Category Archives: Construction Management

Impact of Water and Climate When Talking About CLT

Cross-Laminated Timber (CLT) is the trendy new building material that has received a lot of attention in the northwest this year.  It is by definition a series of thick boards (usually 2×4 or 2×6) glued together in an alternating pattern and Cross-to form a solid wall, post or beam. (A similar concept is glue laminated (glue lam) timber that has been used for beam construction for many years.)

To understand how water affects wood construction, one must understand its function in a tree. Wood is designed to provide structural support for the tree, move water from roots to leaves and store the chemical results of photosynthesis (primarily sugars in the form of cellulose). These properties of wood create some challenges for its longevity when exposed to moisture.  Thus, while wood can be structurally strong, it naturally absorbs and retains water.

Depending on climatic conditions, it can take some time for CLT panels to fully dry after getting wet. So, moisture control is of utmost importance when building from wood, such as cross-laminated timber.

The first source of moisture concern is during manufacturing. While manufacturers try to have all of their boards have the same moisture content, wood is an organic material and each board reacts to moisture differently. Some take water in more quickly and some take longer to dry. The result is that boards swell and shrink at differing rates which can affect the performance of a CLT panel.

Think of the hardwood floors in older buildings where the floor buckles in places. That is what happens when one board expands against another. As the wood dries again, it shrinks. This expansion and contraction process can damage the wood causing new cracks and gaps. On a floor, one can just refinish it. But, CLT is an integrated structural component and any deformity can weaken its life-safety purposes.

Another source of moisture is environmental moisture during construction. Studies indicate that construction moisture wetting is a serious problem in rainy climates like the Pacific Northwest. It can take several years for the wood to fully dry after construction. With rainfall likely in any month of the year, it is vital that proper precautions be taken to prevent the wood from getting and staying wet.

It is not just rainfall that is a concern. Other sources of moisture must also be accounted for, such as sprinklers, groundwater, pressure washing of siding, sidewalks, driveways, etc. decorative ponds, swimming pools, or other sources of moisture.

Wet CLT panels built with wrappings that do not allow vapor to pass through (both interior and exterior) are much more likely to develop bio-deterioration. There are vapor permeable wrapping on the market that can mitigate this concern to some extent.

Exterior cladding also plays a significant role in preventing moisture penetration. Not long ago, there were significant problems with wood frame buildings experience the negative effects of prolonged moisture, especially condominiums. The resulting lawsuits bankrupted many developers and contractors. With CLT, the stakes are even higher. With wood frame, the walls can be exposed, dried and cleaned. Since CLT is a structural system, that process is much more difficult and expensive.

In comparison, concrete, masonry and block all dry very quickly when wetted. Any remaining moisture can be removed easily (similar to using your shop-vac and a floor-squeegee in your garage after a big rain). Thus, construction moisture is of much less concern. And, there is no concern with rot with concrete, masonry and block construction.

One solution is to cover the CLT panels during construction until they are clad with the exterior components (be it siding, brick or stone). This is complicated, expensive and time consuming. Thus, covering panels during construction can undermine the presumed time and cost savings of CLT construction.

Another possible solution is to install water and air barrier membranes when the panels are manufactured. To date, no manufacturer is doing this, presumably because it would be cost prohibitive.

Certainly, more research needs to be done before the public can be assured of the safety and environmental impacts of cross-laminated timber panels used in construction. Also, new additional building code standards and construction practices would need to be developed to mitigate the health, life-safety and structural concerns posed by our wet climate on CLT buildings.  Current building codes allow wood structures to be built up to five stories. The talk is some would like to go up to as many as 20 stories with CLT.

The risk of fire in a wood building has historically been the primary concern with high-rise wood construction, but, water and climatic conditions can have an equally and perhaps more insidious impact on the construction.

CLT may have the potential to be a sustainable building product that lowers the carbon impact of building.  But so do proven products like concrete, steel and masonry.

While fire is an immediate and known risk, water’s impact is subtler, but no less threatening. This is particularly true in the Northwest from the Pacific Coast to the Cascades.

Ah, the smell of concrete in the morning

Westlake wrkrs with blurred rebarI was out recently on a construction site, doing a little photographic work, when I had another one of those blast-from-the-past moments in which we all, I assume, occasionally find ourselves suddenly immersed. In this case, as best as I could determine, it was instantly sparked by the smell of dust and concrete, with notes (I borrowed that term from a wine label) of something metallic. That had to be it — the unique blend of those elements, in just the right blend, that we apparently only take in on an active construction site.

As it has many times, that scent alone instantly took me back to when I was a little guy, fifty years ago (good lord, did I just say that?), occasionally visiting jobsites with my dad, who ran the Richland, Washington branch of Lord Electric (does anyone remember them?). He was The Boss, of course, which was quite cool, but I also really enjoyed those visits because I got to wear the hardhat and see stuff I sure didn’t see elsewhere: big pieces of construction equipment; the bone structure of halfway-built buildings; lots of different tools, fixtures and materials, and lots of people bustling around. Lots of pickup trucks, conduit, extension cords and cool lunch boxes too, as I recall.

This all made me think about what has, and what has not, changed on a construction site in half a century. For the most part, they still look very similar, but of course the materials, tools, techniques and practices – and certainly the pickup trucks and lunch boxes – have evolved quite a bit. In some ways, the people have changed, too, but in others, they have not; people have always, and will always, simply love to build things. That has to bring about a kind of satisfaction and pride that you don’t often find elsewhere.

And all those thoughts went through my head in a matter of moments.

Somewhere out there today, I’m sure there’s a superintendent or foreman or project manager on a jobsite with a son or daughter tagging along, taking in the sights and smells of all that’s going on around them. And likely creating little snippets of memories that will last — well, as we know now — at least half a century.

        – Sean Lewis

See what local firms can do with concrete

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The DJC’s annual special section on concrete is now available. Its focus is on award-winning projects by members of the Washington Aggregates & Concrete Association.

There’s also a great article by Melanie Cochrun of GLY Construction on how her firm used 600 workers to make two record-setting concrete pours earlier this year in Bellevue.

Check it out!

 

Check out what’s happening in the local construction scene

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The DJC has published its annual Construction & Equipment special section. It’s a mix of industry articles, profiles of local award-winning projects and a few interviews with the contractors who make it all happen.

Read all about it at www.djc.com/special/construct2015

 

Washington students shine at CM competition

UWTeam

This UW team won first place in the Mixed Use category.

 

The University of Washington won three awards and Washington State University won one at the recently held regional Associated Schools of Construction student competition in Nevada.

The event is the largest construction management competition in the country, with more than 1,300 students from 44 universities and 17 states participating, according to a WSU news release.

Both schools competed in Region 7, which includes schools from Washington, Oregon, California and Hawaii.

UW won first place in the Region 7 Mixed Use category, and WSU was third in the Commercial category. UW also won two third place awards in the Open competition, in the Integrated Project and Sustainable Building/LEED categories.

Way to go kids!

WSUTeam

WSU’s winning team in the Commercial category.