Components of Heavy Timber & Mass Timber Construction
So in this series, we’ve talked about different approaches to assembling building structure. Here in this module, we’re covering Heavy Timber and Mass Timber Construction.
But why did Heavy Timber ever even become popular? Couldn’t lightweight wood construction be easier to construct? Couldn't it be easier to cut wood to bring to the job site assemble? Couldn't that be all easier?
Heavy Timber is a traditional method of construction that uses large solid wood members as building structure. The need for heavy timber arose from early factories in the Northeast United States.
Because of the early manufacturing techniques, machinery, and vast amounts of combustibles inside factories and mills, fires became more and more commonplace. Lightweight construction methods at the time just didn’t help the situation.
By 1835, heavy timber construction was standardized after fires had decimated New England’s textile mills. Insurance companies, with major losses, began refusing to insure mills and factories. In the wake of this, a group of Rhode Island mill owners formed an organization that became known as the Associated Factory Mutual Fire Insurance Companies to self-insure. Their meeting in 1835 standardized design for mill construction.
Our terms of Standard Mill and Semi-Mill, still found in NFPA 13 today, reflect the outcome from those early meetings.
This new standardized heavy timber construction method sought huge beams and huge columns that could char on the edges along the outside, and still maintain the strength to support the building structure. The idea of the fire-resistive new standard heavy timber construction began to tip and in the next few decades it grew in popularity across the United States.
WHAT IS HEAVY TIMBER?
So, what is Heavy Timber Construction?
Well today, the NFPA line of codes and standards defines it and gives us nominal dimensions to qualify as Heavy Timber. This is defined as Type IV (2HH) construction in the NFPA line of codes and standards.
NFPA 5000 and NFPA 220 gives us minimum dimensions for columns supporting floor loads as not less than 8 inches (or 205 mm) in any direction. Columns that are supporting roofs only not less than 6-inches (150 mm) in any direction.
Beams? Well, if they’re supporting floors not less than 6-inches (150 mm) wide and not less than 10-inches (255 mm) deep. Supporting roofs only not less than 4-inches (100 mm) wide and 6-inches (150 mm) deep.
Floors also need to have some substantial thickness to them. Tongue and groove plank flooring or cross-laminated timber has to be not less than 4 inches (a 100 mm) thick. That’s a lot of wood!
In the ICC line of codes, we have different subclassifications for Type IV construction. This is a new change or at the time of this module, relatively new. These include Type IV-A, IV-B, IV-C, and IV-Heavy Timber.
These relatively new classes have been added with the interest in growth in popularity of mass timber buildings, where even skyscrapers are constructed with predominately wood construction.
ICC gives minimum dimensions under the International Building Code, which is in Section 2304.11, with different albeit similar requirements on how to qualify as heavy timber construction.
COMPONENTS OF HEAVY TIMBER
So, all that being said, what are the components of heavy timber construction?
Well by now, in this series, we're recovering a whole lot of structure. A lot of this terminology should sound very familiar.
A roof deck for heavy timber can be cross-laminated timber roofs of three-inch thickness, or they could be sawn, it could be wood structural panel, or glue-laminated plank roofs with some minimum dimension thickness.
Purlins below that roof deck. Purlins, when they’re provided, run perpendicular to the slope of the roof and take that roof load over to the rafters. The rafters are the sloped members along the roof that direct the load down to a bearing wall, a girt (or technically if it's, you know, we're looking at beam terminology, it's technically a girder), or take that load to a column.
Now by the nature of using wood, the term columns and posts are pretty much interchangeable here.
Some of the descriptions for posts will change a little bit depending upon its purpose. A King Post is a vertical member right at the peak of a timber frame. A Queen Post is one of two vertical members that actually is in tension, not compression. These are offset from the center down the slope of the rafter just a little bit.
Some notes here about the difference between light frame and heavy timber construction.
Obviously, we’ve already covered that there are minimum sizes to qualify for the timber members to be heavy timber construction. But aside from the size of the individual pieces, note the spacing on our columns and our beams.
These illustrations are not great for scale but timber structures not only have bigger individual pieces, they’re also spaced out at much greater distances than we use in light-frame construction. Fundamentally, the big difference between light frame and heavy timber construction is that we're using less pieces that are significantly larger when we're in heavy timber construction.
Eventually, at the bottom of the posts or the columns if you will, we have a foundation wall and we have a footer below the foundation wall that transfers the weight down to the soil.
Now this is just one arrangement for heavy timber construction and our roofs aren't necessarily always peaked like this
But what about the term, Mass Timber? We've heard it. It's relatively new. Is it different than heavy timber? Well, yeah, it is.
Heavy Timber Construction is the traditional construction method that’s been used for centuries. The individual wood members are historically heavy sawn (or which is just a single piece of cut wood). Heavy Timber also traditionally uses tongue and groove decking.
Now Heavy Timber could use glue-laminated beams or could use glue-laminated roof decks (which glues layers of dimensional lumber together), but the distinction between Heavy Timber and Mass Timber for our purposes really doesn’t matter so much. They’re both similar styles. Mass timber is more or less just an evolution of what we've had with heavy timber construction.
Now in general, Mass Timber Construction makes wider use of laminated wood members. It uses dimensional 2x, which is, you know, 2x4, 2x6 wood members that are laminated together. Sometimes in new ways. Some newer engineering lamination techniques include NLT (nail-laminated timber) or CLT (cross-laminated timber), among some others.
WHAT IS MASS TIMBER?
Mass Timber Construction is billed as a new way of construction. It’s environmentally friendly (it produces far less carbon than steel or concrete), it can produce less waste, can be cost-effective, and has earthquake-resistance properties due to the ductile nature of wood where it can bend-but-not-break in a seismic event.
Now the downsides to Mass Timber Construction includes concerns about deforestation and our need to replenish those harvested forests, and our favorite as we identified earlier; combustibility. We need to make sure that we're constructing these buildings in a manner that is still resistive to fire.
WHAT’S THE DIFFERENCE?
So, what makes a mass timber construction different than heavy timber?
Well, much of the fundamental concepts between the two are the same. We have thick wood structural members that have some inherent fire resistance due to their size, use of wood for the main structural frame, and a few other things. But the real concept for mass timber is to use newer laminating techniques for building occupancies that traditionally have not used wood and to use them for building heights that have traditionally not used wood, including high rises.
So when we say mass timber, we tend to think of the new buildings, the new high rises, the new office towers or apartment complexes that are cropping up more and more today. These are the tall buildings that use large, laminated lumber for floor panels, beams, and posts. Much of the descriptors are exactly the same as heavy timber, but they're just used for a different end result.
So, to wrap this one up today, heavy timber and mass timber construction are squarely etched in our fire protection tradition. Use of large, thick wood structural members that can tolerate char along the outside during a fire event provide some inherent fire resistance, they can tolerate high temperatures, and they provide structural stability during a fire.
This tradition of heavy timber construction has evolved with some relatively recent innovations in how we create large wood panels and posts and beams to create modern-day mass timber construction.
Now for sloped roofs, we often find timber roof panels, purlins, rafters, posts, girts, and the foundation.
For level roofs, we have roof panels, floor panels, beams, posts, and the foundation.
That’s about it for this one.
I’m Joe Meyer, this is MeyerFire University.
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