Design to Minimum Dimensions


By Javed B. Malik
First published in Concrete International Magazine, July 2007

Focusing on member size can defeat the purpose
Structural engineers generally strive to optimize the cost of structures, often by minimizing the sizes of structural members. An emphasis on minimizing the size of concrete members, however, can lead to unintended consequences that may defeat the global goal of minimizing the construction cost for the overall project. In short, it’s important to step back to consider how the individual components interact. Although this may seem rather basic, I’ve observed that problems occur often enough to warrant a reminder, especially for younger engineers and detailers.

ISSUES
Concrete members sized purely on the basis of applied loads may not be large enough to accommodate the required amount of reinforcing steel with the proper spacing between bars. Conflicts can be created by the reinforcement for the member in question, reinforcing bars from adjacent members, and embedded anchor bolts or headed studs. These conflicts can potentially lead to honeycombs and voids in the concrete, inadequate cover, and inadequate embedment. Designing individual members to minimum dimensions can also create a large number of similar, but not identical, members. This can significantly impact cost by limiting reuse of the formwork and reducing the efficiencies of workers and inspectors.

EXAMPLES
The following are some common examples where designing to minimum overall dimension can create problems. Addressing these and similar issues during the design phase saves time, reduces requests for information as well as change orders, and avoids headaches for both the contractor and the engineer.

Piers and pier caps
Sizing lightly loaded piers considering only the applied loads and allowable soil bearing capacity can result in relatively small piers. For piers supporting steel columns, this can create a conflict such as shown in Fig. 1(a), where the anchor bolts or bearing plates will not fit inside the steel cage for the pier. Obviously, this can be resolved by increasing the pier diameter as shown in Fig. 1(b), or a wider pier cap at top of piers can be installed to accommodate the anchor bolts as shown in Fig. 1(c). To minimize the number of pier sizes installed at a site, it’s preferable to change the shaft diameters in increments of at least 6 in. (150 mm) and the bell or under-ream diameters in increments of at least 12 in. (300 mm).

Spread footings
To minimize the number of different footing types, the length or width should be changed in minimum increments of 12 in. (300 mm). Before finalizing the footing thickness, the depth required to develop the column dowels or embed anchor bolts for steel columns should be checked because it may control the footing thickness (Fig. 2). An alternative to thickening the entire footing is to locally thicken it at the column location. For practical reasons, a minimum thickness of 12 in. (300 mm) is suggested.

Grade beams
To eliminate formwork, the sides of grade beams are often placed against earth, requiring a clear concrete cover of at least 3 in. (75 mm). To accommodate bend diameters at the corners of stirrups in grade beams, it’s good practice to use a minimum grade beam width of 12 or 15 in. (300 or 380 mm) as shown in Fig. 3. If the sides of the grade beams are formed, clear cover on the stirrups can be reduced to 1-1/2 in. (40 mm), and the grade beam can be made narrower. In these cases, a note should be added on the drawings requiring the contractor to increase the beam width by 1-1/2 in. (40 mm) on each side if the decision is made to eliminate forms.

Columns
It’s good practice to standardize column sizes on a job as much as possible. Ideally, all interior columns should be of one size and exterior columns of another size, if necessary. This will simplify the formwork and steel placement. It’s generally economical to keep the same column sizes for as many floors as possible and use higher strength concrete and more longitudinal reinforcement on the lower floors.

Beams
Beam dimensions, especially depth, should also be standardized on a job. It’s generally economical to use the same depth for all beams at a floor except for heavily loaded girders or spandrel beams. As shown in Fig. 4, making the beams slightly wider or narrower than the columns can help prevent interference between beam bars and vertical column bars. Although beams that are wider than the columns may be preferred to simplify formwork, the designer must also check the beam-column joint for any special reinforcement required in special moment frames for seismic applications.

Walls
Designing to the minimum thickness for walls can produce several problems. Walls are not only reinforced with vertical and horizontal steel, but sometimes have ties enclosing the vertical steel such as at boundary elements. In addition, bars from slabs, floor beams, and link beams terminate in the walls. As shown in Fig. 5, link beam bars placed in several planes can further complicate the placement and congestion of the reinforcement. If these issues are not carefully considered during design, the wall can become heavily congested at locations where several elements intersect and make it very difficult to place the bars and consolidate the concrete properly.

Tilt-up wall panels
For tilt-up walls, panel thickness is often set at about 1/48th the vertical span of the wall.[1] It’s important to note, however, the effect of architectural reveals on the net wall thickness. This is needed not only for design, but also for detailing. For example, to ensure that the wall is thick enough for embedment plates with headed studs, designers must verify that sum of the plate thickness, the stud length, and the cover on the end of the studs doesn’t exceed the net wall thickness (Fig. 6).

Using double mats of reinforcing can significantly increase the moment capacity as well as the cracked moment of inertia (and thus, the axial capacity of slender wall elements). For panels thinner than 6 in. (150 mm), however, double mats of reinforcement are not preferred as they will be located nearly on top of each other. Finally, note that a standard hook may not fit well in a thin wall panel, so it may be necessary to place the hook in the plane of the wall or use welded-bar mats.

STEPPING BACK
Although the size of structural members must be appropriate for the applied loads and material properties, this should only be considered the starting point. By simply taking a step back and looking at how various elements interface with one another, the issues discussed in this article and other, similar issues can often be easily found and corrected. Making this a continuous process during design and detailing can help avoid having to redesign elements when conflicts are found, and it can lead to a better understanding of how the structural elements interact as a whole.

Acknowledgments
The author is thankful to the members of ACI Committee 315-B, Details of Concrete Reinforcement—Constructibility, for their valuable suggestions and contributions.

References
“Tilt-Up Construction and Engineering Manual,” 6th Edition, Tilt-Up Concrete Association, Mount Vernon, IA, Aug. 2006, p. 9-2.


 

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