Structural Loads

Engineering Books for Babies

A Review of Some Interesting Titles

By Linda Kaplan, P.E.

Goodnight Moon (HarperCollins). Chica Chica Boom Boom (Little Simon). The Very Hungry Caterpillar (World of Eric Carle). Great classics of children’s literature written for the youngest babies and gifted to new parents regularly. But how about Future Engineer or Baby Loves Structural Engineering? Also excellent books, written for the youngest babies, but not nearly as well known and primarily gifted to or from an engineer. These deserve a place on every young reader’s shelf, not just those with a direct connection to the field of engineering. You might be wondering how this relates to the mission of STRUCTURE magazine. We strive to “engage, enlighten, and empower structural engineers.” This includes contributing to and encouraging the expansion of the profession and engaging non-traditional populations, such as babies. Just as many of our editorial board members are parents with small children, so are many of our readers; and if you are not, you likely know someone who is. When speaking to students, especially at the high school or early college level, it is common to hear, “I don’t know what an engineer is/does.” We may be missing out on some of the best talent simply because the students do not know the possibilities of engineering, and therefore do not pursue the field. Unless they have a family member working in engineering, or an unusually good high school guidance counselor, they are likely never introduced to it. Education and outreach at the high school level are clearly very important to combat this issue. However, engineering books introduced to babies can have a positive impact as well. Just knowing what an engineer is, even at the most basic level, might be all the push a student needs to investigate further. This obviously takes the long view, but we suspect it is easier to tackle the issue by getting to the young in their formative years rather than trying for a mid-course correction or a turn at the last moment. Future Engineer by Lori Alexander (Cartwheel Books) introduces the concept of an Engineer

by drawing parallels to babies’ natural interests, making the field approachable to anyone. This simple board book features brightly colored paired cartoon drawings of the Engineer working and Baby doing something similar in their own way. A full range of diverse characters, both for the Engineer and for Baby, keep this book current and allow almost any child to recognize themselves within the pages. The story remains neutral to all types of engineering, but the last page does have simple descriptions of six common engineering fields. Baby Loves Structural Engineering by Ruth Spiro (Charlesbridge) takes a narrower approach, focusing solely on structural engineering, specifically building a house. It appeals to the natural curiosity and basic block-building interests of many babies. The story follows Baby as they build a house with blocks and uses slightly more technical language than other books. Throughout the story, baby remains a single character, never given an assigned gender, allowing the focus to remain on the building. The house has a foundation that must hold up the walls and all the loads inside; walls must hold up the roof and help stay strong no matter the weather, and a roof keeps everyone safe and dry. It even goes so far as to label the structure as dead load and the cartoon pigs living inside the house “Engineers want to know how as live load. This early introduction of the language of engineering is valuable things work. So does Baby.” in making the field approachable as the child grows. When the language and “Engineers make things that simple concepts are familiar, even in a basic sense, the “scary” factor of an help others. Baby loves to help!” unknown field is greatly diminished. Possibly more mainstream are children’s books focused on trucks and

construction equipment. While not as directly about engineering, these stories still spark interest in how things are built and the construction activities children see around them, opening the opportunity for further conversation. For example, a kid getting excited about seeing an excavator or crane on the side of the road may someday be the construction engineer directing them. Where Do Diggers Sleep at Night? by Brianna Caplan Sayres (Random House Books for Young Readers) is a simple rhyming bedtime book introducing different types of trucks and construction equipment. The characters here are the trucks; young trucks are presented gender neutral with an almost even split of mom and dad trucks putting them to bed. Each rhyme gives some information about the purpose of the subject truck but does not get technical, and the focus remains on bedtime. This book does a good job of mixing in less common construction vehicles with trucks that children may be more familiar with, like the garbage truck, helping broaden horizons and interests. Goodnight, Goodnight, Construction Site by Sherri Duskey Rinker and Tom Lichtenheld (Chronicle Books) tells the story of five truck friends as they finish up their site work for the day and go to bed. The rhymes here are slightly longer and include far more details about the purpose and parts of each truck and its role on the construction site. The focus here is again on construction rather than engineering design or problem-solving. The story provides an opportunity to learn about the individual trucks and discuss how they work

“Where do giant cranes sleep when they’ve lifted their last beams? Do their moms pick them up and rock them and wish them sweet truck dreams?” together, and the book is written more at a toddler or older toddler level than the others. A series of books have followed, including Mighty, Mighty Construction Site, Three Cheers for Kid McGear!, and Construction Site Mission: Demolition, which expands on the idea of job site teamwork and introduces more trucks to the crew, including a much needed female truck, as the first book, unfortunately, presents all five trucks as male. As a series, the “Scooping gravel, dirt, concepts presented give a nice overview of the and sand, Excavator complex construction field and equipment shapes the land.” at an age-appropriate level, potentially spark“A few more holes to ing future interest. With so many childig and soon he’ll roll dren’s books available, to bed beneath the it is easy to write off ones about engineering moon.” as “niche” or “gag gifts,” assuming they will only appeal to parents already working in the profession. Remember that these books are written for children and not just those whose mom, dad, aunt, or uncle are already exposing them to engineering. Kids are listening and learning about the world and opportunities through books. Let’s present them all with the opportunity to learn about engineering while they are young so that it is not new or scary when they are older. Next month, we will discuss books written for preschool and early elementary-aged

“Baby likes to build. Engineers future engineers. These focus more on the probdo, too.” lem solving, teamwork, and design process involved in engineering. The gender split also plays a more prominent

“What is Baby building? role in these books as well.■

A house! A house is a Linda Kaplan is a Project Engineer with structure. It has a Pennoni in Pittsburgh, PA. She has 2 daughters, ages 3½ and 1½. If anyone foundation, walls, and a roof.” has pictures of a Blue Excavator, the older one keeps asking. (lkaplan@pennoni.com)

structural LOADS A New and Unexpected Roof Snow Drift

By Michael O’Rourke, Ph.D., P.E., and Chris Letchford, Ph.D., F.IEAust, F.SEI, F.ASCE, CPEng

One could argue that drift loads are the most important snow load since they account for roughly 75% of all snow-related structural problems. The various types of roof snowdrifts are reasonably well understood. However, a new snowdrift was recently observed downwind of a run of roof-top refrigeration piping, which did not seem consistent with our current understanding of snowdrift formation. In this article, the drift formation processes for four common snowdrifts – leeward roof step drifts, gable roof drifts, windward roof step drift, and parapet wall/roof projection drifts – are reviewed as well as the apparent formation process for the new roof-top piping-run drift. Finally, an explanation for the apparent inconsistency between the new piping-run drift and common drifts is provided.

Figure 1. Growth of leeward roof step drift with time, t1 < t2 < t3. Wind from left to right.

Leeward Roof Step Drifts Gable Roof Drifts

Leeward roof step drifts are comparatively straightforward. The trapping efficiency (percent of windblown snow arriving at the geometric irregularity which remains at the geometric irregularity) is taken to be about 50%, based on water flume studies. The fetch is the horizontal extent of the roof upwind of the step. These drifts have a nominal right triangular shape throughout the drift formation process, with the peak drift depth located adjacent to the step, as shown in Figure 1. The drift slope is initially about 1 on 4 (14º), which is thought to be the average or typical angle of repose for drifted snow. This slope is maintained while the drift height is less than the step size. When the drift reaches the top of the step, the slope begins to flatten as the drift continues to grow downwind. When the slope reaches about 1 on 8 (7º), the drift shape becomes streamlined, and drift growth nominally terminates as the geometric irregularity has been eliminated. That is, absent the snowdrift, mean wind streamlines attach to the lower level roof at roughly 8 roof steps downwind.

Although often referred to as unbalanced loads, gable roof drifts are actually across-the-ridge leeward drifts. Water flume studies suggest that the trapping efficiency of gable roof drifts is nominally the same as that for leeward roof step drifts. In the American Society of Civil Engineers’ ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, the cross-sectional area of a gable roof drift is close to and based upon that for a leeward roof step drift with the upwind fetch being the eave to ridge distance for the gable. Gable roof drifts have a non-right triangular shape, as shown in Figure 2. Figure 2. Expected shape of gable roof drift. Wind from left to right. The top surface is more or less flat, with the bottom surface matching the roof slope and the downwind surface having a slope approximating the angle of repose of drifted snow. Although observed gable roof drifts have a non-right triangular shape described above, they are currently approximated in ASCE 7 by a rectangular surcharge, with an aspect ratio being a function of the roof slope – longer horizontal extent and shallower depth for lower roof slopes. Gable roof drifts typically do not form if the roof slope is too shallow (less than ½ on 12) or too steep (greater than 7 on 12). At the lower limit, the roof is so flat that there is no flow separation at the ridgeline, and the attached flow eliminates the aerodynamic shadow where the drift would form. This is generally consistent with the maximum angle of about 4° for the expanding portion of a venturi tube or diffuser to avoid flow separation. Note that the total change in roof pitch at the ridge (upslope at ½ on 12 to downslope at ½ on 12) is about 5°. At the upper limit (greater than 7 on 12), the roof is steeper than the maximum (unlikely to be exceeded) angle of repose for drifted snow (taken to be about 30°). In this case, there would be flow separation at the ridge, and an area of aerodynamic shadow would form downwind of the ridge, but the wind transported snow particle will either roll off or Figure 3. Windward roof step drift shape – drift at parapet wall similar. Wind from left to right. not “stick” to the steep roof.