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 and location.
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
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 being represented.
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:
– Overall length of bars bent at each end:
– 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
The variation in bending of a group of supposedly
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.
- Committee on Manual of Standard Practice, Manual of Standard Practice, 27th edition, Concrete Reinforcing Steel Institute, Schaumburg, IL, 2001, p. 6-2.
- ACI Committee 318, “Building Code Requirements for Structural Concrete (ACI 318-08) and Commentary,” American Concrete Institute, Farmington Hills, MI, 2008, pp. 89-90.
- 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.
- Birley, D., “The Tolerance Cloud,” Concrete International, V. 27, No. 6, June 2005, pp. 61-63.
- ACI Committee 315, ACI Detailing Manual—2004, SP-66(04), American Concrete Institute, Farmington Hills, MI, 2004, p. 43.