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New Heights in Mass Timber

As the winter of 2016 approached, all eyes were on Carbon12. The foundations had been poured and a steel frame stood two stories high, waiting for the wood panels that would enable it to climb skyward. The goal was to build the entire eight story structure in just ten weeks. Here is how it happened.

It was essential that the steel and concrete were ready to accept the CLT panels that were ready and waiting in Canada. The wood package required a tolerance of ⅛”, yet acceptable tolerances for concrete are ½” to 1” depending on location, and up to ¾” for structural steel. It was imperative that the trades worked together to achieve a tight tolerance across the board. The first floor wood columns were sized to assume for some shimming to occur at their base. This allowed the framers to make up for some of the tolerances in the steel bases across the ground floor before the CLT arrived.

The wood columns and beams were installed on the east side first. Once the columns and beams were plumbed, squared and leveled, they were screwed together with long diagonal screws on the top. The framers then moved to the other side and installed the columns and beams. The CLT panels took roughly two days to install, including installation of the splines, steel straps and screws. The first level took roughly two weeks, but the upper floors were completed with a floor every four days.

After three floors of CLT were installed, the framers had to stop CLT installation to add the next level of the steel core. They then dropped back and installed the interior walls and stairs to keep the project moving forward and make the floors safe for other trades.

  • How did you achieve tolerance at the ground level between the concrete and the wood?
  • How did you provide tolerance between the steel brace frame core and the wood beams that connected to it?
  • How did the wood-to-wood fabrication provide tolerance?
  • How did you install the exterior wood-framed walls?
  • Why didn’t you use a concrete core?
  • How did you use technology to ensure that everything went together as designed?
In my 30 years of building, I have not seen a building framed as quickly and efficiently as Carbon 12. The structural steel core and mass timber elements fit together seamlessly - with very little corrective work.

Scott Noble, Senior Project Manager, Kaiser+Path

On January 27, 2017 the seventh floor deck was installed, making Carbon12 the tallest CLT structure in the United States.

Did You Know?

The entire wood package arrived as a kit of parts. The building had 234 columns and 336 beams, of which only 4 beams had to be trimmed to fit.

Smaller Crews, More Safety

Due to a chronic labor shortage, finding subcontractors and the crews to work on the project was very challenging. Because of the nature of CLT construction, only four carpenters would be needed to assemble Carbon12’s mass timber components.

Smaller crew sizes mean less cost, improved safety, better coordination and added accountability. To prevent damage and staining, the CLT pieces were lifted directly from the semi-truck trailer with no double handling. The pieces were lifted, or “flown”, with the crane to a crew of three carpenters who maneuvered the pieces into place and secured them.

  • How was the steel installed?
  • Why didn’t you panelize the interior and exterior walls to double the efficiency?
  • How long would it have taken if you constructed the building using post-tension concrete slabs?
  • What kind of equipment did you use to install mass timber and steel components?

The Wood Mock Up

Before the mass timber was fabricated, Kaiser+Path designed a mini-Carbon12 cross-section to ensure the look, constructability and mobility of each piece was taken into account. When the mock-up arrived at the construction site, it was installed and evaluated. Tweaks and modifications were quickly made, comments were compiled and sent back to the manufacturer. Examples included: adding a pick point to the top of the columns so they could easily be craned into place, and identifying areas that needed more or less tolerance to ensure a tight fit. These adjustments proved invaluable not just for speed but also for safety.

Cold Temps And Snowy Weather

Framing took place between December 2016 and February 2017, which was one of the wettest, coldest, snowiest winters in recent history. While most of the construction sites in town were closed for several days at a time, Carbon12 continued to rise.

Snow Inside The Building — Crews arrived on site and spent the first few hours clearing snow so that work could continue.

Snowy Roads — Mass timber can be installed in any temperature and weather condition. It is not limited by cure temperatures or welding torches.

Avoiding Waste

Mass timber members were cut, modified and finished before they reached Carbon12’s site. By taking the cutting, connector install and notching out of the field and putting it into the factory, waste and on-site mistakes were eliminated. The mass timber components went through a rigorous quality control process in the production facility. This saved time on site and avoided the need to order extra attic stock in case of miscuts. Structurlam requires that the wood waste from their factory be used as biofuel, giving it a second life as a power producer.

During framing, only two dumpsters were used. These contained scraps from the interior framing and the protective wrap that covered the CLT panels during transportation. Oregon requires construction dumpsters go to recycling centers to be sorted. However, the materials are co-mingled and there is a large amount of recyclable or reusable material that ends up being damaged or wasted. By lessening the use of dumpsters, money was saved and the likelihood of materials being recycled properly was greatly increased.

Did You Know?

All the mass timber for Carbon12 was trucked from a facility in British Columbia to the jobsite by one driver making multiple day-long trips; swapping out the loaded trailer with an empty trailer for the return trip.

The truck driver left the CLT production facility in Penticton, BC in the early evening. Driving all night, he arrived in Portland around dawn. He slept for a few hours in his truck before arriving at the job site at 6:00am. He dropped his fully loaded trailer, grabbed some breakfast and hitched up the empty one from the previous trip to head back to Penticton.

Acoustic Floor System

Since the acoustic system was a custom design of disparate parts, Kaiser+Path hired multiple subcontractors: one to install the initial insulation and base layers, another to install the gypcrete topping slab and finally one more to install the finished floor products. Securing the initial materials, particularly the insulation board, proved challenging because the size of the material was a special order. Roughly 2,200 sheets of insulation board took 12 weeks to arrive in Portland.

The flooring process began by closing up any gaps in the sub-floor system. Many locations where the CLT was notched around steel columns and beams to allow for installation had to be blocked with expandable foam, sheet metal closure strips and wood blocking. The bottom of the walls received a layer of ⅜” isolation foam to acoustically separate the floor system from the walls. For extra moisture protection, a layer of plastic sheathing was added to the bare CLT, then wrapped up the columns and walls and then seams were taped. The entire floor was closed to all trades except the electricians who needed access to run power through the built up floor system to exterior walls and ceiling boxes. With the electrical work completed and inspected, the first layer of insulation board was installed on the entire floor - omitting areas where electrical raceways were located.

Next, rigid DensDeck board was installed over the insulation board. Then another layer of insulation board was installed, taking care to stagger the joints to avoid gaps through the “sandwich” of insulation.The process took a week per floor to prepare for installation of the gypcrete.

The 1.5” gypcrete topping slab install took three days. The first day consisted of installing a slip sheet on top of the insulation board, and then wire mesh. The second day was pumping and placement of the gypcrete. For efficiency in placement, the gypcrete was placed on two floors at a time. The third day allowed the gypcrete topping slab to cure and set enough to be walked on the following day.

The entire process meant that for almost two weeks two floors were “offline” to all subcontractors and crews except for those specifically working on the flooring system. Because of this, the MEPF rough-in was completed in the space prior to the flooring system and as soon as the gypcrete floors were poured, installation of the drywall could take place. The last layer of the floor system was an acoustic mat that was placed under the hardwood floor through a combination of floating and glue down. The mat provided the benefit of separating the wood from the gypcrete, allowing it to expand and contract and not transferring footfalls directly to the gypcrete. The robustness of the entire floor assembly plus the fact that there are no shared walls between units provides a similar acoustic experience as living in a detached single family home.

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Rain, Rain, Rain

Starting in mid-February, it rained almost constantly for the next six weeks, preventing the roof system from being installed.

Tent — The building was capped with a large tent in late March and into April to keep the rain off the CLT roof deck. Fans circulated warmed air to bring the moisture in the CLT down to acceptable levels so the roof could be installed.

Roofing — Roofers used a breathable SBS sheet directly on top of the CLT that tied into permanent roof vents. A TPO roof was installed on top of the tapered insulation to provide a weather tight and energy efficient system.

Careful Coordination

Like CLT itself, the Mechanical, Electrical, Plumbing and Fire systems (MEPF) requires careful up-front coordination. Carbon12 was built using a design-build approach with the subcontractors. During the longer-than-usual permitting process, time was dedicated to MEPF pre-design to ensure that the shaft pre-cut in the CLT manufacturing facility would accommodate all of the building systems. Hours of meetings were held on each shaft, soffit, and ceiling in an effort to reduce the size of ducts and pipes, ensure structural integrity, and to create a sequencing plan to allow subcontractors to install their systems without impeding others' work.

The meetings and subsequent installation were challenging as each contractor negotiated for the space and location that would be most advantageous for their tasks. The dedicated MEPF teams created workarounds and custom solutions to adapt to the spaces provided. At times this meant less efficiency, either with materials or install time. It helped that the floors were identical so improvements were made as they progressed. In the end, the subcontractors were working together like a well-oiled machine.

Leaving as much exposed wood as possible proved to be a true challenge. To do it, we had to coordinate all the trades into a few small soffits and drop ceilings. Each unit is 1500 square feet but all four trades were vying for space in one 200 square foot soffit.

Taylor Cabot, Project Manager, Kaiser+Path

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