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Civil Drafting Basics

Understanding The Plan Types

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Civil Drafting Basics

A Good Way To Visualize Topographic Contours

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Maps

The most basic form of civil drafting is the map. A map is an aerial view of the physical structures, legal lot designations, property lines, zoning conditions and property boundaries in a given location. In general, there are two types of map data: existing and proposed. Existing mapping conditions are legal verifications of all existing boundaries and facilities within a designated area. They are usually created by a survey firm/group and the information shown on the map is verified accurate by a Professional Land Surveyor. The proposed map is most often overlaid on top of an existing survey map to show areas of new construction/design and the necessary alterations to the existing conditions that the proposed work will entail.

The existing “basemap” is created using a collection of data points that are taken by a survey crew in the field. Each point consists of five pieces of data: Point Number, Northing, Easting, Z-elevation, and a Description (PNEZD). The point number differentiates each point, and the Northing/Easting values are Cartesian coordinates in a particular map zone (state plane for example) that show exactly where in the real world the point shot was taken. The “Z” value is the elevation of the point above a set location, or “datum” that is preset for reference. For example, the datum can be set for zero (sea level), or an assumed datum (such as a building foundation) can be assigned a random number (ie. 100) and the elevation of the points are taken in reference to that. If the assumed datum of 100 is used and a point taken at the bottom of a driveway apron reads as 2.8’ below that level, the “Z” value of the point is 97.2. The Description value of a data point refers to the object being surveyed: building corner, top of curb, bottom of wall, etc. These points are brought into CAD/Design software and connected, using 3D lines, to generate a Digital Terrain Model(DTM), which is a 3D representation of the existing site conditions. Design and grading information can then be extracted from that model. 2D line work, such as building outlines, curbs, drives, etc. are drawn for plan presentation, using the coordinate information from the surveyed points. Bearing/distance for all property lines are added to the basemap, as well as location information for all pins/markers and any existing rights-of-way, etc.

Design work for new maps is done on top of a copy of the existing basemap. All new structures, their sizes and locations, including dimensions to existing property lines and offsets is drawn in as 2D line work. Additional design information is often added to these maps, such as Signage, Striping, Curbing, Lot Annotations, Setbacks, Sight Triangles, Easements, Roadway Stationing, etc.

Topography

Topographic plans are also designated in existing/proposed formats. Topography makes use of contours, spot elevations, and various structures labeled with their elevation (such as the Finish Floor of a Building) to represent the three dimensions of the real world site on a 2D plan drawing. The primary tool of representing this is the contour line. Contours lines are used to connect a series of points on a map that are all at the same elevation. They are usually set to even intervals, (such as 1’, or 5’) so that, when labeled, they become a quick visual reference as to where a site’s elevation goes up/down and at what severity of slope. Contour lines that are close together indicate a rapid change in elevation, whereas those farther apart represent a more gradual change. The larger the map, the larger the interval between contours is likely to be. For example, a map that shows the entire state of New Jersey will not display 1’ contour intervals; the lines would be so close together that it would make the map unreadable. It would be much more likely to see 100’, possibly even 500’ contour intervals on such a large scale map. For smaller sites, such as a residential development, 1’ contour intervals are the norm.

Contours show steady ranges of slope at even intervals but that is not always an accurate rendition of what a surface is doing. The plan may show a large gap between the 110 and 111 contour lines and that represents a steady slope from one contour to the next, but the real world rarely has smooth slopes. It is far more likely there are small hills and dips between those two contours, which do not rise/fall to the contour elevations. These variations are represented using the “spot elevation”. This is a symbol marker (usually a simple X) with an associated elevation written beside it. Imagine that there is a high point for a septic field in between my 110 and 111 contours that has an elevation of 110.8; a “spot elevation” marker is placed and labeled at that location. Spot elevations are used to provide additional topographic detail between contours, as well as at the corners of all structures (building, drainage inlets, etc.)

Another common practice on topographic maps (particularly proposed maps) is to include a “slope arrow” on surfaces that need to meet specific construction code criteria. Slope arrows show the direction and percentage of slope between two points. You commonly use this for driveways, to show that the percentage of slope from top to bottom meets the “walkable” criteria of the governing ordinance.

Roadway

Roadway plans are initially developed based on access needs of the site combined with the requirements of the local construction ordinance. As an example, when developing the roadway design for a subdivision, the layout is developed to maximize buildable properties within the overall site while still conforming to the requirements of the traffic ordinance. Traffic speed, lane size, the need for curbing/sidewalks, etc. are all controlled by the ordinance, while the actual layout of the road can be adapted to the needs of the site. The design begins by establishing a roadway centerline off of which all other construction items will be built. Design concerns along the centerline, such as the length of horizontal curves, need to be calculated based on control items such as the traffic speed, needed passing distance and sight clearances for the driver. Once these are determined and the centerline of road established in plan, items such as curbing, sidewalks, setbacks and rights-of-way can be established using simple offset commands to establish the initial corridor design.

In more complex design situations, you need to take into consideration items such as superelevation around curves, transitioning road and lane widths, and hydraulic flow considerations at intersection and on/off ramps. Much of this process needs to take the percentage of slope along both the sectional and profile lengths of the road, but that's an article for another day!.

Drainage

At the end of the day, all civil design is essentially about controlling the flow of water. All the many design elements that go into a full scale site are all predicated upon the need to keep water from flowing to and/or ponding in locations that will damage your site and instead directing it toward the locations you design for storm water collection. Common methods of drainage control are through the use of storm water inlets: below ground structures with open grates that allow water to flow into them. Theses structures are connected together by pipes of varying sizes and slopes to create a drainage network that allows the designer to control the amount, and flow rate, of the collected water and direct it toward regional collection basins, existing public drainage systems, or possibly into existing watersheds. The most commonly used inlet structures are called Type B and Type E inlets.

Type B Inlets: are used in curbed roadways, they have a cast metal back plate that insets directly into the curb and the grate sits flush with the top of pavement. Road drainage is directed from the crown of road (centerline) toward the curbs and the gutter line is then sloped toward the B-Inlet. This means the water flows from the center of the road, down to the curb on either side, then flows along the curb and into the inlets.

Type E Inlets: are essentially concrete boxes with a flat grate on top. They are used primarily in flat areas where there is no curb to control water flow, such as parking areas or open fields. The open area is designed so that there are E-Inlets at low points in the topography, where all water will naturally flow. In the case of a parking lot, the grading is carefully designed with ridge and valley lines, to direct all runoff to the inlet locations.

Beyond controlling the surface runoff, the designer has to account for how much water can collect in a given drainagenetwork and at what rate it will flow out to its final destination. This is done through a combination of inlet and pipe sizing, as well as the percentage of slope between structures that control how quickly water will flow through the network. In a gravity drainage system, the steeper the slope of the pipe, the more quickly the water will flow from structure to structure. Likewise, the larger the pipe size, the more water that can be held inside the pipes before it starts to overload the network and backflow into the streets. When designing a drainage system, the area of collection (what amount of surface area is collected into each inlet) also needs to be carefully considered. Impermeable areas, such as roads and parking areas, naturally generate more flow than permeable areas such as grass fields, where seepage accounts for a large portion of the water control. You also need to take into account the drainage areas of existing structures and regions and make sure that any alteration of their process is accounted for in your proposed design.

See? Nothing here to be scared of, just simple common sense applied to the needs of the CAD design world. What do you think: ready to jump into the civil CAD world now?

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