What Wood you Choose?

 

By: Adam Carroll

The wise man built his house upon the rock… But what did the wise man use to build the house? As described in the blog Choosing Your Structural Soulmate by Emily Dunham and Win Bishop, there are four main building systems. These four types include concrete, steel, masonry and lightframe (wood and lightgage steel). Of these, wood is the only building material that can be taken from its naturally occurring state and used directly in the form of a shelter. For example, given the appropriate saws, tools, and plenty of nearby trees, a log cabin can be constructed. All other building materials require extreme heat, pressure, and energy just to bring them to the state needed for use in a building system. Once in their final state, strenuous amounts of energy are required to erect these materials due to their abundant self-weight. Because of this, wood has always maintained its place in construction. As a result, the average owner looking to build their “house upon the rock” typically chooses wood.

However, what if an owner were looking to build housing for hundreds of people, all under one roof and on a limited parcel of land? The decision clearly becomes more complicated. Could timber still be used? Timber framing is relatively low-cost, sustainable with a small carbon-footprint, and can offer structural performance and design versatility. Wood is a light-weight building material, not only making it less labor intensive, but increasing construction speed. However, just because wood is considered a light-weight, doesn’t mean it can be taken lightly.

Wood framed structures have evolved from the simple log cabin to mid-rise structures. In general, wood structures are currently limited to a height of 85 feet, or six stories throughout the United States. These mid-rise wood framed developments are often a mixture of residential apartments, condominiums, dormitories, or assisted living spaces with commercial retail, restaurants, or office spaces on the street level. In countries such as Europe and Canada, wood framed systems have been advanced for use in high-rise structures, with the tallest standing at 174 feet. High-rise wood structures require mass timber products, such as cross-laminated timber, laminated veneer lumber, and glu-laminated members. Before competing for the title of the world’s tallest wood building, let’s focus on the timber structures that are more commonly in demand.

Over the last decade, mid-rise wood structures are becoming increasingly popular by developers because of their low construction costs and high density of leasable space. But how do these compare to other building systems? In short, building codes require that structures provide a measurable level of safety, regardless of the materials used. Therefore, if you were thinking there can’t be that much to wood framing, keep in mind that a five-story wood building must be capable of performing to the same standard of a five-story concrete or steel building. This includes resistance to gravity, wind, and seismic loads. So how? Make everything a load-bearing shear wall and call it good? Alternatively, wood structures can be engineered to provide optimal use of materials. An optimized system requires appropriate use of many components, including stud walls, beams, columns, sheathing, and many components that are not wood. Efficient wood buildings make use of numerous types of steel hardware and fasteners. Hardware typically consists of truss/joist hangers, truss press plates, and straps that resist uplift or tension forces. Although most components of a wood building are framed on-site, the most efficient systems usually result in shop-fabricated open-web wood trusses. Trusses support the floor and roof and, depending on the size of the structure, there could be thousands of trusses. Thus, it’s critical these members are constructed in a quality-controlled environment. While there are numerous ways to design a cost-efficient, safe, and durable wood-framed system, timber does have its limitations.

Wood is simply not as robust as concrete, steel, or masonry. And it doesn’t take a caveman to tell you what wood is commonly used for outside of building material….FIRE! Wood is fuel to fire or in more technical terms, a combustible material. This is often of the highest concern and likely the greatest challenge in building safe, yet economical timber structures. Building codes have stringent requirements in place to ensure occupants have time to safely exit a building in the event of a fire. Wood buildings often have a combination of fire-resistant assemblies and fire-protection systems. This may include multiple layers of gypsum sheetrock surrounding wood, fire retardant treated (FRT) lumber, sprinkler systems, fire and smoke detector systems, and firewalls that create fire separations between segments of a structure. For a structural engineer, this requires working with an architect to strategically locate and detail members to achieve the fire ratings required by code.

Another area of concern in wood structures is shrinkage. For a mid-rise building, the top of wood framing may shrink (or decrease in elevation) between one and two inches. If overlooked, this can create major issues where wood interfaces with materials that do not shrink, such as masonry, gypsum sheetrock, and plumbing lines. Detailing the connections at these materials, or sometimes lack of connection, is essential in ensuring serviceability throughout the life of the structure.

 Wood is also subject to rot and decay. If left untreated and exposed to elements, wood will eventually deteriorate. Generally, the best option is to completely encase wood framing to avoid continued exposure to the elements. However, when wood is exposed to elements, it must be pressure treated lumber. This is a chemical process involving liquid preservatives that are absorbed into the wood fibers and help protect wood when continuously exposed to ground and moisture. However, despite these challenges that wood framed systems present, structural engineers strive towards finding solutions.

 In summary, over the last decade, wood frame systems have trended towards taller buildings. Only time will tell if the U.S. takes on high-rise wood construction. Regardless, wood will continue to be a favorable building system due to its low cost and rapid constructability. Ultimately, for a wood framed system to be chosen for a building of any size, the owner must trust that it’s the wise decision. As structural engineers, exceeding the expectations of the client and people occupying these buildings is the key to building that trust and advancing wood framed systems for future structures.

 

Engineering is like a box of chocolates …You never know what you’re going to get.

By: Marcy Williford

Unless you’ve been living under a rock (or in a boat), you’ve probably noticed the tremendous amount of rain we’ve had over the last 6 weeks here in Birmingham, AL. While yes, water is arguably the most important nutrient for any living thing, it is also a pretty important part of a civil engineer’s profession. As Matt Coe mentioned in his blog post “What is a Civil Engineer?,” one very large part of a civil engineer’s job is to design a site not only to function, appeal and provide to a client’s wishes, but also to adapt to the local and even national ordinances, codes and standards. These of which include stormwater quantity and quality management.

Okay, so dig a hole and send the water to it, right? Yep, exactly. But how do you design the water to drain in a way to prevent flooding, reduce stormwater pollution and provide water as a source rather than a waste? Or how about designing it as a functioning system, while ensuring your next-door neighbor isn’t flooding? Well, that’s where a civil engineer comes into play. Our job is to take a site as it comes, accept it for what it is, and design, re-design, and design again to make it the best product it can be. One very important part of that process is considering every drop of water that falls onto our site (or even runs onto our site) and managing this water in a variety of ways to reduce and improve the quality of the runoff. I could talk for days about what all goes into this, but I’ll spare you.

So what do I mean by quantity and quality management? Water quantity involves design which mimics the pre-construction hydrology as closely as possible. This is typically with some type of detention, whether it is above or below ground. Why? To protect your neighbors from seeing more water after your development than they were before. Water quality, on the other hand, comes in many forms and fashions, such as bioswales, green roofs, pervious pavement…the list goes on and on. And the benefits? Healthier natural waters, improved air quality, reduced pollutants. Basically, all-around improved quality of life.

Back to my point: engineering is like a box of chocolates. No two sites are ever the same. The stormwater design of a ½ acre coffee shop in the suburbs and a 100-acre parking lot surrounded by wetlands look pretty different. Geographical location, desired use versus previous use, topography, soil characteristics and size of the site are just a few main players in your design. Even more, there are a variety of ways to interpret the way you manage this water. For example, your coffee shop may need some form or fashion of underground detention whereas your 100-acre parking lot may require 5 detention ponds.

I guess what I’m trying to say is designing stormwater in a way which handles, conforms, and produces the most efficient use of a site means curating a specific design for the site at hand. So many factors present their own challenges and make the site unique. The civil engineer has the humbling job of combining the needs of the client, requirements of the organizations, and existing conditions of the site to ultimately deliver a product that works for everyone involved and continues to keep the public safe. And this is what makes our job so rewarding. To solve problems and provide someone with a place of work, residence, or even just a place to do their grocery shopping. Working as a team with a variety of people with different talents in order to serve a client in the best way possible.

Choosing Your Structural Soulmate

By: Emily Dunham, PE and Win Bishop, PE

At the start of every project, when the architect’s floor plans are only beginning to take shape, structural engineers are making game changing decisions about the building. They are deciding the structural framing system that will bring the architect’s vision to life and make it stand.

Determining the structural system for a new building requires the compilation of all kinds of facts regarding the building, all of which must be considered before selecting the system. Specific considerations start with where the building will be located in order to understand the environmental conditions the structure will have to withstand. This includes the likelihood of the structure to experience extreme wind, earthquakes, or the accumulation of snow. This information is also coupled with the overall size and shape of the building to understand the impact of these environmental factors. Additional considerations include the intended use of the building and the functionality required by the future occupant. This leads to developing the gravity and lateral load requirements that the structure will be required to resist.

Other building considerations that impact the choice of a building system include the likelihood of vibration, required fire ratings, allowable defections, the building self-weight, and building geometry (floor-to-floor heights, building height, overall building dimensions). The cost and availability of different building materials can also play a role in the selection of the building system.

All of this information is used to determine a cost effective, constructible structural system for the building, which is confirmed by collaboration with the architect and contractor.

The four main building systems include concrete, steel, masonry, and lightframe construction (wood and lightgage steel). Each have distinct advantages and challenges.

Concrete systems tend to be heavy structures. This is advantageous when dampening against vibrations is needed, like in hospitals or schools. The high self-weight can also be a detriment as it increases the seismic demands on the structure. Thankfully, concrete is also very stiff, which helps control horizontal drift due to wind and seismic events. Another benefit of concrete is that the fire resistance of concrete is provided with the concrete itself. Additional applied fireproofing is not necessary with properly dimensioned concrete framing. A concrete system can also be advantageous in situations where floor to floor heights are limited as concrete floor systems can often be thinner than a steel system. Cast-in-place concrete framing systems tend to be more economical in structures taller than 3 floors due to the opportunity to reuse the forming system for multiple concrete pours.

Steel systems are light compared to concrete. While the lighter self weight is beneficial in many aspects, this reduces the inherent vibration dampening capacity of the system and specific measures may be needed to minimize the vibrations felt by the building occupants. Steel itself is not fire resistant, but fire resistance can be achieved with applied fire-retardant materials. Steel structures are more flexible than concrete structures and utilize steel frames to resist lateral loads. X-braces are the stiffest and most economical braces, but these can conflict with doors or windows. As an alternative, moment frames can be used which eliminates the possibility of conflicts with doors or windows. The use of moment frames also provides more flexibility for future renovations. Unfortunately, moment frames are much less stiff than x-braces and can be expensive due to the labor involved in the fabrication and installation of moment connections.

Masonry, often in the form of CMU (concrete masonry units), has great bearing and shear capacity and behaves similarly to a concrete system. While CMU has less strength than a concrete system, masonry tends to be cheaper than concrete and the units allow for assembly in situations in which placing concrete would be more difficult. Masonry also does not require formwork and the reinforcement is significantly simpler than a concrete system. As with concrete, masonry does not require additional fireproofing measures to be added. Masonry is limited in that it is only available in discrete sizes which must be considered and worked around during the design process.

Light frame construction is advantageous as it is lightweight, quick to erect, and cheaper than the more robust structural systems. By its inherent nature of being a load bearing wall system, coordination of the structural system and the floor plan is simplified as the walls of the floor plan become the structural system. Lightframe is limited in multi-story buildings as the wall details become more complicated and the additional weight becomes challenging to the material. Fireproofing is also an issue and care must be taken to ensure the details are appropriate to achieve the required fire rating.

There are many factors in choosing a structural framing system, but through the process, one system tends to rise to the surface as being the most logical for the structure being evaluated. Also, as technology advances and buildings are becoming more unique, we find ourselves using multiple systems in a single structure and even hybrid systems in order to utilize the best aspects of each system.

No matter what the intended structure is, we will find a way to frame it and make it stand.