Transcript 5. heavy timber frame construction

5. HEAVY TIMBER FRAME CONSTRUCTION
Chapter 5
Heavy Timber Frame Construction
5.1 INTRODUCTION TO BUILDING FRAMES
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Wood beams have been used to span roofs and floors of buildings
from the beginning of civilization
Earlier roof timbers were combined with load bearing masonry walls
to build houses and public buildings
British and European carpenters built buildings exclusively with
hand-hewn timbers with wood-to-wood connections. This was brought
over to North America
Present day heavy timber construction, with exterior walls of masonry
or concrete, is listed in building codes as Type 4 construction.
Progression of European timber frame construction
Construction of exclusively heavy timber construction
Form of an exclusively heavy timber construction
Timber Construction History
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Till the Late 1800s most
– Construction was Timber (and/or masonry) with
– Wood to Wood Connections
WHY?
Timer (Trees) Plentiful
 Craftsman Available
 Nails & Fasteners Rare & Expensive
 Few Alternatives
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Timber Construction Today
Why so Little Timber Construction Today?
 Alternate Structural Systems Available
 For Both Residential & Commercial
 Commercial Alternatives
 Concrete & Steel (late 1800s)
 Advantages over Timber
– Greater spans
– Lighter structures
– Increased fire resistance
– Increased versatility
– More economical
Commercial Alternatives
Concrete & Steel (late 1800s)
Advantages over Timber
– Greater spans
– Lighter structures
– Increased fire resistance
– Increased versatility
– More economical
Residential Alternatives
Light Wood Framing (mid 1800s)
Fasteners available @ reasonable price
Advantages
* Lighter structure
* Used less material
erect * More economical
* Quicker to
Where/when is timber construction typically used
today?
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Aesthetics, appearance, feel important
– Uniqueness of Timber Construction
Common Uses
– * High end residential
* Restaurants, lodges, etc.
– * Public areas requiring aesthetics
Ski Lodge
Church
5.2 HEAVY TIMBER FRAME CONSTRUCTION ADVANTAGES
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With masonry all around, the building catches fire very slowly
Supports the load even after heavy charring
Code recognizes the fire resistance properties of heavy timber
construction - Type 4 construction in Table 1.1 - Type 5 - Light
Wood Framing
While unprotected steel beam collapses much earlier during a
fire, a heavy timber beam can be sandblasted after a fire and
left in service (after normal fire damage)
Minimum timber sizes are specified in building codes for heavy
timber frame constructions
Edges must be chamfered for better fire resistance
5.3 PRECAUTIONS TO BE TAKEN IN HEAVY
TIMBER CONSTRUCTION
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Wood shrinkage: Wood is subjected to large amounts of expansion and
contraction caused by seasonal changes in moisture content, particularly in the
direction perpendicular to the direction of its grain fiber. These effects must be
minimized by proper detailing. Beams and girders must be allowed to shrink
without causing the floors and roof to sag
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Anchorage of heavy timber beams to masonry walls:
First the beam must be protected from decay due to moisture seepage through
the masonry walls - Achieved by leaving a ventilating airspace of at least 1/2”
between masonry and all sides of beam; this can be eliminated if the beam was
chemically treated to prevent decay. The beam must be securely anchored to
the wall so that it does not pull away from the the wall during normal services;
but it must be able to rotate freely, without prying the wall apart, as it burns
during a heavy fire - Methods of attachment must allow rotation of the support
after the beam falls apart due to fire damage
5.3 PRECAUTION TO BE TAKEN IN HEAVY
TIMBER CONSTRUCTION (Cont’d)
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Mode of covering the floors and roofs with proper
decking: Floor decking to be covered with a finish floor consisting of
nominal 1” thick tongue-and-grooved boards laid at right angles or diagonal to
the structural decking, or with plywood or particle boards
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Lateral bracing of buildings: Building is normally braced against
wind and seismic forces by the shear resistance of its walls, working together
with the diaphragm action of its roof and floor decks
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In areas of high seismic risk, the walls must be reinforced both vertically
and horizontally, and the decks may have to be specially nailed, or overlaid
with plywood to increase its shear resistance. In buildings with framed
exterior walls, diagonal bracing or shear panels must be provided. The
roof must be properly anchored to the building
5.4 Additional Considerations
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Lack of concealed cavities for:
– MPE rough-in
– Building Insulation
Span length limitations (typically limited to 20’)
Alternatives
– Large Beams – often laminated
– Heavy timber trusses