Mass Timber Construction in New Zealand: What Are the Real Structural Benefits?

Sacred Heart College Community Hub — a two-storey education and community facility constructed using an LVL mass-timber structural system.

Mass timber construction in New Zealand is receiving increasing attention for its potential structural, architectural and sustainability benefits.

Engineered timber products such as cross-laminated timber (CLT), laminated veneer lumber (LVL) and glulam have created new opportunities for structural engineers, architects and contractors. These systems can produce buildings that are relatively lightweight, highly prefabricated and architecturally distinctive.

But does that automatically mean timber is better than concrete or steel?

Not necessarily.

The more useful question is:

What does mass timber allow us to do differently, and when does that make sense for a project?

Mass Timber Construction in New Zealand: Prefabrication Benefits

The LVL structural frame during construction, before the completed building concealed some of the structural relationships.

One of the most significant structural advantages of timber is its comparatively low self-weight.

That matters in New Zealand.

A lighter building generally means lower seismic mass. Because earthquake forces are directly related to mass, reducing building weight can reduce the inertial forces generated during a seismic event. Final seismic performance still depends on factors such as stiffness, ductility, structural configuration and the lateral load-resisting system, but lower mass provides an attractive starting point from an engineering perspective.

There can also be benefits below ground.

Reducing the weight of the superstructure can reduce loads transmitted into footings, piles and foundations. On sites with poor ground conditions, the foundation savings can become an important part of the overall project equation.

This is why comparing a timber floor directly against a concrete floor on a $/m² basis can be misleading.

The real question is:

What does the structural system do for the building as a whole?

Much of the Building Can Arrive Ready to Assemble

Engineered-timber components installed as part of the primary structural frame.

Another attractive feature of engineered timber systems is prefabrication.

Structural elements can be digitally coordinated, precision-manufactured and prepared off-site before arriving on site ready for installation. Connections, openings and service penetrations can often be considered and incorporated early in the design process.

This can improve construction efficiency and reduce the amount of wet construction required on site.

However, there is a trade-off.

Decisions often need to be made earlier.

Relocating a penetration through a floor or wall is relatively straightforward during design coordination. It becomes significantly more difficult once a prefabricated structural element has been manufactured.

For that reason, successful mass timber projects typically require close coordination between:

• the architect

• the structural engineer

• building services designers

• the fabricator

• the contractor

• the wider project team.

Coordination of structure, services and internal finishes is particularly important where timber elements remain exposed.

When the Structure Is Also the Architecture

Exposed LVL framing forms part of the finished interior and contributes directly to the character of the learning and community spaces.

One of the most appealing characteristics of mass timber construction is that the structure can contribute directly to the architectural outcome.

A timber beam does not necessarily need to disappear above a ceiling.

A column does not necessarily need to be hidden within a wall.

The structural frame can become part of the completed building.

A good local example is the Mission House redevelopment at Sacred Heart College in Lower Hutt.

The former Mission House was replaced with a new two-storey laminated timber building providing administration and support spaces at ground level, with library, learning and staff facilities above. Publicly available project information highlights the extensive use of engineered timber and the way the exposed structure contributes to both the architectural character and overall user experience of the building.

This illustrates one of the unique opportunities offered by timber construction:

The structure can be both functional and architectural.

What About Sustainability?

The completed interior retains the timber structure as a visible part of the building.

Sustainability is often one of the first topics raised when discussing mass timber.

Timber is a renewable material and can store biogenic carbon during the life of the building. These characteristics have contributed to growing interest in timber construction internationally.

However, the comparison is rarely as simple as saying:

"Timber is sustainable."

Timber buildings still contain:

• steel connections

• fixings and brackets

• concrete foundations

• transport and manufacturing impacts

• fire protection measures

• durability considerations.

If embodied carbon is a significant project objective, comparing complete structural systems through a life-cycle assessment is generally more meaningful than comparing materials in isolation.

There Are Challenges Too

Timber connection detailing is an important part of the structural concept and requires early coordination with architecture, services and construction requirements.

Mass timber is not a magic structural material.

Like any structural system, it requires careful consideration.

Some of the key issues that need to be addressed early include:

• fire performance

• moisture management and durability

• acoustic separation

• floor vibration

• connection design

• service penetrations

• construction tolerances

• coordination between architecture and structure.

Connections are a particularly important example.

A relatively simple timber framing concept can become surprisingly complex if the connection philosophy has not been considered early in the design process.

When Is Concrete Better?

There are many situations where concrete remains the most appropriate structural solution.

Concrete provides mass and stiffness, which can be advantageous where vibration performance or acoustics are significant design drivers.

It is also commonly used for:

• foundations

• retaining walls

• basements

• below-ground structures

• heavily loaded elements.

Concrete may also be preferred where durability, fire resistance or structural stiffness dominate the design requirements.

These are not shortcomings of timber.

They are examples of selecting a material that best suits the engineering problem being solved.

When Is Steel Better?

Steel has its own strengths.

If the objective is to:

• achieve very long clear spans

• minimise structural depth

• transfer large concentrated loads

• create particularly slender structures

then steel may be the most efficient solution.

And sometimes the best solution is not choosing between materials at all.

The Answer Might Be All Three

Efficient structural solutions often combine materials, using each where its properties best suit the building.

Many successful projects use a combination of materials.

For example:

• Timber for floors, beams, columns and exposed structure

• Steel for long-span elements, transfer structures and connections

• Concrete for foundations, slabs, retaining structures and cores

That is not a compromise.

It can be a highly efficient structural strategy.

Each material is used where its properties provide the greatest benefit.

So, Is Mass Timber Better?

The final measure of a structural system is how successfully it supports the building’s intended use.

We do not think that is the right question.

The better question is:

What structural system makes the most sense for this building?

Mass timber can reduce structural weight, support prefabrication, contribute directly to architecture and potentially lower embodied carbon.

Concrete and steel also offer advantages that timber cannot always replicate.

Sometimes timber will be the obvious choice.

Sometimes concrete or steel will make considerably more sense.

Increasingly, the best solution may be a carefully considered combination of all three.

Good structural engineering is not about having a favourite material. It is about using the right material in the right place.