Wind Speeds in Structural Design

Among the least understood aspects of structural design for single-family homes is lateral loading, particularly wind loads. Until roughly 20 or 30 years ago, at least in New England, single-family homes were typically built without involving structural engineering services. This, combined with the fact that the region has not experienced a hurricane since one named Bob in 1991, helps explain why some contractors with 30+ years of experience still scratch their heads when reviewing plans that specify wood shear walls, holdown units, or gulp, steel moment frames.

But it is extremely important to understand, design, and build structures to withstand these lateral loads. Although we have been spared since 1991, the wind events of the past will certainly occur again in the future; one could surmise this even without considering climate change.

So let us explore some definitions and numbers. First, there are different ways to measure and report wind speeds. Weather stations continuously collect weather data (duh), with wind speeds being one parameter of interest, measured in miles per hour (mph). Sustained wind speeds are many individual measurements averaged over a time period of 1-minute or more. Peak gust wind speeds are a similar concept, but the many individual measurements are averaged over a much shorter time period, usually between 1 and 3 seconds. Hurricane Bob struck with sustained wind speeds between 75 and 100 mph, with a peak gust wind speed of 125 mph. Remember this peak gust wind speed because it will help serve as an anchor for current structural design standards.

Structural engineers determine loads on buildings using the American Society of Civil Engineers specification, “Minimum Design Loads and Associated Criteria for Buildings and Other Structures” (ASCE 7). Much progress has been made with each subsequent edition of ASCE 7, as research on wind, seismic, and other loads has advanced considerably. In 1995, the first edition published following Hurricane Bob had a total specification length of 236 pages. The current edition, published in 2022, is a whopping 1,046 pages, with roughly 300 pages devoted solely to wind loads.

To design homes, we use the 3-second gust wind speed, corresponding to a 7% probability of exceedance for a 50-year wind event. In simpler terms, this amounts to designing homes to withstand a 700-year storm, that is, a storm with a 1-in-700 chance of occurring in any given year. This does not mean a 700-year storm would occur exactly once every 700 years; realistically, there could be two 700-year storms within the same year, decade, or whatever time frame you typically default to.

To reference a couple of projects recently completed: in Cambridge, MA, the ASCE 7 wind speed used in design was 120 mph, while in Chappaquiddick, MA, it was 134 mph. Comparing these to Hurricane Bob’s 125 mph, the ASCE 7 derived wind speeds appear reasonable, that is, they are neither too high nor too low (at least in my humble opinion). Yet, with such a seemingly small difference in wind speeds, how could the wind loads for the Chappaquiddick project end up more than twice those for the Cambridge project? For the millions of readers of the Koa Rose blog, we will find out next time.

A Brief History on the Building Code

The first pseudo–building code is often attributed to King Hammurabi, ruler of Babylon, written around 1750 BCE. The Code of Hammurabi is known for its “eye-for-an-eye” style of justice, and it included several provisions related to construction. Most famously, it stated that a builder could be put to death if a house he constructed collapsed and killed the owner. While grim and archaic, this early rule illustrates a central purpose of building codes:  to establish minimum standards of construction quality to safeguard public health, safety, and welfare.

Regulations continued to evolve across ancient civilizations — from Egypt to Greece to Rome — as societies grew and urban populations densified. It soon became clear that structural collapse was only one of many hazards that building regulations needed to address. After the Great Fire of Rome in 64 CE, Emperor Nero (not the greatest guy who ever lived) introduced new urban planning rules that regulated street width and building heights and required wider use of fire-resistant construction materials, such as masonry and concrete. These reforms represent some of the earliest known attempts to control fire spread through material and urban design requirements.

Fast-forward to the Great Fire of London in 1666, which led authorities to require brick and stone construction in place of the highly flammable timber common at the time. Even earlier, the first American building regulations appeared in the early 1600s, primarily addressing fire safety. In 1630, for example, Boston banned wooden chimneys and thatched roofs (seems like a good rule to me). In the late 1770s, George Washington recommended height and area limitations for wood-framed buildings in his plans for the new capital city, Washington DC. By the early 1800s, major U.S. cities began establishing formal building codes, often motivated and financially supported by insurance companies burdened by enormous fire-related losses.

The early 1900s marked the beginning of modern building codes as we know them. As cities industrialized, local codes gradually consolidated into regional model codes. Three major code organizations emerged in the United States:

• Building Officials and Code Administrators (BOCA) for the Eastern and Midwestern states

• International Conference of Building Officials (ICBO) in the West

• Southern Building Code Congress International (SBCCI) in the South

Another major consolidation occurred in 2000 with the formation of the International Code Council (ICC), which unified BOCA, ICBO, and SBCCI into a single national model-code body. That same year, the ICC published the first International Building Code (IBC), creating a comprehensive, consensus-based model code now used throughout the United States and adopted, often with local modifications, by jurisdictions around the world. Today, the IBC serves as the primary reference for structural design, fire safety, accessibility, and material standards.

Quality is Everything

Introduction

Countless companies across all industries market themselves as quality oriented. It is legal to advertise in almost any way - Harbor Freight’s slogan is “Quality Tools, Lowest Prices”. It is easy to claim that quality is important. Meanwhile, providing the highest quality products and services is simultaneously the most important aspect of business while being the most difficult to achieve. It requires a genuine love for the work and drives all thoughts, decisions, and actions. Many companies are more concerned with the bottom line and intentionally bypass quality by choosing instead to cut costs, advertise well, and pray for a good market. However, the bottom line could (or should) be viewed as a result of the quality of products and services provided. If you take care of quality, quality will take care of you.

Some people in the Architecture Construction Engineering (ACE) industry say that everyone does quality work, so quality is not a differentiator. While quality cannot be a single leg to stand on, not everyone does quality work. Within structural engineering, like many professions and trades, there is a quality spectrum wherein lies the difference between excellent, average, and oh-my-word. The structural engineer, like the architect and builder, is not a commodity. In fact, the quality of work provided by the structural engineer affects the quality of work provided by the architect and builder, which impacts the finished product itself.

In Koa Rose’s niche within consulting structural engineering for custom single-family homes, our product is the Contract Documents that specify the foundation and framing to keep these buildings standing and functional; our service is the process to achieve this product. Our products and services are integrated into the architectural and construction teams’ products and services. Everyone relies on each other while each team member affects the other, and the homeowner as well. Understanding that our work impacts that of others and the finished product itself, there surfaces two important questions:

1. How do we define quality for a consulting structural engineer?

2. How does this quality affect the architect, builder, and homeowner?

Product

Let us first discuss these two questions as they relate to Koa Rose’s product - the Contract Documents.

Inherent in quality of product is simplicity, where “simplicity is complexity resolved” (quote by Constantin Brancusi). Is the structural solution for the given architectural intent as simple as possible? Or is it convoluted and over-designed? Structural engineers often make conservative assumptions resulting in over-design for several reasons. One, they do not understand a design concept for lack of knowledge. Two, they deliberately reduce their design effort due to their own time and budget constraints. In both cases, the product suffers.

An excellent structural engineer either already understands the design concept, or more often, spends the necessary time and effort learning on the job. This deliberately focused experiential learning results in an important concept called engineering judgement. Structural engineering is truly a blend of science and art. Science is what we know, while art is what we do not know. There is a lot we know, while there is probably more we do not. Learning the science is relatively easy; it involves taking classes as well as reading building codes, specifications, textbooks, and so on. It is the necessary cookbook stuff that teaches you the profession’s basic vocabulary. Learning the art is hard, and the art here is engineering judgement. Quality of engineering judgement is not entirely based on years of experience. One engineer might have twenty years of experience while another might have one year of experience twenty times. Years of experience only helps the conscientious individual, giving them time to deliberately ponder concepts (to which answers within books or the internet cannot be found), observe real-life built conditions across multiple projects, learn the historical context of construction, ask other engineers their opinions, compare and contrast these different opinions, and so on. Note, if you ask five engineers their opinion, you might get six different answers (not including your own of course). The quality of engineering judgement is a key differentiator in quality of product.

An excellent structural engineer gives themselves sufficient design time by charging appropriate fees and setting clear deliverable deadlines before starting a project; this eliminates the temptation to reduce design. Money and time saved by the structural engineer pale in comparison to the associated construction costs and delays that result. 10 to 20 % of project costs are incurred during the design phase while 80 to 90 % of costs are irrevocably committed. Cutting costs at the design phase inevitably results in higher overall costs. The decision to charge low and provide low affects the architect, builder, and homeowner. Sometimes a structural engineer makes an honest mistake and inadvertently charges too low a fee. We all make mistakes. When this happens, we must not reduce quality for the sake of recovering our own costs. Our moral obligation is to always deliver quality because our work affects everyone else’s money, time, and well-being.

To reiterate, inherent in quality of product is simplicity, which can only be achieved through sound engineering judgement and sufficient time spent on each individual project. But what does simplicity of structural drawings look like to the architect and builder? There are so many examples, and here are a few. Are the drawings neat and clean, or sloppily drafted with careless line-work, misspellings, etcetera? Will the builder make a mistake reading those drawings? Will an architect spend more of their own design time decoding those drawings? Is the final design the most buildable solution while simultaneously economizing materials? Was steel necessary, or could engineered lumber or kiln-dried lumber have been specified? Is the architect aware that a $30,000 steel moment frame could be eliminated if this wall shifted two inches and plywood sheathing is installed on that interior wall? Does the final framed condition require extensive shoring or is there a carefully thought-out detail that may preclude shoring altogether? Is there a proper number of sections and details produced with relevant context, or were these omitted to reduce design time?

The essence of these questions can be summarized by the following quote by Steve Jobs:

“When you start looking at a problem and it seems really simple, you don’t really understand the complexity of the problem. Then you get into the problem, and you see it’s really complicated, and you come up with all these convoluted solutions. That’s sort of the middle, and that's where most people stop. But the really great person will keep on going and find the key, the underlying principle of the problem - and come up with an elegant, really beautiful solution that works.”

Service

Now let us discuss the original two questions as they relate to Koa Rose’s service - the process.

Inherent in quality of process is the golden rule - “treat others the way you want to be treated”. It is okay to expect your structural engineer to be reliable, proactive, and kind considering we expect this from our architect and builder. The process starts with the project proposal and ends with the final structural affidavit. Everything in between, which is the process itself, must meet the following criteria.

Reliable means you can be trusted to follow through. Do calls and emails receive timely and complete responses? It is disappointing to hear stories about the unresponsive engineer as bottleneck delays often prevent the architect and builder from completing their own work. Are delivery dates being met? It is disappointing to hear about the late engineer as missing deadlines affects the workflow of others. Did you expect a fluid design process, or did you receive a request for added services based on the first minor change? In residential construction where each home is a custom one-off, it is usually impossible to complete the structural design in one-pass. To make the best product, collaboration which begs iteration is a necessity. The structural engineer who puts in a low fee to win work does so with the expectation that a one-pass quick and dirty design is either expected, or added services will be acceptable.

Proactive means taking initiative and ownership to see a project through. It is incumbent on the structural engineer to flag current coordination items and forecast future ones that may arise; this should occur early and often. Loose ends should be tied up before final drawings are issued and construction begins. It is easier for a structural engineer to change lines on a computer screen compared to a builder altering or replacing their work. Providing options and being clear about pros and cons for each option empowers the architect or builder to make the right decision for their own work. Because everyone’s work is integrated into the whole, asking around to gage preferences is not only polite, but necessary. Structural engineers cannot live in their own world but must have some working knowledge of architecture, construction, mechanical systems, and so on. This enables us to not necessarily know the answers, but to ask the experts the right questions and then integrate these answers within the structural solution.

Kind means being a joy to work with. We love interacting with our clients because they are great people, and we want to return the favor. Work is worthwhile if you love the work you do and the people you do it with. Against conventional wisdom, we prefer doing as much business with as few long-term, repeat clients as possible. Being dependent on another company’s performance can be risky and it is important not to push this concept to the extreme. But when done properly, our success becomes interlinked. We care about our clients’ success and hope they care about ours. Some structural engineers accept too much work from too many clients, which dilutes their focus on each client. At Koa Rose, we would rather know a few individuals extremely well, providing them with the highest quality product and service. The alternative is being loosely acquainted with many individuals and providing them with mediocrity.

Conclusion

We often work for well-to-do homeowners able to spend a fortune; patrons and matrons providing opportunities to work on complex, challenging projects. However, even the wealthy do not like wasting money. When they spend, they expect value for each dollar spent. A wise builder we work alongside said it well. If a homeowner budgets for a 10-million-dollar renovation, they want it to cost 10 million dollars. They do not want to spend 12 million, or 10.5 million, or 10.2 million. They want to spend what the renovation is supposed to cost. The structural engineer must stay honest with their design no matter who the client or what the project. It is easy to haphazardly design an inefficient building, throwing money at drawing details for the sake of conservatism or speed of design. And after the fact, it is easy to claim that quality is important.

Yet it is our moral obligation to actually provide the highest quality product and service possible because peoples’ time, money, and well-being should not be wasted. Quality of product cannot be achieved without first perfecting quality of process. Maybe “perfecting” is the wrong word because there will never be a perfect product or service (although we could and should try). You can design and build a beautiful home by gutting through a “project from hell” scenario. But the product, that beautiful home, would not meet its full potential; it could have been better. This essay is a long way of saying that the quality of each project team member matters a lot. At least if you intend to build a quality product. Quality really is everything.