Detailing & Fabrication Tolerance Cloud


By Dick Birley, President of Condor Rebar Consultants, Inc.
First published in Concrete International Magazine, February 2006

Some practices can lead to problems

Because it's inherently difficult and expensive to fabricate reinforcing steel and formwork to precise dimensions, workers must place bars to avoid conflicts with items that simply can't be moved, such as openings and embedded hardware. Tolerances on placement, as well as fabricating and forming, are therefore needed to make concrete construction physically possible and economically practical.


AVOIDING CONFLICTS

In addition to this normal interplay of tolerances in the field, detailers sometimes use tolerances to avoid conflicts. For example, to simplify formwork construction, beams and girders are commonly designed with a common soffit elevation. However, if the engineering drawings don't show how to avoid crossing the beam and girder reinforcing bars in the same planes (Fig. 1), there will be a constructibility problem. In such a case, detailers might detail the girder stirrups to the maximum vertical dimension and the beam stirrups to the minimum vertical dimension. While this won't completely eliminate the problem, it can mitigate it significantly. Of course, the detailer must "flag" these dimensions as critical, and the fabricator must exercise special care to fabricate the stirrups precisely to the detailed dimensions (normal fabricating tolerances wouldn't be acceptable).

Occasionally, ironworkers find a run of lapped temperature bars in a slab that doesn't terminate with the proper embedment at the end support. Rather than trying to find some extra bars to extend the run, ironworkers may opt to adjust the splices within the run to their minimum length. Quite often, this will extend the run the few inches required to achieve proper embedment at the support.

I've seen fabricators, especially during times of steel shortages, adjust the length of certain bars by the plus or minus tolerance to get exact cuts out of stock bars. For instance, to obtain 13 ft 5 in. bars from a 40 ft bar, a fabricator would need to make two cuts—producing two 13 ft 5 in. bars and one 13 ft 2 in. crop bar (Fig. 2). If the fabricator adjusts the bar length to 13 ft 4 in., however, two cuts would produce three 13 ft 4 in. bars, and there would be no crop bar. If the ironworker installing the bars cooperates and adjusts the end locations of the bars by 1/2 in., everything will be within tolerance.

In my opinion, these examples are acceptable, practical uses of tolerances to solve specific constructibility issues. They do, however, require mutual cooperation among the trades involved and are recognized as one-off instances. Therefore, they should not be considered acceptable common practice.


CAUTIONS

Unfortunately, I've also seen practices that I consider to be abuses of tolerances. While these practices may be followed because of a perceived economic gain, such practices are often detrimental to other trades involved (and even to their own trade).

Although it might seem that routinely cutting all reinforcing bars to an acceptable tolerance of 1 in. less than the detailed length would save thousands of feet of reinforcement each year, this really isn't the case. Many of those 1 in. savings would actually lead to longer pieces of crop bars, and produce no real "savings." For instance, if 13 ft 3 in. bars were required and cut to the proper length, the fabricator would get three usable bars plus a 3 in. crop bar out of a standard 40 ft bar (Fig. 3). If the lengths were reduced by an inch to 13 ft 2 in., the fabricator would get three usable bars plus a 6 in. crop bar—nothing would be gained.

Apart from a perceived economic benefit, others argue that cutting bars to the detailed length inevitably produces some bars that are longer and that these longer bars cause clearance problems for the ironworker. It's a poor argument. It's true that the ironworker would have to be more careful while placing the longer bars than if they had been cut to the detailed dimension. However, if the fabricator is cutting the bars to the minimum dimension, normal variation will cause some bars to be shorter. These shorter bars will create excessive clearance problems that are not easily rectified.

In fact, the practice of cutting bars to the minimum length can actually turn out to be very costly. A few years ago, I toured a large construction project and noticed some ironworkers coupling 8 in. pieces of No. 14 bar to the tops of a number of column bars. I inquired, and found that these were the last of four lifts of reinforcement for the columns. Because the fabricator had cut each lift of No. 14 vertical bars short by the minus tolerance (2 in.), and because No. 14 bar splices must be coupled rather than lapped, the full run of each vertical bar was 8 in. short of the required embedment into the supported beam. Because the short run required additional short bars, field labor for end-preparation to install extra couplers, and the cost of the couplers themselves, the potential "savings" turned into major additional costs.

During my years as an ironworker, I occasionally encountered contractors who routinely used tolerances to reduce the thickness of slabs. As Fig. 1 illustrates, even with a full-thickness slab, the multiple layers of crossing reinforcing at beam-to-girder connections may force the ironworker to use the minimum cover. However, if the contractor has already formed the slab to the minimum thickness, even the minimum cover may not be possible, and problems such as surface cracking along the length of the bars can result. What was thought to be a savings in concrete often increased costs required for ironworkers as well as finishers.


DESIGN CONSIDERATIONS

Fabricating bars to minimum dimensions within tolerances frequently creates annoying or even significant problems for the unsuspecting ironworker. Because detailers calculate the lengths of bars, they are usually the first ones faulted for the problem and must present calculations to prove that their dimensions are correct. Detailers quickly get to know which fabricators are involved in the practice of shortcutting bars. To reduce or eliminate potential problems in critical situations, many detailers will dimension bars to maximum tolerances (or even longer).

Unfortunately, the person reviewing the detailing for the designer often spots these longer dimensions and marks them for correction. Of course, this presents a dilemma for detailers: do they correct the dimensions to satisfy the designer, or do they ignore the designer in the interest of better constructibility? Perhaps the best course of action for a designer is to allow longer dimensions, provided they will not cause a structural problem.

- A Paradigm Shift

- Constraints on Reinforcing Bar Modeling

- Reinforcing Bars Exceeding Stock Lengths

- Rebar and Waterstops

- Design to Minimum Dimensions

- Shearwalls & Boundary Elements

- Sloped vs Stepped Footings

- Calculating the Length of Bent Bars

- Beam-Column Joints

- Avoiding the Dead Zone

- Placing Drawings are not Shop Drawings


- The Tolerance Cloud

- Placement Tolerance Clouds

- Forming Tolerance Cloud

- Detailing & Fabrication Tolerance Cloud

 

Terms of Use | Privacy Policy © 2012 Condor Rebar Consultants Inc. All rights reserved