What Is Heavy Timber Construction?

Heavy timber construction is the process of using natural timber beams or glulaminated beams that have a cross section larger than 6″x6″ to form the structural frame work of a building. It is classified as Type IV Construction for fire protection purposes. The beams can be joined with traditional wooden joinery, like mortise and tenons with hardwood pegs, or with steel gusset plates or knife plates connected with bolts. It is a building system that has been used for thousands of years. The term “Heavy Timber Construction” is similar to timber frame construction and post and beam construction, but tends to refer to larger buildings with longer spans and higher loads.

 

What is Glue Laminated Timber (Also Called Glulam)?

Glue laminated timber refers to large timber beams that have been glued together from strips of common 2x construction wood like 2×4’s, 2×6’s, 2×8’s and 2×10’s. Relatively short  2x’s are finger jointed together at the ends to make long strips of wood. The strips are flooded with glue, then laminated into larger beams in a specially designed press that clamps the strips together under high pressure. The resulting beam is planed down on the sides by three eights of an inch, or so, to even up the laminations. A typical size for a glulam beam is 6 3/4″ x 13 1/2″. Any depth can be achieved by simply adding more laminations; 10 3/4″ x 46 1/2″ is possible as is 3 1/8″ x 7 1/2″.

The advantage of using glulam beams is strength and length. The beams can be 60 feet or longer and can be made very strong by increasing the number of laminations and the depth of the beam. Because the beam is made up of many laminations, it is also more uniform in its strength characteristics.  A natural timber might have large knots or checks in the middle that reduces its overall strength, where as a glulam timber has limited defects relative to its overall size. Therefore a high modulus of elasticity can be used for glulams when the engineers calculate their bending capacity.

The disadvantages of glulam timber is the higher cost compared to natural timber and the aesthetics. The laminations are visible and don’t look like a natural wooden beam.

 

When Is Heavy Timber Construction Used?

Heavy Timber is used in all types of public and private buildings – Churches, Welcome Centers, Pavilions, Schools, Museums, Houses and Barns . Just about any building can use heavy timber. The term heavy timber is mostly associated with larger commercial or public projects, where houses and barns might be more likely to be called timber framed buildings.

 

What is the Difference Between Heavy Timber Construction and Post and Beam Construction?

Post and Beam Construction and Heavy Timber construction are very similar, almost to the point of be synonymous. There are nuanced differences however. When people talk about post and beam construction, they are usually referring to barns, sheds and houses that use natural wooden beams joined together with simple half lapped joinery and mortises and tenons with wooden pegs. People use the term Heavy Timber Construction when they are talking about commercial construction that may still use natural wood beams, but also may use glulaminated beams or larger douglas fir beams. the connections in heavy timber construction can steel or wood. Bolted steel gusset plates are used. Knife plates that are slotted into the center of the timber are used and often the timber is reinforced with steel rods. Many times heavy timber framing will be used with steel tie rods or steel framing. Both types of construction would be considered Type IV Heavy Timber Construction for fire protection purposes.

 

What is Type IV-HT Construction and Its Relevance to Fire Protection?

All buildings are classified according to their ability to resist collapsing during a fire and to protect the inhabitants of the building as well as the fire fighters who go into burning buildings to save people. The fire code is very specific on the burn protection times that are required. Low occupancy residential buildings have lower required burn times than high occupancy public buildings like churches. It all makes sense from a safety standpoint.

Heavy timber is unique in the way that it burns. Small wood members, like 2×4’s, will burn quickly once they catch fire and even more quickly if they are spaced closely together because the fire will jump from one to the next. 2×4’s can be weakened structurally to the point of collapse in 30 minutes or less. Heavy timber beams, with a cross section of eight inches or more, burn very slowly. They develop a “char” on the outside that further slows the rate of burn. Heavy Timber can burn for two hours or more before it reaches the point of structural collapse, therefore it has its own category under the code. The added burn time allows more time for people to egress (get out of) the building and gives firefighters more time to save the people inside.  Because heavy timber takes longer to burn, sprinkler systems may not be required with Type IV construction, or occupancy can be increased – reducing the overall cost of construction without sacrificing safety.

The IBC (International Building Code) defines the types of construction and considers the burn time of all building components when determining what occupancy and separations are allowed for all buildings. Commercial and residential builders must follow the IBC to get building permits for their projects.

 

Does Heavy Timber Construction Always Use Steel Gusset Plates to Attach the Beams Together?

Steel gusset plates are often used in heavy timber construction, particularly when the beams are glue-laminated, but traditional wood joinery or steel knife plates can also be used to join the beams together. Sometimes, on trusses that span more than forty feet, steel gusset plates are required at the seat connection, where the top chord meets the bottom chord, because the forces are so large that they cannot be resolved with wood to wood connections.