Detail: Mass timber structure of the Bjergsted Financial Park

Upon entering the competition to design the new office building, Stavanger-based Helen & Hard, in collaboration with architects SAAHA, Swiss engineering team Création Holz and Norwegian engineering office Degree of Freedom, sought to create an innovative timber construction with a reduced carbon footprint and distinctive interior spaces. The ambitious project, completed in 2019, required extensive specialty engineering to achieve the structural spans and timber expression the team had in mind.

The result is an outstanding workplace made seemingly all from natural wood elements. The interior environment is healthy and inspiring, not only for its finishes but also for the ample sunlight that floods its spaces. For all this and more, the building obtained a BREEAM Outstanding certification for its environmental, social and economic sustainability.

Mass timber design is not new to Helen & Hard, who has used it as the basis for projects ranging from cultural centers to public libraries and student housing across Norway. “As an office, we try to use timber as much as possible, because we think it’s the future,” says project architect Njål Undheim. From the start of the Financial Park competition, the architects engaged the innovative engineering firm Création Holz, with whom they have a long working history. Ideas related to timber construction were thus applied in the earliest concept phases of the project.

The realized building consists of an all-glass envelope that clearly reveals its warm inner wooden structure and finishes. The building takes advantage of the flexibility of timber to produce a triangulated form and massing that responds to its urban context. Its roof slants towards the small neighboring wooden houses to the west, where a small park signals the main entrance. The form gradually increases in height towards the east, where it grows to 7 stories and takes on a more urban character as it points to an historic concert hall and adjacent park.

Timber: its benefits and costs

Mass timber is associated with several well-known benefits to a building project. Among them are trees’ ability to store carbon from the atmosphere, therefore contributing to a project’s overall reduced embodied carbon when incorporating wood products. Other benefits include reduced chemical byproducts and reduced site-related waste, when compared with steel and concrete construction. Timber is also associated with lower on-site construction costs due to a quicker erection period, since all components are typically prefabricated and delivered ready for installation. Physical properties of timber such as its ability to absorb and release moisture can help to improve indoor air quality by moderating humidity levels; its porosity makes it acoustically beneficial for an open plan office space; and its lightness reduces overall structural loads for large buildings. And studies have also shown that interior environments with expressed wood yields have positive psychological effects on building users. 

But added costs for both the material and the complexities associated with its design and construction are often of legitimate concern to clients, as is long-term maintenance. Helen & Hard was fortunate to work with a client, SR Bank, that was intent on making a sustainable building that also benefited its surrounding community. There were, nonetheless, initial reservations about timber: “The client was a bank, so they were extremely preoccupied about cost and risk,” says Undheim. “We took them for a study trip to experience large wooden buildings, feel their atmospheres and compare that with the standard office buildings they are used to.”

The material choice was ultimately made once a competitive cost was guaranteed. An economic analysis was made early in design development to compare the overall project costs of the timber structure with that of a more conventional structure. The difference between steel and timber, for example, was 1.4%, according to the report(1). The bank weighed the environmental benefits with the difference in costs and chose timber, which it also intended to become a symbol for its company-wide sustainability values.  

As a natural material, timber is certainly a more cost-effective option for projects with access to large producers, and in countries where forestry practices can meet project needs. The United States, Canada and Scandinavia are among the major centers of timber production. And yet, despite Norway’s ample timber resources for a project like the Financial Park, regional supply was not adequate. At the time of the construction, according to Undheim, there was not a producer with the experience to fabricate the building’s non-traditional CLT elements; so these were instead imported from Germany.

Climate is another factor that may need to be considered extra carefully when planning for timber projects. The erection period of the wooden construction for the Financial Park was scheduled at one year. “We were lucky in the first part of the year with not much rain,” says Undheim. “But at the end, before the building was closed, there was a lot.” Certain damaged components of the timber required sanding and re-treatment, and as a result, some color variation is evident in the interior building finishes.

Combining multiple timber structural solutions

The Financial Park structure rests on three basement levels of reinforced concrete. Four service cores are also of concrete and provide lateral stability to the structure. The remaining above-grade structural system is in timber. Horizontal loads are transferred to the cores by means of cross laminated timber (CLT) floor slabs and an inclined CLT roof slab, all dimensioned at 200mm in thickness. Floors are supported by glue laminated (glulam) timber beams and columns. When needed for additional strength and stiffness, laminated veneer lumber (LVL) made of beech is also used.

Glulam columns are continuous from level three to the roof, with the tallest standing 24 meters in height. The standard column is dimensioned 380x500mm, with the LVL columns at the ground floor sized larger at 480x800. The structural grid is set at 5.4m to allow for current and future flexibility. All of the structural timber elements are designed with a fire rating of 90 minutes. Columns and beams feature a 70mm char layer to insulate section cores from extreme heat, meaning the framing elements are oversized to allow outer layers to burn in case of a fire, thereby protecting the structure and keeping it intact.

Much consideration was given to the fabrication of the timber elements, which were held to strict dimensional tolerances and designed to minimize erection time. Multi-storey sections were constructed in the factory and transferred to site to be simply lifted into place by crane.

Crafted timber connections

According to Undheim, the design intent for the building was to maximize the use of timber and minimize that of steel. Even the joints, which are typically supported with steel plates in timber construction, are instead timber-to-timber connections without metal fasteners. LVL inserts, for example, were innovatively used at floor beams where they bear on the columns, specifically to avoid the need for metal plates. All the connections are internal and therefore protected in case of fire.

A unique building characteristic can be found in the timber joints at each of the grid lines, which feature a buildup of several wood plies and a series of visible 80mm-diameter beech dowels. Here a pair of 380mm-wide glulam beams bear directly on either side of the notched glulam columns. The beams consist of an inner layer of LVL with a constant depth, and an outer glulam layer of variable depth. These are organically shaped in profile to include openings for the passing of mechanical and electrical equipment. The larger surface towards the slab edge also provides more area for the insertion of the four dowels. The dowels were dried to 8% moisture and then expanded to lock as they were exposed to moisture in the air. 

The principal circulation on all seven levels connects through the atrium by means of a series of sculptural stairs. The load bearing stringers are double-curved and cantilevered, projecting five meters into the atrium, with the longest stair measuring almost 20 meters in length. Their fabrication required precision as well as expertise. “This is the most difficult part of the building,” says Undheim. “The stringers could not be produced in Norway, so we had to go to Germany, where they could slice the wood up in 4mm plies, glue them in one direction, slice them up again and glue them in the other direction.” 

Each stair is composed of two parts divided in the middle. These were transported separately, then mounted and connected on site. Connections were designed to transfer forces directly between timber members, though self-tapping timber screws and steel threaded rods are used for local reinforcement. “The stairs had to be brought in from the roof,” says Undheim. “And we could not close the building until we lifted them in by air, so the building was delayed by a couple of months as we waited for the stairs to arrive,” first by boat, then by truck and finally by crane.

To produce curved steel rails and glass railings that accurately follow the as-built geometry of the stairs, a 3D scan of the atrium was produced. The steel for the railing was then fabricated based on the findings of the scan.

Electric wiring runs inside the base of the stringers to feed LED fixtures that accentuate each stair’s curvilinear form. The stringers connect back to the building base structure at a perimeter beam along the CLT slab, where a slatted hung ceiling of ash is also embedded with LED fixtures for visual continuity. 

It is in the atrium that the full range of the building’s wood species – mainly spruce, beech and ash – are on display, as are the various structural systems. The result is an intelligent composition of materials that speaks to the natural warmth of timber, its structural integrity and its versatility.