By Dick Birley, President of Condor Rebar Consultants, Inc.
First published in Concrete International Magazine, November 2008
In a three-dimensional (3-D) model, structural, mechanical,
electrical, and architectural components can be
brought together to form a composite, digital prototype.
These models can be used to resolve conflicts and
interference issues between components earlier in the
design phase more than ever before.
The greatest success with 3-D modeling is usually
found for components with very precise tolerances. For
instance, the tolerance for the outside diameter of steel
pipe is measured in thousandths of an inch, and the pipe can be located in the field to within fractions of an inch. If
a pipe is accurately modeled, it can be a reasonably true
representation of the real pipe in the structure. Software that can analyze a composite model and find interferences
produces the greatest benefit in terms of conflict analysis
and resolution when the 3-D model contains these
types of components with tight tolerances on dimensions
Given their successful experience with these elements,
modelers often assume they will have similar success
with 3-D modeling of reinforcing bars. Unfortunately, this
is not necessarily the case.
3-D bar modeling Problems
Reinforcing bar models are great for illustrating the
general layout of the bars, especially in congested areas,
and are useful for identifying possible problem areas
that might require special attention from the detailer,
fabricator, or placer. The range of tolerances for the
manufacture, fabrication, and placement of reinforcing
bars, however, is larger than for most other components
in the construction industry. Therefore, current reinforcing
bar models tend to be only general representations of
the bars, not accurate or realistic representations. A
3-D model using precise dimensions for reinforcement
without considering tolerances, therefore, cannot be
used in the same manner as models for other components.
Reinforcing bars are manufactured to dimensions
specified in ASTM standards.
The nominal bar diameter
excludes the deformations and is used to calculate the
cross-sectional bar area for design and weight calculations.
When considering constructibility, the important dimension is the overall diameter of the bar, which is measured to
the outside of the deformations. Overall reinforcing bar diameters from the CRSI Manual of Standard Practice1 are
shown in Fig. 1.
Using a No. 11 (No. 36) bar as an example, the nominal
diameter is 1.41 in. (35.8 mm), but the overall diameter
is 1.625 in. (40 mm) or 15% larger than the nominal
diameter. The overall diameter, however, is also
approximate for at least two reasons. First, if the
reinforcing bar was rolled at a mill using worn rolls, the
overall bar diameter will tend to be larger. Second,
deformation patterns used by some mills have higher
deformations and, therefore, larger overall diameters,
than other patterns. For the No. 11 (No. 36) bar in this
example, at least another 1 or 2% difference between
nominal and overall diameter should be allowed to accommodate for these possibilities.
Some modelers use the nominal diameter for the bar
size. Clearly, those models are not as accurate as a model
that uses the overall diameter relative to the actual bar
Reinforcing Bar Fabrication Tolerances
Reinforcing bar fabrication and placement is not
precise. This is reflected by the liberal fabrication and
placing tolerances specified by ACI Committees 318,
Structural Concrete Building Code, and 117, Tolerances.
Trying to model bars in a structure without incorporating these tolerances will produce a graphic model with much
less precision than expected.
Here are a few of the tolerances that, if not considered,
can affect the accuracy of the model:
- Sheared length:
No. 3 to No. 11 = ±1 in.;
(No. 10 to No. 36 = ±25 mm);
No. 14 and No. 18 = ±2 in.;
(No. 43 and No. 57 = ±50 mm);
- Overall length of bars bent at each end:
No. 3 to No. 11 = ±1 in.;
(No. 10 to No. 36 = ±25 mm);
No. 14 = ±2.5 in.;
(No. 43 = ±65 mm);
No. 18 = ±3.5 in.;
(No. 57 = ±90 mm); and
- Angular deviation on 90-degree bends:
The designer cannot assume that the dimensions of the
modeled 3-D bar will be the same as those for the actual
bar in the structure. If an adjustment is made to a particular
bar to solve an interference problem in the graphical model,
it will not necessarily translate to the actual placed bar.
Reinforcing bar bends are also difficult to accurately
model. Bends are typically modeled with a constant radius. For the actual bar, the bend radius varies
depending on whether the bar is bent normal to the plane
of the main longitudinal ribs or parallel to the plane of the
longitudinal ribs. Many other factors affect the curvature
of bends, such as the speed of the bending machine or the
state of wear of the mandrel that the bar is bent around.
The variation in bending of a group of supposedly
identical bars can be significant.
For example, consider
the No. 8 (No. 25) bar with standard hooks at each end.
Figure 2 represents the perfect bar case. Figure 3 shows
the possible geometric variations for the bar in Fig. 2 that
are within acceptable fabrication tolerances. Based on
the potential variations shown in Fig. 3, Fig. 2 is not an
accurate representation of reality.
The issue of finished bend diameter is another concern
for the reinforcing bar modeler. The ACI Detailing
Manual–20045 contains the recommended end hook
dimensions shown in Fig. 4. The “D” dimension is the
finished bend diameter for various bar sizes. A springback
effect is mentioned that will produce a finished
bend diameter slightly larger than the pin that the bar
was bent around.
As an example, the finished bend diameter for a No. 11
(No. 36) bar is 12 in. (305 mm). A reinforcing bar modeler
could reasonably be expected to use this dimension to
model bent No. 11 (No. 36) bars. The finished bend
diameters listed, however, are minimum dimensions. To ensure their bars are bent to the proper minimum
diameter, many fabricators use the “D” dimension as the
actual size of their mandrels. If the 3-D model is used to
investigate interferences, this potential dimensional
difference can be significant, especially for larger bars.
The longitudinal ribs on reinforcing bars present
another complicating issue. First, when bars are rolled in
the mill, they pass through the rolls onto the rolling
(cooling) bed, gradually twisting to the right or left. The
result is that the longitudinal ribs tend to spiral around
the longitudinal axis of the bar. This spiraling of the
longitudinal ribs can have an effect on fabrication.
A bend made at right angles to the plane of the main
ribs will be tighter than a bend made parallel to the plane
of the main ribs. The difference can be significant. The
first bend on a bar can always be bent at right angles to the plane of the main ribs, but due to the spiraling of the
ribs, there is no guarantee subsequent bends on the same
bar will be at right angles to the plane of the longitudinal
ribs. In fact, there is a high probability that they won’t.
Thus, there can be two different bend diameters on the
actual bar, while identical bend diameters would be
assumed in the 3-D model.
Per ACI 117-06, the tolerance on cover varies from
–1/4 to –1/2 in. (–6 to –13 mm), depending on the member
thickness. This is unlikely to be included in a reinforcing
bar 3-D model. The tolerance for bar spacing in slabs
and walls is ±3 in. (±76 mm). Moving bars in a model
fractions of an inch to avoid a perceived interference in
the model doesn’t accurately reflect field conditions.
A journeyman ironworker has the knowledge, tools,
and skill to solve in the field almost all of the minor
interferences encountered in a model.
Use with caution
Reinforcing bar 3-D modeling is a beneficial tool for
providing a general perspective of the bar arrangement
in a structure, and how the bars may interface with the
other components. While many components are reflected
in a relatively precise manner, reinforcing bar fabrication
and placement typically is not. Software that detects and
identifies interface conflicts is useful for resolving
problems in most components because the model closely
reflects reality. Because many bar models don’t accurately
depict real conditions, however, resolving bar fabrication
and placement problems in a graphical computer model
is not recommended and should be used with caution.
1. Committee on Manual of Standard Practice, Manual of
Standard Practice, 27th edition, Concrete Reinforcing Steel Institute,
Schaumburg, IL, 2001, p. 6-2.
2. ACI Committee 318, “Building Code Requirements for Structural
Concrete (ACI 318-08) and Commentary,” American Concrete
Institute, Farmington Hills, MI, 2008, pp. 89-90.
3. ACI Committee 117, “Specifications for Tolerances for Concrete
Construction and Materials and Commentary (ACI 117-06),”
American Concrete Institute, Farmington Hills, MI, 2006, 70 pp.
4. Birley, D., “The Tolerance Cloud,” Concrete International, V. 27,
No. 6, June 2005, pp. 61-63.
5. ACI Committee 315, ACI Detailing Manual—2004, SP-66(04),
American Concrete Institute, Farmington Hills, MI, 2004, p. 43.
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