Ministry of Municipal Affairs
PROPOSED CHANGE TO THE 2012 BUILDING CODE
O. REG. 332/12 AS AMENDED
CHANGE NUMBER:
B-04-01-25
SOURCE:
Ontario-NBC
CODE REFERENCE:
Division B / 4.1.8.15.
Division B / 4.1.8.13.(2)
Division B / 4.1.8.16.
DESCRIPTION OF THE PROPOSED AMENDMENT
4.1.8.15.(8) This proposed change is intended to delete a design force limit that was incorrect but would not apply
in practice. It also clarifies that the design must satisfy the R d R o requirements for the type of SFRS listed in Table
4.1.8.9.
4.1.8.13.(2) This proposed change is intended to add a requirement to the calculation of displacements to include
increases due to foundation movements.
4.1.8.15.(9) This proposed change is intended to emphasize that foundation movements can affect the
displacements and forces in a structure and that they must be considered in the analysis and design.
4.1.8.16. This proposed change is intended to incorporate the results of new research on footing deflections and
movements.
EXISTING 2012 BUILDING CODE PROVISION(S)
4.1.8.15. Design Provisions
(7) Except as provided in Sentence (8), the design forces associated with the lateral capacity of the SFRS need not
exceed the forces determined in accordance with Sentence 4.1.8.7.(1) with R d R o taken as 1.0, unless otherwise provided
by the applicable referenced design standards for elements, in which case the design forces associated with the lateral
capacity of the SFRS need not exceed the forces determined in accordance with Sentence 4.1.8.7.(1) with R d R o taken as
1.3. (See Appendix A.)
(8) If foundation rocking is accounted for, the design forces for the SFRS need not exceed the maximum values
associated with foundation rocking, provided that R d and R o for the type of SFRS used conform to Table 4.1.8.9. and that
the foundation is designed in accordance with Sentence 4.1.8.16.(1).
4.1.8.13. Deflections and Drift Limits
(2) Lateral deflections obtained from a linear elastic analysis using the methods given in Articles 4.1.8.11. and 4.1.8.12.
and incorporating the effects of torsion, including accidental torsional moments, shall be multiplied by R d R o /I E to give
realistic values of anticipated deflections.
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4.1.8.16. Foundation Provisions
(1) Foundations shall be designed to resist the lateral load capacity of the SFRS, except that when the foundations are
allowed to rock, the design forces for the foundation need not exceed those determined in Sentence 4.1.8.7.(1) using an
R d R o equal to 2.0. (See Appendix A.)
(2) The design of foundations shall be such that they are capable of transferring earthquake loads and effects between
the building and the ground without exceeding the capacities of the soil and rock.
(3) In cases where I E F a S a (0.2) is equal to or greater than 0.35, the following requirements shall be satisfied:
(a) piles or pile caps, drilled piers, and caissons shall be interconnected by continuous ties in no fewer than two
directions, (See Appendix A.)
(b) piles, drilled piers, and caissons shall be embedded a minimum of 100 mm into the pile cap or structure, and
(c) piles, drilled piers, and caissons, other than wood piles, shall be connected to the pile cap or structure for a minimum
tension force equal to 0.15 times the factored compression load on the pile.
(4) At sites where I E F a S a (0.2) is equal to or greater than 0.35, basement walls shall be designed to resist earthquake
lateral pressures from backfill or natural ground. (See Appendix A.)
(5) At sites where I E F a S a (0.2) is greater than 0.75, the following requirements shall be satisfied:
(a) piles, drilled piers, or caissons shall be designed and detailed to accommodate cyclic inelastic behaviour when the
design moment in the element due to earthquake effects is greater than 75% of its moment capacity, and (See
Appendix A.)
(b) spread footings founded on soil defined as Site Class E or F shall be interconnected by continuous ties in no fewer
than two directions.
(6) Each segment of a tie between elements that is required by Clause (3)(a) or (5)(b) shall be designed to carry by
tension or compression a horizontal force at least equal to the greatest factored pile cap or column vertical load in the
elements it connects, multiplied by a factor of 0.10 I E F a S a (0.2), unless it can be demonstrated that equivalent restraints
can be provided by other means. (See Appendix A.)
(7) The potential for liquefaction of the soil and its consequences, such as significant ground displacement and loss of
soil strength and stiffness, shall be evaluated based on the ground motion parameters referenced in Subsection 1.1.2. and
shall be taken into account in the design of the structure and its foundations. (See Appendix A.)
A-4.1.8.16.(1) Rocking Foundations.
Information on foundations that are allowed to rock can be found in the Commentary entitled “Design for Seismic Effects” in
the User’s Guide - NBC 2010, Structural Commentaries (Part 4 of Division B).
A-4.1.8.16.(3)(a) Interconnection of Foundation Elements.
Information on the interconnection of piles or pile caps, drilled piers, and caissons can be found in the Commentary entitled
“Design for Seismic Effects” in the User’s Guide - NBC 2010, Structural Commentaries (Part 4 of Division B).
A-4.1.8.16.(4) Earthquake Lateral Pressures from Backfill or Natural Ground.
Information on methods of computing the seismic lateral pressures from backfill or natural ground can be found in the
Commentary entitled “Design for Seismic Effects” in the User’s Guide - NBC 2010, Structural Commentaries (Part 4 of
Division B).
A-4.1.8.16.(5)(a) Cyclic Inelastic Behaviour of Foundation Elements.
Information on the cyclic inelastic behaviour of piles or pile caps, drilled piers, and caissons can be found in the Commentary
entitled “Design for Seismic Effects” in the User’s Guide - NBC 2010, Structural Commentaries (Part 4 of Division B).
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A-4.1.8.16.(6) Alternative Foundation Ties.
Alternative methods of tying foundations together, such as a properly reinforced floor slab capable of resisting the required
tension and compression forces, may be used. Passive soil pressure against buried pile caps may not be used to resist these
forces.
A-4.1.8.16.(7) Liquefaction.
Information on liquefaction can be found in the Commentary entitled “Design for Seismic Effects” in the User’s Guide NBC 2010, Structural Commentaries (Part 4 of Division B).
PROPOSED CODE CHANGE
4.1.8.15. Design Provisions
(7) Except as provided in Sentence (8), tThe design forces associated with the lateral capacity of the SFRS need not
exceed the forces determined in accordance with Sentence 4.1.8.7.(1) with R d R o taken as 1.0, unless otherwise provided
by the applicable referenced design standards for elements, in which case the design forces associated with the lateral
capacity of the SFRS need not exceed the forces determined in accordance with Sentence 4.1.8.7.(1) with R d R o taken as
1.3. (See Appendix A.)
(8) If foundation rocking is accounted for, the design forces for the SFRS need not exceed the maximum values
associated with foundation rocking, provided that R d and R o for the type of SFRS used conform to Table 4.1.8.9. and that
the foundation is designed in accordance with Sentence 4.1.8.16.(1). Foundations need not be designed to resist the lateral
load overturning capacity of the SFRS, provided the R d and R o for the type of SFRS used conform to Table 4.1.8.9. and
that the foundation is designed in accordance with Sentence 4.1.8.16.(4).
(9) Foundation displacements and rotations shall be considered as required by Sentence 4.1.8.16.(1).
4.1.8.13. Deflections and Drift Limits
(2) Lateral deflections obtained from a linear elastic analysis using the methods given in Articles 4.1.8.11. and 4.1.8.12.
and incorporating the effects of torsion, including accidental torsional moments, shall be multiplied by R d R o /I E and
increased as required by Sentences 4.1.8.10.(6) and 4.1.8.16.(1) to give realistic values of anticipated deflections.
4.1.8.16. Foundation Provisions
(1) The increased displacements of the structure resulting from foundation movement shall be shown to be within
acceptable limits for both the SFRS and the structural framing elements not considered to part of the SFRS. (See
Appendix A.)
(12) Foundations shall be designed to resist the lateral load capacity of the SFRS, except that when the foundations are
allowed to rock, the design forces for the foundation need not exceed those determined in Sentence 4.1.8.7.(1) using an
R d R o equal to 2.0. (See Appendix A.) Except as provided in Sentences (3) and (4), foundations shall be designed to have
factored shear and overturning resistances greater than the lateral load capacity of the SFRS. (See Appendix A.)
(3) The shear and overturning resistances of the foundation determined using a bearing stress equal to 1.5 times the
factored bearing strength of the soil or rock and all other resistances equal to 1.3 times the factored resistances need not
exceed the design forces determined in Sentence 4.1.8.7.(1) using R d R o = 1.0 except that the factor of 1.3 shall not apply
to the portion of the resistance to uplift or overturning resulting from gravity loads.
(4) A foundation is permitted to have a factored overturning resistance less than the lateral load overturning capacity of
the support SFRS, provided the following requirements are met:
(a) neither the foundation nor the supported SFRS are constrained against rotation, and
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(b) the design overturning moment of the foundation is
(i) not less than 75% of the overturning capacity of the supported SFRS, and
(ii) not less than that determined in Sentence 4.1.8.7.(1) using R d R o = 2.0
(See Appendix A)
(25) The design of foundations shall be such that they are capable of transferring earthquake loads and effects between
the building and the ground without exceeding the capacities of the soil and rock.
(36) In cases where I E F a S a (0.2) is equal to or greater than 0.35, the following requirements shall be satisfied:
(a) piles or pile caps, drilled piers, and caissons shall be interconnected by continuous ties in no fewer than two
directions, (See Appendix A.)
(b) piles, drilled piers, and caissons shall be embedded a minimum of 100 mm into the pile cap or structure, and
(c) piles, drilled piers, and caissons, other than wood piles, shall be connected to the pile cap or structure for a minimum
tension force equal to 0.15 times the factored compression load on the pile.
(47) At sites where I E F a S a (0.2) is equal to or greater than 0.35, basement walls shall be designed to resist earthquake
lateral pressures from backfill or natural ground. (See Appendix A.)
(58) At sites where I E F a S a (0.2) is greater than 0.75, the following requirements shall be satisfied:
(a) piles, drilled piers, or caissons shall be designed and detailed to accommodate cyclic inelastic behaviour when the
design moment in the element due to earthquake effects is greater than 75% of its moment capacity, and (See
Appendix A.)
(b) spread footings founded on soil defined as Site Class E or F shall be interconnected by continuous ties in no fewer
than two directions.
(69) Each segment of a tie between elements that is required by Clause (3)(a) or (5)(b) shall be designed to carry by
tension or compression a horizontal force at least equal to the greatest factored pile cap or column vertical load in the
elements it connects, multiplied by a factor of 0.10 I E F a S a (0.2), unless it can be demonstrated that equivalent restraints
can be provided by other means. (See Appendix A.)
(710)
The potential for liquefaction of the soil and its consequences, such as significant ground displacement and
loss of soil strength and stiffness, shall be evaluated based on the ground motion parameters referenced in Subsection
1.1.2. and shall be taken into account in the design of the structure and its foundations. (See Appendix A.)
A-4.1.8.16.(1) Rocking Foundations Foundation Movement.
Information on foundations that are allowed to rock can be found in the Commentary entitled “Design for Seismic Effects” in
the User’s Guide - NBC 2010, Structural Commentaries (Part 4 of Division B). The bearing stress distribution in soil or rock
that is used to determine the factored overturning resistance of the foundation influences the rotation of the foundation, which
occurs due to the forces applied by the SFRS. Generally, all foundations will rotate on soil or rock. In particular, footings (a
type of foundation unit) often undergo uplift at one end, and if the factored bearing stress at the other end is only over a short
length, then the uplift and rotation of the footing can be significant. CSA A23.3, “Design of Concrete Structures,” contains
design requirements for footings that rotate and uplift; see also the Commentary entitled Design for Seismic Effects in the
“User’s Guide – NBC 2015, Structural Commentaries (Part 4 of Division B)” for guidance and methods to account for
foundation movement.
A-4.1.8.16.(2) Actual Lateral Load Capacity of the SFRS.
The actual lateral load capacity of the SFRS includes the effects of member overstrengths similar to those used to determine
the Ro factors. The applicable CSA design standards include requirements on calculating the overstrengths and capacities,
which may be based on the members’ nominal or probable resistance. The actual capacities are larger than the factored loads
and factored resistances and, in many cases, can be significantly larger. Note that the foundations designed to develop the
capacity of the SFRS will undergo movements and Sentence 4.1.8.16.(1) still applies.
A-4.1.8.16.(4) Overturning Resistance of the Foundation.
For the special case where the foundation is a footing, and where it and the attached SFRS are not constrained against
rotation, it is permitted, with certain limitations, to size the footing to have a factored overturning resistance less than the
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overturning capacity of the supported SFRS. This approach results in a smaller footing, increased footing rotations, increased
drifts in the structure, and increased soil stresses, all of which are over and above those associated with footings sized to have
a factored overturning resistance equal to or greater than the overturning capacity of the SFRS. The footing itself must have a
factored resistance capable of developing the required soil or
rock reactions. An example of a footing and SFRS that are not constrained against rotation is an SFRS on a footing near the
ground surface such that it can rotate freely and is attached to a gravity-load-resisting system (non-SFRS) that is laterally
flexible and provides little lateral resistance. For this case, the SFRS is usually analyzed on its own and the resulting
displacements are imposed on the non-SFRS elements in order to assess the effects on them. Cases where the footing and
SFRS are attached to a system that has significant lateral stiffness require careful analysis and engineering judgement, or the
footing can be capacity-designed.
Limiting the overturning moment on the foundation and the RdRovalue provides some control on the increase in lateral
displacement, drift and stress in the soil or rock. Cases that exceed these limits require special study.
For the common case where the SFRS and/or the footing are constrained in some way against rotation, the footing’s factored
resistance must be equal to or greater than the capacity of the supported SFRS. An example of an SFRS constrained against
freely rotating with the footing is an SFRS attached to adjacent foundation walls by below-grade diaphragms. Examples of
footings constrained against free rotation are footings that use soil anchors to resist overturning, footings on piles, and raft
foundations. Note that Sentence 4.1.8.16.(1) still applies.
See CSA A23.3, “Design of Concrete Structures,” and the Commentary entitled Design for Seismic Effects in the “User’s
Guide – NBC 2015, Structural Commentaries (Part 4 of Division B).”
A-4.1.8.16.(36)(a) Interconnection of Foundation Elements.
Information on the interconnection of piles or pile caps, drilled piers, and caissons can be found in the Commentary entitled
“Design for Seismic Effects” in the User’s Guide - NBC 2010, Structural Commentaries (Part 4 of Division B).
A-4.1.8.16.(47) Earthquake Lateral Pressures from Backfill or Natural Ground.
Information on methods of computing the seismic lateral pressures from backfill or natural ground can be found in the
Commentary entitled “Design for Seismic Effects” in the User’s Guide - NBC 2010, Structural Commentaries (Part 4 of
Division B).
A-4.1.8.16.(58)(a) Cyclic Inelastic Behaviour of Foundation Elements.
Information on the cyclic inelastic behaviour of piles or pile caps, drilled piers, and caissons can be found in the Commentary
entitled “Design for Seismic Effects” in the User’s Guide - NBC 2010, Structural Commentaries (Part 4 of Division B).
A-4.1.8.16.(69) Alternative Foundation Ties.
Alternative methods of tying foundations together, such as a properly reinforced floor slab capable of resisting the required
tension and compression forces, may be used. Passive soil pressure against buried pile caps may not be used to resist these
forces.
A-4.1.8.16.(710) Liquefaction.
Information on liquefaction can be found in the Commentary entitled “Design for Seismic Effects” in the User’s Guide NBC 2010, Structural Commentaries (Part 4 of Division B).
RATIONALE FOR CHANGE
Problem/General Background
4.1.8.13.(2) Currently, foundation flexibility and footing rotational stiffness are not typically modeled and recent
research has shown that this effect can have a noticeable effect on increasing the expected displacements in a
structure. This could create unsafe conditions for other structural elements not part of the SFRS if their integrity is
sensitive to displacements.
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4.1.8.15.(8) The Sentence contained an incorrect limitation on design forces which would not get used in practice.
The change removes the limit and clarifies the clause with respect to design requirements.
4.1.8.15.(9) A common assumption in modeling a structure is to use a model fixed at the foundation. This clause,
along with others in a group of proposed changes, stresses the fact that a fixed base assumption may not be
conservative for the calculation of forces and displacements in the structure and foundation movements need to be
considered in the design. Ignoring these additional displacements could result in an unsafe design.
4.1.8.16. Recent research on “rocking” footings indicates that the previous limits of RdRo=2.0 in the Building
Code worked for many cases but when the footing is much weaker than the wall (say a wall with a large
overstrength) the building displacements and soil stresses start to become quite large. Experience in recent
earthquakes has also emphasized that deflections and drifts are critical in assessing the integrity of parts of the
structure other than the SFRS as failure of these elements has led to collapse.
Justification/Explanation
This proposed change would harmonize requirements with the model National Building Code of Canada.
4.1.8.13.(2) This change is part of a group that introduces requirements to consider these effects in the design of
the structure.
4.1.8.15.(8) Corrects and clarifies the Sentence requirements.
4.1.8.15.(9) Require the designer to consider conditions that may be un-conservative if ignored.
4.1.8.16. The revised Sentence limits the ratio of footing to wall capacity and requires the designer to consider
how foundation displacements and rotations affect the deflections and forces in the superstructure, and how these
increased deflections affect the integrity of the columns and slab/column joints. Ignoring these deflections could
result in an unsafe condition.
Cost/Benefit Implications
4.1.8.13.(2) The cost may vary from zero to a small cost due to increased detailing required to deal with increased
deflections on the structure. There may be an increase in cost to designers if they change their models, but the
CSA A23.3 standard allows a simple method to adjust the displacements without requiring a new analysis model.
4.1.8.15.(8) None.
4.1.8.15.(9) There may be no cost implications or a small one on the construction side if additional detailing is
required. Extra design time may be required for modeling the structure but CSA A23.3 has a simplified method
added in the 2014 version which gives a simple solution for the calculation when a fixed base is used.
4.1.8.16. There may no cost implications or small costs associated with the analysis and design, but the CSA
A23.3 requirements provide simple solutions and allow use of fixed base analysis. The increased displacements
may also require additional detailing of columns and slab column joints to maintain life safety, but the associated
costs are typically minor.
Enforcement Implications
None.
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Who is Affected
Building officials, consultants, builders, building owners.
Objective Based Analysis
Provision
Objective/Functional Statement
Division B 4.1.8.15.
(7)
(8)
(9)
Division B 4.1.8.13.
(2)
[F22-OS2.3,OS2.4]
(2)
[F22-OP2.3,OP2.4]
Division B 4.1.8.16.
(1)
[F22-OS2.3,OS2.4]
(1)
[F22-OP2.3,OP2.4]
(2) 1
[F20-OS2.1]
(2) 1
[F20-OP2.1]
(3)
(4)
(5) 2
[F20-OS2.2,OS2.4]
(5) 2
[F20-OP2.2,OP2.4]
(6) 3
(a), (b) [F22-OS2.4]
(6) 3
(a), (b) [F22-OP2.4]
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Provision
Objective/Functional Statement
(6) 3
(c) [F20-OS2.4]
(6) 3
(c) [F20-OP2.4]
(7) 4
[F20-OS2.1]
(7) 4
[F20-OP2.1,OP2.4]
(8) 5
[a][F20-OS2.1]
(8) 5
[a][F20-OP2.1]
(8) 5
[b][F22-OS2.4]
(8) 5
[b][F22-OP2.4]
(9) 6
[F20-OS2.4]
(9) 6
[F20-OP2.4]
(10) 7
[F20-OS2.2][F22-OS2.4]
(10) 7
[F20-OP2.2][F22-OP2.4]
OTHER SUPPORTING MATERIALS
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