) Lesson 3

Foundations

(Based on the 2014 ORSC)

of 93 Updated: August 2016 1 Oregon Building Codes, All Rights Reserved

Introduction

Chapter 4 provides the requirements for the design and construction of

foundation systems for buildings regulated by the code. Provisions for seismic

load, flood load and frost protection are contained in this chapter. A foundation

system consists of two interdependent components: the foundation structure

itself and the supporting soil.

This chapter provides prescriptive

requirements for constructing footings

and walls for foundations of wood,

masonry, concrete, and precast concrete.

In addition to a foundations ability to

support the required design loads, this

chapter addresses several other factors that can affect foundation performance.

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Today we will cover:

Material requirements for concrete and precast

concrete foundations

Minimum requirements for footings

Minimum requirements for foundation walls

Foundation drainage requirements

Foundation waterproofing and damp proofing

Underfloor area requirements

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Foundation requirements

R401.2 Requirements: Foundation construction shall be capable of

accommodating all loads according to Section R301 and of transmitting the

resulting loads to the supporting soil. Where a construction joint is created

between a concrete footing and stem wall, a means of connection shall be

provided to accommodate lateral displacement. The connection shall be made

by the use of a key-way or other method in accordance with accepted foundation

design practices. Fill soils that support footings and foundations shall be

designed, installed and tested in accordance with accepted engineering practice.

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Foundation requirements cont.

Foundation tie-in/connection

Where a construction joint is

created between a concrete

footing and stem wall, a means

of connection shall be provided

to accommodate lateral

displacement.

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Foundation requirements cont.

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Good Compaction

Poor Compaction

Compacted fill requirements

Fill soils that are intended to support a

residential structure are required to be

placed under an engineers direction

and have the appropriate testing (R401.2).

When placed properly, soils will be brought

up in lifts over a compacted original ground

surface.

When placed loosely then

surface compacted the structure can

settle into the un-compacted soils and

cause structural damage to the building.

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Foundation requirements cont.

Proper compaction of the support soils for the structure as shown in the cross-

section below will prevent soil slough failure as seen below and the potential for

slab cracking and sinking due to soils settling below the slab.

Compacted or

native soils of 93 8

Site soils

Section R401.4 delineates

the requirements for allowable

soil bearing pressures for

typical residential applications.

On rare occasions the inspector

might encounter soils on site

that do not meet the basic

provisions as outlined in the

table and the sections. In these

cases the developer may need

to provide soils testing and

geotechnical evaluation in

order to assure stability and

safety for the residence.

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Foundations on expansive soils

R403.1.10 Foundations on expansive soils: Foundation and floor slabs for

buildings located on expansive soils shall be designed in accordance with Section

1805.8 of the Building Code.

Exception: Slab-on-ground and other foundation systems which have performed

adequately in soil conditions similar to those encountered at the building site are

permitted subject to the approval of the building official.

R403.1.10.1 Expansive soils

Classifications: Soils meeting all four

of the following provisions will be

considered expansive:

1. Plasticity Index (PI) of 15 or greater.

2. More than 10 percent of the soil

particles pass a No.200 sieve.

3. More than 10 percent of the soil

particles are less than 5 micrometers in

size.

4. Expansion Index greater than 20.

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Foundations on expansive soils cont.

Potential ways a structure will react to expansive soils

The inspector must be able to recognize potential expansive soils so that appropriate

measures may be taken when encountered in the field. Undetected expansive soils

can lead to severe structural damage in the future.

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Surface drainage

R401.3 Drainage: Surface drainage shall

be diverted to a storm sewer conveyance

or other approved point of collection that

does not create a hazard. Lots shall be

graded to drain surface water away from

foundation walls.

The grade shall fall a minimum of

6 inches (152 mm)within the first

10 feet (3048 mm).

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Surface drainage cont.

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Surface drainage cont.

There will obviously be times when the lot configuration does not lend itself well to

being able to direct drainage 10 feet away from the structure. On these occasions

the code has given the exception which can be accomplished in a variety of

different ways.

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Surface drainage cont.

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Surface drainage cont.

These conditions are what the code

is trying to avoid when stipulating the

minimum drainage provisions.

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Seismic design foundation provisions

R403.1.3 Continuous footing in Seismic Design Categories D1 and D2: The

braced wall panels at exterior walls of buildings located in Seismic Design

Categories D1 and D2 shall be supported by continuous footings. All required

interior braced wall panels in buildings with plan dimensions greater than 50 feet

(15 240 mm) shall also be supported by continuous footings.

The provisions in Section R403.1.3 are reiterated

in Section 602.10 for brace wall support.

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Seismic design foundation provisions cont.

R403.1.4 Footing and stem wall reinforcing in Seismic Design Categories D1

and D2: Concrete footings located in Seismic Design Categories D1 and D2, as

established in Table R301.2(1), shall have minimum reinforcement in accordance

with this section and Figure R403.1(1)(A). Reinforcement shall be installed with

support and cover in accordance with Section R403.1.5.3.

Lateral loading from seismic or wind forces also affects the foundations

system. Loads are transmitted to the foundation via brace walls or shear

walls and this loading must be transferred to the supporting soils.

The higher seismic areas require special consideration for the foundations.

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Seismic design foundation provisions cont.

Lateral loading from seismic or wind forces also affects the foundations

system. Loads are transmitted to the foundation via brace walls or shear walls

and this loading must be transferred to the supporting soils.

Section R602.10.7 provides the braced wall panel support criteria.

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Seismic design foundation provisions cont.

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Seismic design foundation provisions cont.

Due to the potential for foundation failure induced by soil movement during a seismic

event Section R403.1.4.1, R403.1.4.2 and R403.1.4.3 provide special reinforcement

requirements in foundations in Seismic Design Category structures in D1 and D2 regions. The details in Figure R403.1(1)(A) give pictorial representations of the

written provisions, below is detailing for a slab floor with turned down footings using

a construction joint at the top of the footing.

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Frost protection

R403.1.5 Minimum depth: All exterior footings shall be placed at least 12 inches (305 mm)

below the finished grade on undisturbed ground surface. Where applicable, the depth of

footings shall also conform to Sections R403.1.5.1 through R403.1.5.2.

R403.1.5.1 Frost protection: Except where otherwise protected from frost, foundation walls,

piers and other permanent supports of buildings and structures shall be protected from frost by

one or more of the following methods:

1. Extended below the frost line specified in Table R301.2.(1);

2. Constructing in accordance with Section R403.3;

3. Constructing in accordance with ASCE 32; or

4. Erected on solid rock.

Exceptions:

1. Protection of freestanding accessory structures with an area of 600 square feet

(56m2) or less, of light-frame construction, with an eave height of 10 feet

(3048mm)or less shall not be required.

2. Protection of freestanding accessory structures with an area of 400 square feet

(37 m2) or less, of other than light-frame construction, with an eave height of 10 feet

(3048 mm) or less shall not be required.

3. Decks not supported by a dwelling need not be provided with f