DRAWING ON EXPERIENCE … BUILDING ON STRENGTH

Six Storey Wood-Framed Buildings

Unique Structural Opportunities & Challenges

Kurt Ruhland, P.Eng.Director, Building Structures

MTE Consultants

Overview

• OBC changes came into effect Jan. 1, 2015

permitting wood-framed building up to six

storeys

• Adopted previously in other provinces:

• British Columbia (2009), Quebec (2014) and

Calgary (2014)

• Code changes are “Made in Ontario”

• They do not exactly match the code

provisions in other provinces

Opportunity

• Opportunities for Cost Savings

• Use of renewable resources

• Structural Design Factors of Safety as high or

higher than comparable structures

• New options for Builders

and Developers

Structural Engineering Design Considerations

Increased loads

• Gravity

• Lateral

Wood shrinkage

• Cladding

• Mechanical

• Shafts

Fire and life safety

• Sprinklers

• Shafts

• Exits

• Roof Anchors

Possible system

constraints

• Shear wall layouts

Increased Gravity Loads

• Increase Loads are Simply Due to

Added Floors to Building; Two Extra

Floors

• Since Floors are typically heavier than

roofs:

• 50% more floor area

• Results in about 60% more weight in the

building

• Requires Engineered Design using

same design codes and factors of

safety as previously used

Increased Gravity Loads

• Design loads can be resisted by conventional

wood framing

• Alternatively, Engineered Lumber or Concrete

Block could be used on lower levels

• Due to “heavy” wood in walls consideration

should be given to plumbing, electrical, etc. in

bearing walls.

• Also need to consider sound transmission and

insulation

• Recommend use of shop fabricated panelized

walls

Increased Lateral Loads

• Increased lateral loads (seismic)

• Seismic loads are proportional to weight of the

building, so 60% increase due to increase in

building mass

• OBC 4.1.8.10 (4): In higher seismic regions there

are added requirements for vertical alignment of

shearwalls through the building (shearwalls must

stack full height)

• OBC 4.1.8.11 (11): Provides additional restrictions

on use of a dynamic analysis

Wood Shrinkage

• Actual Code Change: OBC A-5.6.2.1. Sealing and Drainage.

As a consequence of increased building height, wood construction buildings exceeding 4 storeys may experience increased loadings on environmental separators and may require different design considerations than common approaches used by industry for buildings of 4 storeys or less. These considerations include but are not limited to, the following:

air barrier assemblies, fenestration selection, protection from precipitation, differential movement due to wood shrinkage, roofing selection and design, and risk of deterioration due to longer exposure of materials to the elements

during construction.

Wood Shrinkage

Proposed Code Change: A-4.3.1.1. Design Basis for WoodThe design criteria for wood, CAN/CSA 086 “Engineering Design in Wood”, makes

assumptions that the wood products being used are in a condition as intended by their grading. This includes the limits of moisture content as specified by the grade. However, conditions such as transportation, site storage, and construction conditions can impact the original design assumptions.

Design considerations should include, and be specific to, shrinkage that may occur due to changes in moisture content of the wood. This is of particular concern where the building height can be up to 6 storeys, including those built under Articles 3.2.2.45., 3.2.2.52., and 3.2.2.58. The potential building movement due to shrinkage should be indicated to other design professionals for their considerations such as cladding systems and mechanical systems, hold-down devices for structural walls and connections to non-shrinking elements including firewalls and elevator shafts.

Municipalities may require, as part of the building permit process, that the designer provide analysis satisfactory to the building official to demonstrate that shrinkage of the wood framing will not have adverse effects on the structure or any building services installations due to excessive shrinkage or differential movements caused by shrinkage. Computations to determine the required size of wood members should be based on the actual net dimensions and not nominal size.

Wood Shrinkage

Nevertheless: Current OBC 4.1.3.5 (1)

• In proportioning structural members to limit serviceability problems resulting from deflections, consideration shall be given to (d) creep, shrinkage, temperature changes and prestress.

The existing language of the code requires the engineer to deal with potential problems caused by shrinkage.

Wood Shrinkage

Wood Shrinkage

• Wood will shrink on the order of ¾” per floor for stick-framed structures to ¼” per floor for structures with precast floors.

• Effect is cumulative over height of the building• Regardless of how it is regulated in the Code

designers must consider the interaction of wood with other elements of the building that will not shrink1. Brick Cladding2. HVAC Ducts3. Plumbing Stacks4. Non-Combustible Shafts5. Shear wall hold-downs

Wood Shrinkage

Shrinkage Compensating Tie-Down Rods

Challenges

Combining block or concrete wall in a wood structure

• Magnitude of seismic loads depend on the lateral force resisting system

• Max. V=2/3 x S0.2 x IE x W/(RdRo)

• Wood Shearwalls: RdRo =3.0x1.7=5.1

• Concrete Shearwalls: RdRo =1.5x1.3=1.95

• Masonry Shearwalls: RdRo =1.5x1.5=2.25

• OBC 4.1.8.9: When combining lateral force resisting systems the lowest RdRo shall be used

Challenges

Combining block or concrete wall in a wood structure

• For instance: • S0.2 =0.16

• IE =1.0

• W=50,000 kN

• The seismic load V is equal to • 1050 kN for wood shear walls (resisted by several long walls)• 2750 kN for concrete shear walls (resisted by only the shafts)• 2350 kN for concrete block shear walls (resisted by only the

shafts)

• Using non-wood shear walls negates some of the savings of wood

Challenges

Stair Stair

Non-Combustible Shafts Exiting on the Ground Floor

Other Considerations

• Brick support• Requirement for non-

combustible cladding

• Roof Anchors• OBC 4.4.4.1

• NFPA13R vs. NFPA13• Limitations on Roof

Construction