Category Archives: Design Guidance

The First Rule of Design

Science and math education focus on finding direct solutions to direct problems. In practice, though, design is iterative and questions and answers are rarely direct, absolute or obvious, leading to the first rule of design: Design is iterative. Despite your best efforts, your first solution will almost never be right.

Right now you are thinking that I have an overly negative view on this topic. You might be right! Before deciding, though, let me tell you why I think this statement isn’t negative at all, why I think this is actually a hopeful idea, and why I think this idea frees us from having to be “right” every time. Further, this idea protects creativity from being overcome by practicality at the first sign of weakness. As we get into the topic, let’s first explore a bit of back story.

Good engineering is the foundation of the built environment, and is itself founded on the realities that we call math and science. Math and science education, then, are essential building blocks of creative design in the engineering and architecture fields. Think back to your own math and science education. If it was at all like mine, it began with math and the understanding that math can be reduced to a series of rules. Science then followed as a related field, with its own set of rules.

Under this system, presented with a problem, a student applies these rules within a structure defined by more rules. After applying all of the rules, the student arrives at an answer. When the rules were all applied correctly, in the right order, we call this the “right” answer. Everything else is “wrong”. Plug and chug. Close only counts in horseshoes and hand grenades – an actual quote that I learned from an actual math teacher. Except, close counts in almost everything else, too, and plugging and chugging relies on having something to plug into and chug along with.

Bringing us back to engineering, consider the process for sizing pipes in a sewer system. The set of equations that governs modeling fluid flow in a pipe network was developed over centuries and is very complex. Thus, directly calculating the exact pipe sizes needed to carry water through a sewer can be almost impossibly difficult. Fortunately for us, pipes only come in certain sizes! Instead of spending hours determining that we need a 13.625 inch diameter pipe, we can just guess that we need a 12 inch, then use a much simpler calculation to check that assumption. That won’t carry the flow? Then try an 18 inch pipe. That seems too big? How about 15 inches? Perfect!

In this example, we knew that there was a very good chance that our first solution – the 12 inch pipe – wouldn’t be right, but the process saved us time. What could have taken hours of careful modeling, took us 15 minutes in a simple spreadsheet.

Engineering education stands in stark contrast to the key concepts that guide an engineer’s daily work. Simply put, design is iterative and your answer – or at least your first answer – is less a solution and more a starting point for a process that could well lead the design in an entirely different direction. This is the power of creativity, something that we should work our hardest to protect. This is also a direct challenge to our way of thinking.

What then is the intent of the first solution if it is not an attempt to solve the problem? I would say that the first solution less needs to solve the problem and more needs to define the box that the eventual solution must exist in. In protecting the creativity of the design, it is to our interest to define this box in the biggest, most flexible terms possible.

Let me be clear that I am not advocating for sloppy math or ignoring the importance of understanding the mechanics of problem solving. Quite the opposite, these things are very important and too many engineers rely on computer programs as substitutes for understanding them. This appears in my own work as a path of least resistance temptation to view myself as a well trained CAD technician and permit form filler instead of striving to become a thoughtful, experienced engineer. This is a topic for another time, but for now my point is this: Understanding how an internal combustion engineer works is only one part of understanding what a car is and does and can be used for. So to are math and science rules to their fields.

As stated at the beginning of this post, knowing that your first answer can’t be right isn’t actually discouraging at all. True, the engineering mind has been taught to love correctness, exactness and direct problem solving and in this case it won’t find any of these for awhile. This is disappointing, but by acknowledging that your first answer can’t be right, you also accept that your first answer doesn’t need to be right. You aren’t the gatekeeper though, responsible for keeping out far fetched, impractical ideas. You are a member of a creative team responsible for continually defining and refining a concept that will one day grow up to become “the solution”.

Letting go of the outcome frees us to focus on the process, and focusing on the process puts us in a position to serve the design team in a way that only we can. We became engineers because we are interested in the puzzles created by the realities of math and science. By looking for direct, exact solutions to these puzzles, we turn our realities into limitations. When we accept that there aren’t any direct, “right” solutions and make the choice to instead engage in the creative process, these realities become challenges and opportunities and we free ourselves to truly do our best work.

To stay up to date with future Civil PDX content, subscribe to the blog by entering your email address at the top of this page. New posts will automatically be delivered to your inbox.

If you found this post interesting, you might also like these past posts from Civil PDX.

Engineering Concepts in LID
The Engineer and the Fisherman
Achieving Zero and the Value of Experience

Advertisements

Passive vs. Proactive Design

Greeley

Each weekday nearly 300,000 Portlanders take to the streets as they make their way to work, and most of them are driving. Despite the city’s mostly successful efforts to promote public transit and active transportation, most commuters still prefer the convenience of their own cars.

MODE COUNT PERCENT
DROVE ALONE 180,107 64%
CARPOOL 25,736 9%
TRANSIT 34,289 12%
BIKE 18,912 7%
WALK 21,336 8%
OTHER 3,247 1%
TOTAL 283,627

 

This isn’t a revelation. We know that Americans rely on driving to get where they need to go. This preference has shaped our communities for almost two hundred years and defines many of the habits of our everyday lives. As an example, my family and I live about one mile from the nearest grocery store and without much trouble or effort could walk there every day or every other day to buy what we need. It is much more convenient though to make the trip once a week, and while I can walk to the store in about fifteen minutes, what I can’t do is carry a week’s worth of food back with me. Isn’t it great that we have a car to help us with that!

To go further, every aspect of the typical supermarket grocery store – from the size of the carts to the size of the parking lot – is designed to support the weekly shopping trip. This is just one piece of our car centered culture. During my one mile drive to the store, I also pass three gas stations, three auto parts stores and two rental car agencies. If I’m willing to make an extra fifteen minute commitment, my options open up to several more stores that might offer better selection, lower prices or both. None of this surprises you, of course, because this is normal to us and normal things aren’t usually that surprising. There is a good chance that my routine is not that different from your own.

There is no community in the United States that isn’t in some way designed around cars. Cars affect every place and every moment of our lives. As our reliance on them has grown, so too has our need for safe streets. To this end, engineers determine street widths, speed limits and a host of other factors according to well established guidelines with the goal of preventing cars from crashing into each other as they move from place to place. While this framework for design has been intentionally evolved over time, many of its effects on our communities have not.

As traffic speed increases, so does the frequency and severity of traffic accidents. Research tells us this, and we expect it from our own driving experience – so, we each limit our speed to our own comfort range. The street’s designers are also trying to manipulate this range to what they feel is safe not only through posted speed limits, but also with more subtle traffic calming measures like lane widths, lane configurations, curve radii and building setbacks. Your driving speed, then, is controlled by two factors: it is directly controlled by how fast you are comfortable driving and indirectly controlled by how fast the designers would like you to be comfortable driving.

The sometimes ignored aspect of this conversation is that there is a reciprocal relationship between these factors. Imagine I am designing a street through an already developed part of your town. I want to understand how the road can better serve your community, so I perform a traffic study to determine how people use the existing street and how that relates to transportation in the surrounding area. From this study, I learn that while the posted speed limit is 25 mph, most people drive closer to 35 mph. This is a big difference and represents a decision point for my design. I want to create a street that is safe for the people who use it, but I also want to create a street that encourages people to use it safely. There are two general approaches that I can take from here.

Approach 1 – Passive Design

Seeing the discrepancy between posted and actual speeds, I have decided that while the street’s classification doesn’t warrant raising the speed limit, it is prudent to design the road to accommodate traffic travelling at a higher speed. Having made this decision, I use some of my available right of way to provide slightly wider lanes. I also lengthen the curves along my corridor to increase how far up the road you can see and give you more time to make decisions while you are driving. Finally, I provide moderately more protection for both drivers and pedestrians by setting street trees and sidewalks away from the edge of the road.

This is of course oversimplified, but the decision making process that I have followed is very reasonable. Each design choice was made to better protect you – the citizen, tax payer and user – based on my understanding of how you drive. The problem is that space in the right of way is limited. Each of my decisions came with a trade off, and because my approach was entirely vehicle-centric, I prioritized driver safety over other street uses at every step. I gave you slightly wider driving lanes so you have room to safely travel at your desired speed. This likely meant making a choice like building narrower bike lanes, leaving cyclists with less room to travel. I set the streetscape features and sidewalks away from the road so they would be less likely to obstruct your driving. This likely left less width for sidewalk and other pedestrian facilities.

Approach 1 has been dubbed “passive design”, a term that is not generally used in a good way. By focusing solely on vehicle needs and safety, we provide less room for cyclists and walkers. We are then encouraging or at least accommodating higher speeds in closer proximity to the street’s most vulnerable users, who are forced to use less robust facilities. At the very least this presents a serious safety concern. At the most it discourages people from using the street for anything other than driving a car. There has to be a better approach to designing streets that accommodate multiple uses, and fortunately there is.

Approach 2 – Proactive Design

Seeing the discrepancy between posted and actual speeds and deciding that the street’s classification doesn’t warrant increasing the speed limit and understanding the goals for the area, I have decided that something must be done to discourage speeding. I create lanes that will still accommodate traffic, but will also feel tighter to drivers, encouraging them to slow down and drive more precisely. Having chosen the lower posted speed for my design, I can bring streetscape features closer to the curb line, reinforcing the narrower feel. Right of way width is also less of an issue now and I can include safer, more generous bike lanes and sidewalks, making the street a more attractive corridor for active transportation.

By creating a safe environment for cyclists, pedestrians and other non-vehicular users, we build on our goal of creating streets that better serve our communities. By encouraging more people to use active transportation options and by giving them safe places to ride and walk, we create new growth opportunities and bring a different type activity to business corridors. The service for car traffic is still there, but the newly imagined street can also be so much more. From public health to business development to transportation, proactively designed complete streets are an important component of strong communities.

None of this is meant to suggest that we can avoid difficult design decisions by being proactive. If there were no difficult design decisions, there wouldn’t need to be engineers to help make those decisions. But our long term visions for how we would like our communities to grow and thrive are fragile things. Civil works projects represent great opportunities to strengthen these visions, as they focus on bringing some kind of change. These opportunities are lost, though, if we stop at passively identifying and accommodating existing uses. By proactively reviewing these uses against long term goals, we still allow ourselves to take the steps needed to maintain our infrastructure, but we also capitalize on a rare opportunity to strengthen and build our communities.

How have passive and proactive approaches affected development in your community? I would love to hear your stories and encourage you to leave them in the comments section below.

[From photo above] The design of Greeley Avenue in North Portland attempts to balance the needs of passenger car, freight, transit, cyclist and pedestrian users, all on a relatively high speed road. With the high projected growth, the City is developing proactive designs that would significantly change traffic patterns here, including a greenway trail along the Willamette River that would move cyclists and pedestrians off the road.

Detailing Vegetated Riverbanks

South-Waterfront-Jute

Awhile back, in “The Engineer and the Fisherman”, I explored the unique challenges posed by trying to stabilize urban riverbanks using natural or vegetated measures. This post generated a lot of interesting feedback – much of it through the Urban Design Network LinkedIn group. This interest combined with the fact that the subject is so broad and complex presents an opportunity to deepen the conversation and more specifically target some of the engineering issues contained in these designs.

Approaching such a complex issue is daunting, but as with most engineering problems the discussion can be simplified by breaking the original topic into understandable pieces. This approach is the basis of my “Framework for Understanding Details”. Under this framework, we segregate constraints into one or more of three categories – programmatic, environmental and physical constraints – with the intersection of the categories representing the available design choices. This relationship is represented in the Venn diagram below.

Framework-Venn

Programmatic constraints are the limitations imposed by the project owner. Looking at the topic of naturalized riverbank design through the lens of this framework, there are two likely programmatic constraints that should concern the designer. First – and especially in urban settings – it is very likely that the project owner wants a bank that will not move in the long term. When some bank movement is acceptable, the amount is usually small and more or less only serves as an indication of the level and urgency of necessary maintenance. In these cases, the bank is usually returned to its originally designed configuration with routine maintenance.

Second, by targeting a naturalized riverbank, the project team adds a programmatic constraint that severely limits the group of available designs. Native vegetation provides the primary long term scour resistance needed to hold these banks in place, but its presence is more an indication of the underlying bank structure than it is the cause of that structure. In the same way that a building’s architecture is supported by a tailored structural design, many factors must be just right before vegetation can be sustained over a long period of time. Of course, the degree of this constraint can vary. There is a big difference between a design that uses natural elements as the primary structure and a design that has the desired outward appearance but is supported by a more conventional engineering approach below ground level.

These programmatic constraints are at least to some degree at odds with each other. Natural riverbanks are flexible, moving and changing over time – a feature that does not mix well with urban development. Project owners spend huge amounts of money developing property and they hire engineers to give them a level of assurance that their investments will be protected through storms, floods, earthquakes and whatever else time might bring. Very few options are left after constraining the design to solutions that use natural elements and create a relatively inflexible bank, and this is after only one category of constraints.

Environmental constraints are the limitations imposed by the interactions between the project and its environment. Not surprisingly, the driving interaction on riverbank projects is that with the river. From the river’s view, most projects occupy a very small amount of space – sitting high on the bank. This can mean that there are global stability problems that can’t be dealt with in the scope of a typical project. Many of these problems radiate from deep within the river’s main channel and can only be dealt with by creating a last line of defense – often soil improvements or deep foundations – at the face of the development. The sketch below illustrates one possible way that a global problem could affect the stability of a project.

Riverbank-Global-Stability

These sorts of topography considerations are one of many factors that contribute to a river’s flow characteristics, which in turn define the stresses that the river imposes on its banks. Variations in the bank line can result in an infinite number of scenarios in this regard, but longitudinal stress – the stress imposed by water moving along the bank or down the river – and perpendicular stress – the stress imposed by waves moving up the bank – are the two parameters that will combine to drive the design. These values depend on the precise conditions for each project and vary from site to site, but generalizations can often be made for specific stretches of a river. For example, bank design on large river systems that carry shipping traffic may be controlled by the waves thrown by passing barges, while design on smaller creeks or streams might be controlled by the speed of the current during times of high water.

Many of the river related habitat and scour issues that our communities face today result from designs that did not account for their impact on downstream systems. The limitations imposed by downstream interactions are very much environmental constraints and need to be quantified and addressed as such. That said, most vegetated bank designs result in slower, less channelized flows and positively affect downstream properties. This leaves global bank stability, river flow related shear stress and wave related shear stress as the key environmental constraints for typical naturalized riverbank designs.

Physical constraints – the limitations imposed by such aspects as location and constructibility – make up the third category in the framework. The most global of these constraints for this conversation involves slope. Over time, many of the riverbanks that support our cities have been steepened to create more developable land. Returning these banks to their natural state usually requires returning them to something closer to their natural slope, creating two challenges.

Just as banks were originally steepened to create more developable land, flattening these banks requires giving some of that land back to the river. This is possible in some cases, but can be problematic on sites that host buildings or other features that are slated to remain. It can also be difficult to transition the re-flattened bank to meet adjacent properties. These transitions often have to be made gradually to avoid creating scour pockets and can lead to a scenario where most of the site is transition and very little is actually a fully naturalized design.

Along with topography and slope considerations, plant establishment time is a very real limitation that must be addressed in order for a naturalized design to succeed. Most natural riverbanks rely on native vegetation for scour protection, without it these banks would wash away. But even in the best cases it takes several years for a newly planted bank to develop the root system necessary to provide this protection, meaning that an interim solution must be integrated into the design. Ideally, this interim measure is biodegradable so that the support and structure it offers will lessen as the vegetated system gets stronger.

Coir mat* – made from coconut shell fibers – can be a useful cover in these situations as it is natural, biodegradable and available in many forms. Like most erosion control blankets, coir mat can be found in both woven – think burlap – and nonwoven – think furnace filter – forms. The woven version is strong and provides good resistance to shear stress. At the same time, though, it has large openings and on its own will allow soil to wash away. Nonwoven coir mat has small openings and is very effective at preventing soil loss, but it is not very strong and would likely rip under the shear stress imposed by swift river currents. Because each type of coir matting has a strength that supports the other’s weakness, an obvious solution is to layer both woven and nonwoven matting on the bank. In this application, nonwoven matting is placed directly on the graded bank to provide soil retention, then woven matting is laid over the top to provide shear resistance.

Of course, even the best matting design will be of little help if it isn’t properly attached to the bank. The attachment system has to be simple and constructible while at the same time providing resistance to horizontal and vertical movement across the entire bank. After working through several designs, the process that I feel best meets these goals is to lay the matting on the bank, stake it in place with a close grid of long, square stakes, then weave twine around and between these stakes. In this configuration the stakes offer resistance to horizontal movement and hold the blanket in place during construction while the twine offers vertical support, preventing the mat from floating or warping. The photo at the beginning of this post shows one example of this type of installation, taken at Portland’s South Waterfront.

With all of this as background, the following is a revised version of the previously presented Venn diagram summarizing the programmatic, environmental and physical constraints that are imposed on a typical naturalized riverbank design. Each project will include many more constraints than those discussed here, but this set is meant to be a fairly standard collection for this type of project. In order to succeed, a riverbank design must address all of these constraints.

Framework-Venn-Riverbank

You can see that while there are many design options that can be categorized as naturalized or vegetated solutions, there are really very few choices that will work with all of the constraints laid out here. This is not meant to imply that designers shouldn’t try to solve this problem though. The civil engineering profession exists to develop creative solutions to problems like this one and civil engineers have to some degree sidelined themselves in recent years by avoiding the risks necessary to confront these challenges. By dissecting and analyzing the project constraints, we can better understand both the steps we need to take to address them and the implications of pushing these boundaries.

Have I overlooked any common constraints that you have encountered on these types of projects? Please feel free to post them along with any other thoughts you might have on the topic in the comments section.

*The original version of this post recommended jute mat, which is also a good product for this application, but is not as strong as coir.

A Framework for Understanding Details

Desk

Well thought out details are the essence and foundation of good engineering design. Too often engineers are tempted to reuse something that has already been developed without analyzing whether it is the right solution for the project at hand. Details must respond to the specific programmatic, environmental and physical constraints that are imposed on the project. When details miss this goal, the project falls short of what it could have been, even if no real problem is created. When they are successful though, the project has the opportunity to reach its full potential.

My vision in developing this blog is to use my perspective and experience as a civil engineer to promote and demonstrate the design processes behind the infrastructure that surrounds us. By analyzing engineering details we see a microcosm of these processes – a slice of design that can be understood independently from the larger project. Before we can approach this analysis though, we need a framework for understanding what a detail is and what it does.

Fight as we may, the truth is that every project has a list of solutions that would be great, but for different reasons don’t quite work. For example, by building roads out of permeable pavements we could significantly change how our infrastructure affects natural water quality and flow patterns, but the materials presently available just can’t support the level and type of traffic carried on even a typical city street. Maybe instead we could put our road network underground; then stormwater would never come in contact with pavement and we could use any material we like. It may not surprise you that the price alone of such a project would prevent it from being considered, particularly in an age when transportation funding can barely keep up with road maintenance costs.

It is then not hard to argue that many of the most significant design criteria for a project are in place before the design team ever begins work. For the sake of our framework, I am going to break these constraints into three categories.

Programmatic constraints are the limitations imposed by the person or group that oversees the project. This could be a public agency, a private developer or people in any number of other positions. By the time the design team has begun work, this owner has usually established much of the program for the project including what functions the site will serve, how much area will be committed to each of these functions and how much money will be made available for the project. Programmatic constraints must be observed because it is difficult to justify spending money to develop a project if that project does not meet them.

Environmental constraints are the limitations imposed by the interaction between a project and the surrounding environment – both upstream and downstream. This is not limited to the ecological environment – though that is certainly included – but extends to all outside interactions including elements like traffic. Upstream interactions – how the environment affects the project – are created independently from the project, and the designer has no control over them. The designer has more control over downstream interactions – how the project affects the environment – but this control is still limited. In either direction, the project will not fit into the bigger picture without close attention to environmental constraints.

Physical constraints are the limitations imposed by such aspects as location, topography and constructibility. These are real world issues with an active presence on the site. Physical constraints are primarily determined by the project site, the properties of the selected materials and the means and methods available to the contractor. A good idea is a wonderful thing, but that idea won’t become a reality if it doesn’t work with the physical constraints.

In order to be a good solution for a particular project, a design must simultaneously respond to all of these constraints. Showing this requirement as a Venn diagram results in the following infographic.

Framework-Venn

Projects are rarely equally constrained in all three categories. One category or another will often dominate the system. In general, imposing more constraints will lead to having fewer options, shrinking the circle and its intersection with the other categories. Conversely, imposing fewer constraints results in more options, expanding both circle and intersection. This change can be shown visually by making a few small changes to our original diagram. The main point here is that fewer constraints and more available solutions will usually lead to more flexibility in design and a better final solution.

Framework-Venn-Resize

In addition to representative circle size, the amount of overlap between categories can vary. In a perfect world you would find a long list of solutions that work for multiple categories. Occasionally one category will even be completely contained as a subset of another. On the flip side though, it is possible for projects to be constrained to the point that one category is completely separated from the others. Such an overly constrained system leaves no design options that fit the overall project and means that a change has to be made to some programmatic, environmental or physical element for the project to be viable. Again revising our original Venn diagram results in the following graphic, showing how a more closely aligned set of constraints leads to more flexibility in design.

Framework-Venn-Shift

In applying this framework to design work, it is important to note that there is a difference between given constraints and self imposed constraints. Again, by the time the design team begins work on a project, many of the constraints are already established. Seeking flexibility and creativity in our solutions and design work, we should avoid adding to these constraints when possible or at least recognize when we are doing so. For example, by limiting ourselves to designs that have a standard detail or designs that we have worked through on previous projects, we can artificially make the circles in our Venn diagram smaller across the board. This is not always a problem, but it is important for this to be a conscious decision.

Next, while it is important to identify self imposed constraints, it is equally important not to dwell on things that can’t be changed. It can be tempting to argue editing a project element to make a seemingly otherwise perfect idea fit, and doing this can be an important part of an open creative process. There is a point though where we have to realize what is possible and what isn’t and direct our energy and creative effort toward the former.

Finally, whether you agree with the way I have categorized project constraints or not, hopefully it is clear that grouping these things in some way will allow you to see the options you have in approaching design challenges. Design is a creative process and it is easy to hit a dead end and not know what to do next. By stepping back and approaching the project through the lens of a different group of constraints, you can continue to make progress while giving your mind the time and space it needs to work through the more difficult problems.

I hope this post has given you some new ideas that you can put to work in your own design efforts. I would love to hear how this framework compares to your own views on the subject and I encourage you to post your thoughts in the comments section below. Moving forward, I plan to use this framework to explore the process that goes into developing details, to analyze common designs and to continue to explore the form and function of the infrastructure around us.

Engineering Concepts in LID

Last week’s post compared how the forest ecosystem and Portland’s municipal sewer manage stormwater. The vision offered for closing the gap between these systems is one spin on what has been dubbed low impact development (LID) – a stormwater management approach that emphasizes using natural features and systems to protect water quality. LID carries a promise of improving water quality and lessening our impact on the natural environment while saving municipalities money by reducing the demand on our aging sewers. Because of this potentially big benefit, public agencies have grown increasingly interested in adopting LID methods, especially here in the Portland area.

As a part of the “Stormwater Cinema” series, KPFF has produced an excellent short film that introduces LID and the engineering behind it, the stormwater toolbox.

So, with that, how should we as engineers approach LID design? What are the key design issues that will help create a successful LID project? To oversimplify things a bit, the goal of LID design is to create a man-made environment that mimics the natural one. The LID designer can best address this goal by decentralizing and layering the design and lengthening the time it takes stormwater to pass through the system. The impact of the resulting design can best be analyzed using three unique but closely related dimensions: time of concentration, stormwater quality and stormwater disposal.

Time of Concentration

Time of concentration (TOC) is a site characteristic, much like the amount of shade or amount of vegetation are. While it can change over time, TOC is independent of intensity, duration and other storm characteristics. Measured in time, TOC is defined as the length of time between when it starts raining and when the flow through that site’s storm drain system reaches its peak.

An interesting relationship exists between TOC and the performance of a storm drain system. As TOC increases, the peak flow rate in the system decreases. This is not to say that increasing TOC will reduce the total amount of water leaving the site – though depending on the method it can – but instead that it will result in the same amount of runoff leaving the site over a longer period of time, creating a longer, lower peak. This relationship is illustrated in the Intensity-Duration-Frequency curve at the link below. Such curves are commonly used to calculate peak flow rates.

Portland-IDF-Curve

Stormwater Quality

The quality of stormwater runoff is a measure of how much pollution that water contains. Higher quality stormwater has less pollution while lower quality stormwater has more. Most municipalities require that the pollutant level in stormwater be brought below some defined level before it can be discharged into a public sewer. Many LID designs – including rain gardens, vegetated swales and flow through planters – have become widely accepted methods of addressing this need. Mechanical systems for treating water are also available and are good options when site constraints preclude the use of LID designs.

Less concentrated, slower moving flows typically can help in the quest for higher stormwater quality as they are less likely to result in erosion and more likely to allow suspended sediments to settle out. While pavement and pipes allow for efficient drainage designs, they also increase velocities and concentrate flows, leading to a need for additional downstream water quality treatment. Project designers often have to take conscious measures to prevent water from concentrating in order to keep velocities down.

Stormwater Disposal

Every drop of water that falls on a site is ultimately disposed of in one of three ways.

  • Offsite disposal – which could include connecting to an offsite sewer system or sending water directly to a water body
  • Infiltration – which could include sending water to a rain garden or infiltration gallery
  • Onsite use – which could include rainwater harvesting for irrigation and greywater systems as well as evaporation and use by plants

Before large scale human development, much more runoff infiltrated and the current thrust of design is to return to this condition. Of course, this goal can be at odds with the need to have hard, durable walking and driving surfaces, so it is important to carefully consider how these surfaces are designed and what areas can be reserved for stormwater management.

The space available for stormwater management can play a large role in determining which methods of disposal can be considered on a particular project. While connecting to an offsite sewer system is usually the least desirable outcome, it is often the only viable option, especially when stormwater management is not a priority in design decisions. Infiltration is a low cost option for disposing of stormwater, but requires space and works best when facilities can be decentralized and runoff can be directed to them without the use of catch basins and pipes. Onsite use is an attractive disposal option, but the reality for most projects is that the money saved by using onsite water will outweigh the added cost of rainwater harvesting. Further, there is usually much more water available during storms than can be captured and stored for future use, especially considering that there can be little need for irrigation during the wetter times of year.

LID Layering

As stated earlier – and to wind the points above together a bit better – a solid approach to LID design includes:

  • Lengthening the amount of time that it takes for runoff to pass through the storm system
  • Avoiding concentrating flows and increasing velocities
  • Maintaining a decentralized approach to stormwater management

Because individual LID techniques often can not address all of the stormwater needs on a site, many projects can benefit from a decentralized, layered approach to stormwater management. By layering LID elements, designers can address TOC, stormwater quality and stormwater disposal issues in a simpler form, before flows have had the chance to concentrate and accelerate. Many elements – like eco-roofs, trees, pervious pavements or check dams – can add significant value at the beginning or middle of a drainage path, leading to a less complicated design challenge at the end of that path.

Since it can be helpful to approach problems from several different angles, try imagining you are a rain drop falling on your site. What is your first contact with the project? Do you land directly on pavement, where you immediately start picking up speed and pollutants; or is there a chance that your first contact could be with the canopy of a tree? Once on the ground, are you allowed to move in a well spread sheet flow; or are you directed toward a drainage structure where you will become part of a larger, concentrated flow? As you move across the site, do you pass through any areas – either in the air, at grade or underground – that could be considered a missed opportunity for bettering the storm drain system?

Hopefully consideration of these questions and the core design issues raised in this post will help you in developing high quality, innovative designs that address the unique challenges presented by your projects.

Tactile Warning Strips

Tactile-Warning-CIP
The sun was out the other day – a rare treat in January, as you may know – so we decided to take a family walk around our North Portland neighborhood. Many of the sidewalks in Portland are decades old and pre-date modern standards, lacking curb ramps and other features that are now required on new construction.

Always wanting more speed, the kids opted for scooters over walking. After all, who wants to run when with half the effort you can get twice the speed and recklessness? At the first intersection, our 5 year old son bottomed out trying to jump the curb and nearly landed on his face in the middle of the street. We had the following conversation.

Me: Careful there, you almost lost it.
5 year old: What, dad?! That’s just the way I do it!

This was a good reminder that our streets are used by all kinds of people, for all kinds of purposes and – 5 years old or otherwise – everyone has their own idea about what they are doing. The ADA Standards for Accessible Design set a basis for designing and constructing projects that accommodate all of these people, purposes and ideas. Curb ramps are one of the more visible elements of these standards.

Aside from the ramp’s geometry, the most significant consideration in designing a curb ramp is how to best incorporate the required “tactile warning strip”, the part of the ramp that addresses ADA surface texture and color contrast guidelines. These guidelines were established to ensure that people with impaired vision know that they are leaving the sidewalk and entering the street.

This post will give you a general overview of design considerations for tactile warning strips, including: color selection, installation methods, material types and available products. Throughout the post, you will find links to related sites, should you want additional information.

Color Selection

Armor-Tile-Colors
Some of the more common color options

A surprising amount of research and debate has gone into color selection for tactile warning strips, but it is safe to say that yellow is the standard or default color. An in depth study by the Federal Highway Administration looked at ten different colors and three black/white zebra patterns and generally supports this.

The problem, though, is that designers, owners and developers spend a lot of time and money carefully selecting materials and colors for their projects and aren’t always enthusiastic about scattering yellow stripes across their site. So, the debate will continue and really no particular answer is always right.

As FHWA’s research shows, contrast with the surrounding walkway is the most important color consideration.

Installation Method: Cast in Place vs. Surface Mounted

Cast in place tiles could have saved this curb ramp

The method of installation for tactile warning strips varies significantly from product to product. Some tiles are designed to be set in wet concrete while others are applied to dry concrete like giant stickers. Others still are bolted down with anchors that are either set or drilled into the underlying ramp. Properly selecting a system can be the difference between an installation that stands up over time and one that needs to be replaced a short time later.

In general, tactile warning strips installed on new curb ramps should use products that are designed to be cast in place. The resulting installation will be less likely to be damaged by movement and incidental wheel loading. Surface mounted products are better saved for retrofit applications, where a warning strip is needed on an existing curb ramp.

Material Types

As with drainage pipe, different tactile warning products are often made of different materials. Plastic systems make for easy installation and are engineered to stand up against pedestrian and vehicular traffic. Concrete panels and pavers can offer a more architectural look, but this can come with higher installation costs, less durability and more long term maintenance. Thermoplastic panels are very inexpensive and easy to install, but may not be the right choice for maintenance reasons.

Durable, well designed products that are easy to install can be more expensive then their competitors. It is the project designer’s job to evaluate the specific project requirements and determine if using a slightly more expensive product would benefit the project in the long run. In the area of tactile warning strips, materials usually have a far smaller impact on life cycle project costs than does the maintenance that comes along with them. Selecting the right long term solution is the key to a successful project in this respect.

Available Products

Several manufactured products exist. Most are available in a wide range of colors and all have nearly identical texture patterns. Where these products differ is in installation method and materials. Properly evaluating these differences during design often determines the long term performance of the ramp.

This is not meant to be a comprehensive list, but only a selection based on my research during design.

Armor Tile

Armor-Tile-Front
Surface of Armor Tile plate

My favorite tactile warning products are made by Armor TileArmor Tile products are made of a polymer composite that resists wear and UV degradation. The flexibility of the composite combined with the structure of the tiles creates a surface that can support heavy loads without being prone to catastrophic cracking. Armor Tile products have an aggressive surface texture to prevent slips. 

Armor-Tile-Back
Back of Armor Tile Cast in Place System

The perforated, ribbed back of Armor Tile’s Cast in Place System works well for both locking the panel in place and supporting occasional wheel loads. Their Surface Applied System is installed using screws and concrete anchors and is a solid choice for retrofit installations.

Cast in Tact

Made from a fiber reinforced, cementitious material, Cast in Tact products are a good solution for projects that require a more architectural look than can be achieved with Armor Tile’s composite panels. These panels have a fairly good reputation for quality and durability, but there are a few recurring issues that should be considered.

Essentially large concrete pavers, Cast in Tact panels are less forgiving than Armor Tile’s during construction. It is not uncommon to see installations with chipped edges or cracks through the panels. Technically, the owner should have the right to require the contractor to replace broken panels. However, this usually requires replacing part or all of the ramp – a major repair for what amounts to a relatively minor problem – and isn’t always realistic.

Looking at different installations, you might also notice that the domes on concrete panels tend to wear down over time. Though the Cast in Tact panels are durable, they are not immune to this problem. The supplier has attempted to address this problem by designing a method for replacing individual domes. I don’t know of any installations where this has been used, but it seems like a fairly time consuming – therefore expensive – repair.

Arcis Corp.

Arcis
Samples showing the front and back of the Arcis tile (note the nut cast in the back)

One tactile warning product that is interesting, but that I have never used is that made by Arcis Corp. The Arcis panel looks very similar to the Cast in Tact product – a cement based panel that sets much like a paver – but also has bolt studs that extend into the underlying concrete to anchor the panel in place. As engineers we like a factor of safety, and Arcis’ product seems give us that.

Thermoplastic Tactile Strips

Stick-Down-Damage-2
A not-too-old thermoplastic strip (note the new looking concrete)

Several makers of thermoplastic pavement marking materials also offer tactile warning strips. Much like thermoplastic legend striping, these strips are laid on the concrete or asphalt ramp and torched. The resulting installation provides good visibility and can easily be fit to the specific installation.

The problem with thermoplastic tactile strips is that they perform very poorly over time. Wheel drag and weather loosen the tiles’ bond to the underlying surface and the tiles eventually begin moving. These tiles are pretty common, which means the problems that come with them are, too. You will probably be able to spot damaged or missing thermoplastic tiles without too much looking.

Because of the maintenance problem posed by thermoplastic tactile warning strips, I suggest you avoid them for anything other than temporary installations.

Suggested Design Considerations

In designing curb ramps, you may find it helpful to weigh the pros and cons of different options using the following questions.

  • What color is the surrounding walkway? What colors of tactile warning strip would provide an acceptable level of contrast?
  • What kind of traffic will come in contact with the curb ramp? Is there a chance that truck wheels could drag across the ramp while turning?
  • Would a cast in place product or a surface mounted product be best for the installation?
  • How will the chosen product perform long term?

There are a lot of tactile warning systems on the market, and the temptation can be to leave the decision to the contractor. Some flexibility in material selection is a good idea and can benefit the project, but that is only true to a point. Too little direction can lead to a project that costs about the same amount of money, but doesn’t perform as well as expected over the long term.