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