Designing for timber-framed buildings

by Elaina Adams | September 1, 2011 1:34 pm

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By Matthew Reid, MASc., P.Eng., and Cory Zurell, PhD, P.Eng.
For more than a decade, urban renewal has seen the renovation of former manufacturing facilities into trendy loft-style offices, condominiums, and apartments. Most of these buildings belong to the ‘brick and timber beam’ vernacular constructed in the first half of the 20^th century. Many constraints govern the design of any building, but a renovation involves the most significant—the building already exists.

These brick-and-beam buildings vary widely in terms of construction quality, materials, past performance, and ongoing durability. With older buildings, one must be cautious of developers who have contracted ‘strong old building’ syndrome—just because the building is old does not justify the assumption it was well-built and will last forever. As a result, educating owners and architects sometimes becomes part of the structural scope of work.

In repurposing former manufacturing facilities, the renovation usually involves a change in occupancy. In Ontario, such work falls under the Ontario Building Code’s (OBC’s) Part 10, Change of Use, and Part 11, Renovations.

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Adapting an historical structure for today’s needs, considering current building standards and architectural intent, involves several significant issues. Three particular ones—fire, sound, and structural capacity—are more sensitive in the reuse of buildings when the existing structure is timber, or when timber is the desired solution for an addition. Inter-related, these all directly impact the architectural design and performance of the renovation or addition.

Fire safety
The Ontario Building Code requires evaluation of the building’s fire safety when a renovation occurs. An experienced code consultant is valuable for establishing the fire protection and occupancy safety requirements, even when a change of major occupancy does not occur. The code consultant writes a report outlining how the architect should apply OBC Part 3, Use and Occupancy, which relates to the fire protection and occupant safety of any extension made to the existing building, alongside the requirements of OBC Part 11, which outlines the requirements for the existing portion of the building. Part 11 uses the term “performance level” for structural evaluation, early warning/evacuation system requirements, and fire protection to determine whether an upgrade is required.

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Employing a code consultant can produce significant returns in some cases. On a project worked on by one of the authors, the code consultant was able to navigate through the code requirements such that the change from industrial to residential occupancy produced a reduced hazard index (HI) and, thus, the work was regarded as a “minor renovation.” This result allowed a non-combustible partial fifth storey to be added to the four-storey timber-framed building. Without the “minor renovation” result of the code report, this addition would not have been possible.

When there is a change of “major occupancy,” as defined by OBC Part 10, the building is required to be classified as to its construction type, which is called its construction index (CI), and as to its occupancy type, which is called its hazard index. The CI is compared to the HI to determine if an upgrade to the building is necessary; if the HI is higher, an upgrade is required.

The building’s construction index is given a number between one and eight, where one is the lowest fire protection performance level. The construction index has two parameters: fire-resistance rating (FRR) and type of construction. The type of construction is a choice between combustible and non-combustible.

The hazard index, like the construction index, is measured on a scale of one to eight, and is designated as the life safety hazard to occupants. The hazard index is defined by the occupancy group and building size. For adaptive reuse, one commonly sees an occupancy Group F (low-hazard industrial, manufacturing) change to either Group A (assembly areas such as schools) or Group C (residential areas).

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As a combustible material, the current code limits timber with respect to its construction index. The highest level timber can receive is a CI of five. It does not matter if the building controls flame spread, as defined in Part 3 and not considered under Part 11. The common case is an historic manufacturing building of heavy timber with a CI of five, which requires an HI of six. Part 11 has compliance alternatives, so an alteration to the building can still occur. A common solution is to provide a sprinkler system.

The construction index also does not consider the potential of a non-combustible assembly that includes combustible material. For example, a floor assembly of concrete topping on plywood and wood joists with two layers of fire-resistant gypsum board ceiling is considered combustible. However, an alternative solution could be submitted to demonstrate the floor assembly meets the objectives of the fire protection clauses.

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The 2005 National Building Code of Canada (NBC) has taken its first steps toward a truly objective framework by introducing objectives and function statements, thereby allowing alternative solution submissions. If one wants to preserve and expose historical elements during an adaptive reuse conversion, then designers must be more comfortable knowing how to formulate an alternative solution submission. As time progresses and the construction industry becomes used to objectives and function statements, alternative solution submissions will be completed quite easily, and become just one more step in the design process.

Sound transmission
The material benefits of timber include a relatively high strength-to-weight ratio. This acts against performance in terms of sound transmission; the lack of mass hurts the structure’s ability to attenuate both ambient and impact sounds. In historic manufacturing facilities, sound transmission would rarely have rated consideration. In repurposing such a facility, upgrading sound transmission ratings between floors is frequently necessary when residential occupancy is considered, and usually affects structure.

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The NBC stipulates a sound transmission class (STC) rating of 50 between units for multi-family residential buildings (increasing to 55 at elevators). No impact insulation class (IIC) is specified, but good practice is a rating of 55 to 65. (See “Wood Frame Construction, Fire Resistance, and Sound Transmission,” by Forintek Canada Corp, Societe d’habitation du Québec and Canada Mortgage and Housing Corporation, 2002). For other occupancies, there is no guidance provided in the code, but best practices are established. There are several possible methods to increase the STC and IIC ratings of an existing timber structure, including:

• adding a concrete topping for additional mass, affecting structural capacity, including seismic performance;
• installing a suspended gypsum board ceiling (again, for additional mass) to cover timber that may otherwise be exposed and esthetically appealing; and
• providing a floating floor such as hardwood on a resilient underlay (a minimal effect structurally, though less effective in terms of attenuation than the other options).

The sound performance ratings of various assemblies are provided in NBC. For example, ratings of select light-frame wood assemblies are given in Figure 1 for comparison.

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Depending on the condition and capacity of an existing structure, a substantial increase in dead load may not be of tremendous concern. If the original load capacity was high, there will not be a problem. However, this is precisely the issue—old buildings do not necessarily have the capacity one would expect, regardless of the previous occupancy.

Structural capacity
Under Part 11, OBC requires the completed building to maintain its level of structural performance. An existing performance level is considered to be reduced if the existing structural systems cannot adequately support the proposed loading that is caused by the renovation, and when:

• there is a major occupancy change;
• the occupant load has been increased by more than 15 per cent; or
• the live load has been increased due to change in use within the same major occupancy group.

Assessing the structural capacity of existing buildings is simple when the original structural design drawings exist. When it comes to pre-1950 buildings, this rarely happens with any type of construction, much less with the brick-and-beam type of buildings of the early 1900s. It is likely a structural engineer did not design the building in the first place and structural drawings may never have existed. With the original design intent and criteria lost to the ages, and complicated by varying quality of regulation at the time of construction and inconsistent structures, assessing the capacity and adequacy of the structure falls to the design team to complete the renovation.

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While in some respects it is reasonable to assume a building has stood the ‘test of time,’ one cannot assume all inadequacies will have revealed themselves over the years. While the building may have performed satisfactorily in its previous life, even if the change in occupancy brings a theoretical loading reduction, there is no guarantee the building will be suitable for its repurposed life.

Repurposing can involve various changes, but with respect to the timber structure, one can generally break down the work into two broad categories: minor changes and repairs, and major renovation and remediation. Minor changes may involve alterations for new mechanical equipment and services as well as a change in occupancy, but not significant changes in the loading of the building. In general, this would require the engineer to get a feel for the structure’s capacity. Timber varies in grade and species, but some basic reasonable assumptions, based on the building’s approximate age, can expedite the process and preclude a full and costly grading assessment of the timbers.

For light framing (usually full-dimension, rough sawn lumber), one can typically assume Spruce-Pine-fir #2/Northern Select Structural when assessing minor changes. (These two species gradings are conveniently close in strength and past gradings have rarely been found to deviate.) (The noted assumptions for species and grade are particular for Southern Ontario, but similar trends can no doubt be determined for other regions based on common historic building practice).

[9]For heavy timber construction, historic buildings are generally found to fall into two possible scenarios as far as grading is concerned. For structures in the range of less than 80 to 100 years old, one generally finds timbers to be Douglas fir, grading to No. 1 or better. Older timber structures (and particularly those built before the completion of the Canadian Pacific Railway [CPR]) tend to be of Northern species—Select Structural being common. However, these assumptions are just a starting point, and though such findings are very common, one can just as easily encounter timber framing that includes, for instance, maple and elm in the mix.

Major renovations, residential conversions in particular, frequently include the addition of a concrete topping to deal with sound and other performance issues. In this case, if assumptions do not provide an obvious and favourable result—such as a load capacity well in excess of what is required by current codes—a full grading assessment of the timbers, complete with sampling for determination of species, may well be required.

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In-situ grading of timbers is a specialized skill. Licensed timber-graders employed at a sawmill will not grade timber beams and columns in an existing building. Since all four sides of beams are rarely exposed to view, a complete grading cannot be performed. This is where engineering experience and judgement combine with grading skills to assess the in-situ condition. (The Ontario Forest Industries Association (OFIA) offers timber-grading courses).

In addition to generally accepted engineering principles and conventions, building codes provide additional guidance for assessing existing structures. NBC includes Commentary L, “Application of NBC Part 4 of Division B for the Structural Evaluation and Upgrading of Existing Buildings.” This commentary principally addresses criteria for the ultimate limit state (ULS) affecting life safety. Commentary L includes a method of evaluation based on past performance (for all loadings except for seismic) and provisions for reduced load factors based on risk category and a calculated reliability level. Reduced load factors are used as a measure of the performance of the existing structure against new loading requirements. If upgrades are necessary, the intention is one would then revert to the full requirements of Part 4, Structural Design, for any upgrade. When an upgrade is required, there is no significant potential for savings to use reduced factors as most of the cost will be labour, not materials.

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An interesting creative aspect to structural engineering is upgrading a building’s structural capacity. There are three basic options: simply replace the member, reinforce the member, or unload the member. Replacing the member seems simple on the drawings, but the design team needs to consider the feasibility of construction. Requirements for shoring and the accessibility of cranes or lifts need to be carefully considered.

Likewise, reinforcing a member can be quite simple or complicated, depending on the context. Considerations include how the load is transferred into the new reinforcing, how the load then gets transferred back out of the reinforcing and into the building structure, and potentially jacking of the existing structure or pre-loading of the reinforcing to balance the load distribution.

Unloading a member is certainly easier said than done. Shortening the span of a beam (adding a column for instance) will usually have significant implications—particularly from an architectural standpoint.

The bottom line
The costs on any building project are always closely scrutinized. Of course, owners and developers like fixed fees, especially when it comes to paying for consultants. For most consultants—architectural, mechanical, electrical, civil, landscape—involved with repurposing an historic building, a fixed fee is relatively easy to determine. For instance, renovations usually involve completely new mechanical and electrical systems, so from that perspective it is comparable to new construction.

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A fixed fee for structural engineering services related to the assessment and renovation of an historic building is tremendously difficult to determine because a complete scope is nearly impossible to determine before performing the work—old buildings tend to be full of surprises. Who could predict a four-storey, four-wythe brick bearing wall would just be sitting on a slab-on-grade and have no foundation wall or footing?

In the authors’ experience, fees from past projects have ranged from as low as a couple of thousand dollars up to many tens of thousands—and the fees do not necessarily directly relate to the building size. From a structural engineer’s perspective, the best practice is to proceed on an hourly basis until the faults of the building are known, and then attempt to carefully define the scope and corresponding fees.

Adaptive reuse of historical buildings is not the cheapest form of construction and better returns on investment (ROIs) can likely be found elsewhere. However, cheap construction costs are not the driving force behind adaptive reuse. From heritage conservation of prime locations to recycling buildings and creating unique and uniquely marketable spaces, such endeavours can be profitable. Historic timber buildings are worth preserving. They just sometimes require a little work, particularly when sound and fire and the effects on structure are concerned.

Matthew Reid, MASc., P.Eng., is an associate with Blackwell Bowick Partnership Limited Structural Engineers in the Toronto head office. He has worked at Blackwell Bowick for five years, completing mainly renovations and additions to institutional, commercial, and residential projects. Reid sits on the Canadian Standards Association (CSA) Technical Committee A307 on Solid and Engineered Wood Products, and completed his Master’s thesis in 2004 on Bolted Connections in Glued Laminated Timber. He can be contacted via e-mail at mreid@blackwellbowick.com.

Cory Zurell, PhD, P.Eng., is a senior associate with Blackwell Bowick Partnership Limited Structural Engineers and directs the firm’s Waterloo, Ont., office. He is also an adjunct assistant professor at the University of Waterloo School of Architecture and sits on the APA–Engineered Wood Association’s Standards Committee on Cross-laminated Timber Panels. Zurell has been involved in structural consulting and education for 14 years and has a particular affinity for timber structures. He can be reached at czurell@blackwellbowick.com.

Source URL: https://www.constructioncanada.net/designing-for-timber-framed-buildings/

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