Building up with lightweight wood frames

by Katie Daniel | December 11, 2015 3:11 pm

[1]

By Michael Baldinelli, P. Eng., MESc.
Lightweight wood-framed (LWWF) construction has gained significant popularity over the past decade as a sustainable, cost-effective solution for low-rise multi-level buildings in Canada. More recently, the push has been to extend use of LWWF to mid-rise construction for increased density on urban or rural projects. In 2009, British Columbia amended its building code to allow five- and six-storey residential wood-framed buildings; in September 2014, legislation was passed so the next Ontario Building Code (OBC) amendment (effective January 1, 2015) would permit wood-framed buildings up to six storeys.

Up until now, very little research has been focused on the three-dimensional behaviour of these structures under lateral loads. Additionally, the current structural analysis and design of these LWWF buildings is performed on an element-by-element basis, neglecting the overall behaviour of the building’s structural system. The result of this lack of information led to the development of a three-dimensional structural analysis program for LWWF
created by Strik Baldinelli Moniz Civil and Structural Engineers. The program:

• calculates all wind and seismic loading on the building;
• takes into account both rigid and flexible
  floor diaphragms;
• calculates the overall building deflection and inter-storey drift at any point in the building;
• accounts for the stiffness of other building materials (e.g. concrete masonry units [CMUs], cast-in-place concrete, and steel cross-braced framing);
• calculates building natural frequency (NF), overcapacity ratio (OCR), and torsional sensitivity;
• takes into account the cumulative effect of hold-down anchor slippage and nail slippage in deflection calculations;
• meets Canadian Standards Association (CSA) O86-14, Canadian Wood Design Manual, and the 2012 OBC code requirements; and
• optimizes shear wall selection based on material and labour costs.

One-dimensional design approach to multi-level LWWF buildings
In the past, the structural design of smaller (i.e. four storeys or fewer) LWWF buildings demanded a less robust design approach. Internal spreadsheets in conjunction with the Canadian Wood Design Manual and the OBC were used to assist in the design.

A one-dimensional approach was taken with each shear wall isolated and reviewed, without incorporating the interaction of other shear walls in the design. The shear loads were distributed assuming a flexible diaphragm distribution. Once they were assigned to each wall, strength and deflection checks were completed. While the one-dimensional approach may have been satisfactory in the past for smaller LWWF buildings, it is no longer valid with the advent of taller, larger buildings.

The need for a three-dimensional, robust, accurate and all-encompassing design program is required for the design of six-storey wood buildings under wind and seismic events. When the developing firm evaluated the marketplace for such a program, limited options were available.

The approach: fixing the problem
In developing a plan to fix the problem, the first step was to model a LWWF building in existing finite element (FE) programs—the same program used to model concrete and steel structures. Researchers found the FE program could not properly model nail slippage and hold-down elongation. Additionally, the interaction between the floor diaphragm and wall elements was not always accurate. The interaction of nail slippage and hold down elongation account for 25 to 35 per cent of the total building deflection, and thus must be properly modeled.

The researcher’s next objective was to conduct a detailed literature review. The group poured over approximately 60 papers/articles to gather as much information on the topic as possible. A majority of the papers complied were from Europe and the United States where these types of buildings have been approved and constructed for more than 20 years. The group found little research had been conducted on the three-dimensional lateral design of six-storey LWWF buildings. While there were excellent articles on localized one-dimensional analysis of wood shear walls, there was no research on the design process, from beginning to end. Additionally, limited software existed to assist in developing a three-dimensional approach to these types of buildings.

[2]

Once the literature review was completed, highlights of each paper/article were compiled for possible use in the design of the program. A detailed flow chart (a simplified version is shown in Figure 1) was developed by the group. It was instrumental in designing the working parts from beginning to end, and it allowed several different components to be designed individually which were then checked, debugged, and incorporated into the main design software.

The program
How the program concludes structural design for lightweight wood-framed buildings can be broken down into five steps.

Step one: Shear wall layout
First, the user must define a shear wall layout. A wood shear wall is a wall composed of wood studs, wood sheathing, nails, and tension/compression devices. The layout has x, y, and z components to provide a three-dimensional design approach to the problem. The first layout of shear walls may change depending on the building’s performance (e.g. added or subtracted shear walls). This initial layout is used as a starting point based on the firm’s LWWF experience and is ‘tweaked’ once the program has completed its design (Figure 2).

Step two: Calculate building loads
The program calculates the seismic and wind loads on the building using OBC requirements. In regards to the seismic loads, one of the main benefits of wood-framed buildings is their reduced mass (i.e. dead load). Wood buildings, generally, have a dead load—three to four times less than other conventional building materials. The seismic shear loads are directly proportional to the mass of the building, thus inducing much smaller loads on the building. Additionally, the ductility factor (RdRo) for wood-framed buildings is more than two times larger when compared to concrete or steel. This increased ductility reduces the shear loads even further, making a strong case for LWWT mid-rise construction.

Step three: Distribute loads to shear walls
The Association of Professional Engineers and Geoscientists of British Columbia (APEGBC) put together a best practices guide to assist engineers in the design of these types of buildings.(More information can be found in the guide, Structural, Fire Protection and Building Envelope Professional Engineering Services for 5 and 6 Storey Wood Frame Residential Building Projects (Mid-Rise Buildings), published by APEGBC) Section 3.5.2 (j) explicitly states both flexible and rigid analysis be performed to determine the maximum loads on each shear wall. If the rigid diaphragm forces increase more than 15 per cent when compared to the flexible case, design for the envelope of forces is required.

The flexible diaphragm distribution is straightforward and only requires one iteration. The flexible diaphragm depends on the tributary area on each shear wall. Once the shear wall layout is provided, this tributary area never changes (unless shear walls are modified). The rigid diaphragm, on the other hand, becomes very onerous. In the rigid diaphragm case, the floor is assumed to move as one entity. Thus, the deflections of all shear walls should be the same, regardless of material properties, force, and length of each wall. The problem becomes challenging because the deflections of the shear walls are dependent on the load and stiffness in the wall. Each time a shear wall is modified (i.e. stiffness or length changes), this process must repeat itself until the deflections converge (Figure 3).

In addition to the load distribution analysis, the forces dissipated in a flexible versus rigid diaphragm load case are different (Figure 4). For a simple two-span floor with three shear walls, the forces for each case could be either over- or underestimated. For the exterior wall, the flexible analysis would underestimate the wall force while for the interior shear wall the flexible load case would govern. It became clear through research and analysis that both the rigid and flexible load distributions were mandatory in the design of six-storey LWWF buildings.

Step four: Evaluate for deflection and strength
Once the loads are properly distributed to each shear wall, the next step is to check the strength and deflection elements to determine whether the walls perform as intended.

Strength
Shear loads are cumulative through the shear wall. In simple terms, the shear load at the sixth floor is the ‘least,’ while the load at the bottom floor is the ‘largest.’ The shear properties of a wood panel shear wall vary depending on the following properties:

• sheathing thickness, material (i.e. oriented strandboard [OSB] or plywood), wood panels on one side of wall or on both sides of wall;
• nail spacing (i.e. 152-mm [6-in.] spacing versus 102-mm [4-in.] spacing, etc.); and
• nail size (i.e. length) or shank diameter.

The moment component is somewhat similar in nature. The moment is ‘least’ at the sixth floor and gathers at the bottom floor where it becomes the largest. The moment resistance of a wood panel shear wall is resisted by the tension/compression couple at the end of the shear walls, where the tension and compression loads are equal to the moment. The tension/compression load—T6/C6—would be much less than the tension/compression load of T1/C1, as noted in Figure 5.

Tension and compression loads are designed in various ways. For six-storey buildings, the design and research team found a continuous threaded rod tie-down system helps control building drift and deflection much better than other traditional tie-down systems used for smaller four-storey buildings. Additionally, the tension loads found in six-storey buildings are much higher than that of four-storey buildings, requiring the use of this tie-down system for tension. The compression value is resisted by wood posts on either side of the tension tie (Figure 6).

Deflection of a wood panel shear wall is made up of four components. These components are for additive and cumulative. Strik Baldinelli Moniz Civil and Structural Engineers found for most Ontario buildings, wind loads govern the deflection criteria while the seismic loads govern the strength design requirements. The deflection components include:

• bending of shear wall (Figure 7, below);
• shearing of shear wall, where Bv is the shear-through-thickness rigidity (Figure 8, below);
• slip of the nails in the sheathing, which is a function of load per nail and nail deformation (Figure 9, below); and
• slip/elongation of the hold-down anchorage, da, is provided by the manufacturer of the hold-down (Figure 10, below).

[3]

[4]

[5]

[6]

 

 

 

 

 

O86-14 and OBC 2012 code confirmation
In combination with the strength and deflection checks, the program confirms all CSA O86-14 and OBC 2012 code requirements and is adaptable to other building codes around the world. If one of these design checks fail, the program will reiterate until the checks are satisfied. In accordance with these design checks, the program calculates the overcapacity ratio (OCR). This ratio is to confirm the assumed response of the structure is as intended between the first and second floor. The OCR values must be between 0.9 and 1.2 (Figure 11). The program also calculates torsional sensitivity. If it is sensitive to torsion and located in a high seismic zone, the building must be either stiffened or a dynamic analysis needs to be completed.

Seismic inter-storey drift is one of the other considerations calculated by the program. If the seismic inter-storey drift exceeds one per cent of storey height, gypsum cannot be used to brace studs or resist lateral loads. It is also important to note gypsum cannot be used to resist seismic loads in buildings greater than four storeys.

[7]

The final calculation of the program is the natural frequency (NF) of the building. Wind loading in the OBC is based on an assumption the natural frequency (Fn) is greater than 1 Hz. Determination of the NF is based on the deflection of the building—if the NF is less than 1 Hz, the building will either need to be stiffened or designed for additional dynamic wind loading (Figure 12).

Step five: Shear wall optimization
To take the program to the next level, material costs were incorporated. In many situations, firms are asked to change sheathing types, hold-down types, nail spacing, and shear wall locations long after the building has been designed. The design is complex and changes are not easy. To optimize the cost in conjunction with the design, a list of 180 different shear walls was assembled based on material and labour costs (panelized walls, supply only).

The shear wall types vary based on:

• stud size and spacing (e.g. 38 x 89 at 400 c/c 2-38 x 89 at 305 c/c, 38 x 152 at 305 c/c etc.);
• species type (SPF [spruce-pine-fur] or LSL [laminated strand lumber]);
• sheathing type and thickness; and
• nail size and spacing.

Based on these 180 wall configurations, a price per linear foot for each wall type was assembled, and the walls are ranked from least to most expensive. While the program selects walls to meet the strength and deflection requirements, it selects the least expensive wall first. If that wall fails, the next least expensive wall is chosen. This process continues until the program completes its design. Manual ‘tweaking’ is performed by the user to optimize the shear wall layout even further. Shear walls not participating efficiently are removed and the program is rerun. This process continues until the most cost-effective structure is designed, meeting all code requirements.

Cost analysis and case studies
To better satisfy the needs of clients, a detailed cost analysis was conducted to determine the costs of LWWF building versus traditional building products. The cost study was based on an existing four-storey LWWF that was just completed. Two levels were added to the structure and then re-designed. The designs were priced by a local wood panelization company. The following results showed the:

• cost of a four-storey, panelized wall/floor supply was $10.50/sf; and
• cost of a six-storey panelized wall/floor supply was $10.95/sf.

[8]

In an attempt to be impartial to other building materials, those values do not include installation costs, and other miscellaneous materials such as additional drywall to meet fire rating requirements, and concrete topping on floor joists to improve sound and fire ratings. When taking these items into consideration, an additional $8/sf should be added to both above costs for a total of $18 to $20/sf. Comparatively, Strik Baldinelli Moniz Civil and Structural Engineers recently completed design on two six-storey buildings composed of steel and concrete products, the framing costs averaged $28.85 /sf for the two buildings.

While it is clear there are savings in using LWWF construction, substantial savings can also be found in foundation design. To get a better handle on costs, the six-storey wood building was modified into a concrete building (same size and layout). A design was completed to get a better idea of foundation costs. The four- and six-storey wood building foundations were compared to the six-storey concrete building. When the ‘volume’ of concrete used for all three buildings was compared the:

• six-storey wood building had 10 per cent more concrete in its foundations compared to the four-storey wood building; and
• six-storey concrete building had 70 per cent more concrete in its foundations compared to the six-storey wood building.

During all comparisons, the soil conditions were considered to be ‘fair.’

The true costs of the foundation between the six-storey wood and six-storey concrete buildings would not be 70 per cent. The volumes do not take into account forming costs and labour, which would normalize the gap between the two costs. A significant savings in the foundation design can be achieved when comparing the two materials.

Similar studies have been performed on structures in British Columbia.(For more information about the British Columbian structures, see the Canadian Wood Council’s (CWC’s) Mid-rise Construction in British Columbia, WoodWorks’ A Case Study on the Remy Project in Richmond BC, WoodWorks’ Wood Solutions in Mid-rise Construction, and Altus Group’s Wood Brings the Savings Home: Construction Cost Guide 2014. ) In all studies, a savings of 10 to 12 per cent on the overall building construction costs were realized when compared to traditional building materials.

Next steps
The program has been used on six LWWF projects and has great success in controlling the building costs while maintaining all relevant code compliances. The ability to accurately calculate building deflections, inter-storey drift, overcapacity ratio, natural frequency, and strength parameters provides confidence that designs meet all CSA O86-14 and OBC requirements. The developers of the program have future plans to:

• create a Windows-based user interface for easier input and output capabilities;
• incorporate material take-offs for preliminary pricing purposes, prior to tender;
• use the program on an eight-storey LWWF building to determine sensitivity to building deflection and shear load interaction at floor diaphragms;
• prepare a paper for technical submission into a variety of engineering journals; and
• develop standalone finite element method (FEM) design software for LWWF buildings suited for all major building codes across North America.

SBM, in conjunction with the National Research Council (NRC) and Western University (London, Ont.) are commencing research into this design method. Phase I results are expected by the spring of 2016.

[9]Michael Baldinelli is a principal with Strik Baldinelli Moniz, Civil and Structural Engineers. He has been involved in the design of more than 35 commercial (LWWF) lightweight wood-framed buildings in Ontario. Baldinelli recently lectured on behalf of the Ontario WoodWorks group on the structural design and optimization of multi-level wood buildings. He can be reached at mike@sbmltd.ca[10].

Source URL: https://www.constructioncanada.net/building-up-with-lightweight-wood-frames/

---