Concerns over ’fiber reinforcement’

I have some issues with the article published in the December 2007 edition of Structural Engineer, entitled, "Following the rules of concrete construction: A new standard allows use of synthetic macro fiber reinforcement on steel decks."

With regard to the statement in the fourth paragraph—"Typically, a 20-percent overlap of wire fabric sheets is needed, resulting in wasted material"—the writer(s) are correct if a structural splice considers two wire spaces at 6-inch-plus overhangs, but that is an old specification. Currently, ACI 318 states a structural splice to be one space plus 2 inches, which calculates to 10 percent additional square feet of WWR (in fact, the material is called welded wire reinforcement today, and is now recognized in current codes and standards as such).

We see many composite deck projects where tables in decking supplier’s catalogs are used by engineers. Those catalogs state that the reinforcement gives capacity to the slabs for unknown applied loads. Consequently, if fibers are then allowed to replace the WWR, the strength of these slabs will decrease (or not exist at all) up to the instant just prior to concrete cracking. When engineers and contractors believe that the WWR is just for temperature and shrinkage steel reinforcing and allow it to be replaced with fibers, then they are overlooking the fact that reinforcement provided by steel is used for more than just shrinkage and temperature stresses. In many slab/deck designs, it is used to carry applied loads.

Another issue: The writer(s) are discussing the visual cracking with the macro fiber in the early stages of shrinkage (plastic shrinkage), the statement was made "…it was the most crack-free job I’ve ever seen." In the next paragraph it is mentioned that the facility is being built; so it appears that the writers have not viewed long-term cracking in the project (drying shrinkage), only plastic shrinkage in the early placement of concrete is occurring. Drying shrinkage will continue for up to one year and even longer, and generally crack widths are greater. My experience has been that if steel reinforcing does not exist, wide cracks develop and displacement occurs. I believe that fibers have provided an improved quality of concrete mix design today, but it is a stretch to label fibers as concrete reinforcement.

We hear from many contractors after they have placed plain concrete slabs or with some volume of fibers in it (rarely specifying a length or volume of fibers) and after drying shrinkage of over a year, cracks are wide and displacement occurs. They tell us that slabs supported on metal deck and slabs-on-ground that contain fibers have more failures than successes and that they plan to place steel-reinforced concrete slabs in the future.

The article is a reminder of some other issues of note. Fibers here, and in other articles, are labeled as concrete reinforcement, which suggests that fibers will perform the same function as steel reinforcement. Steel reinforcing provides a tensile strength to help distribute loads in the concrete both concentrated and uniform, and as mentioned before, that it tends to hold cracks together and help resist displacement. But at the same time, if the reinforcing is too close to the decking or laid directly on top of it, or is placed on supports that provide less than 1-1/2 to 2 inches of cover, the steel reinforcing may cause the cracking or not restrain the concrete from developing wider cracks. Additionally, I believe that slabs on metal decks should be thicker than 3 to 3-1/2 inches in depth to provide proper cover when placed on supports.
Roy H. Reiterman, P.E., FACI
Technical consultant, Wire Reinforcement Institute, Inc.


Dispute Resolution Board

[Regarding "Keeping a claim out of court" by Michael J. Baker, Esq., in the January issue of Structural Engineer, page 15], I suggest that greater consideration of use of the Dispute Resolution Board (DRB) process be employed on complex projects prone to future claims by the contractor of "changed site conditions." In my experience as DRB chair and as a member on several projects, I have "batted 1000" by deciding disputes without attorneys and avoiding the costly legal process. The advantage inherent in the DRB process is that the engineering- and construction-experienced board is active from the start of construction; meeting quarterly before problems arise and thereby knowledgeable of the works, design, construction techniques, and personnel. This allows for a rapid solution of issues when they occur without the usual time investment of the owner and contractor’s staff when the legal arbitration or mediation process commences. The DRB costs are minimal when compared to the alternate methods of resolving disputes.
Edward S. Plotkin, P.E., F.ASCE


Technical time capsules

I have been following the coverage of the collapse of the I-35W Bridge that occurred on Aug.1, 2007. I am shocked and dismayed that the design calculations for a major modern bridge were lost. There was no earthquake or hurricane or even some kind of excessive loading as evidenced so far. [In this type of failure], the design calculations must be part of the investigation process in that they, too, must be reviewed and inspected. Design assumptions, loads, technique, comments and resolutions, and final as-builts (and inspection reports) must be archived properly for the simple fact that, in most cases, a structure will last longer than all the entities involved except for the public agency. As data storage becomes more compact (CDs, DVDs, hard drives) and digital storage becomes even cheaper, there is no excuse not to have consistent policy of archiving the design process and details for all public projects.

I am suggesting that a "technical time capsule" of the calculations must be kept in secure locations—maybe at multiple locations—known to the appropriate parties! Some time ago, I suggested to the Washington State Board of Registration that all public structures have a technical time capsule that is available to both the user of the structure and the owner for inspections and modifications as necessary. To have a major freeway structure fall down should be a wake-up call for more attention to this issue. The costs are just beginning to be incurred with this incident for we now will have the lawyers involved for years to come; on the contrary, as compared to the legal costs, the expense of archiving design documents—including the structural calculations—as well as dollars associated with checking or verifying the original design assumptions during the project’s maintenance and lifetime, will be minuscule.
Gene Grieve, P.E.


A good start

"Understanding adhesive anchors: How to use and select products suitable for long-term sustained load applications," by Ryan Vuletic, P.E., and John Pearson, P.E., S.E., which appeared in the January 2008 issue of Structural Engineer, was a good article as far as it went. Having spent many years providing expertise in structural adhesives to resolve and prevent anomalies for Air Force space systems, there is so much more to the issue that has not been covered. Your audience may be interested in an article I wrote for the Composites & Adhesives Newsletter’s FLASH News Report last August.

Editor’s note: Click here to read the full article, which touches on some of the factors.
George Epstein
Adhesives and composites consultant

A motto to work by

I have been a structural engineer for more than 30 years, and have seen poor design and very heavy steel used for columns and beams and no space for concrete mix to pour. It seems that every time there is a code change, the structural members are becoming larger and stronger than the existing structure standing next to the new one, and it is hard to convince the owner why it will cost more.

[At left] is a clip from a past issue of Structural Engineer magazine that I keep hanging on wall [in my office] so any visitor can read and think.
Thank you very much for keeping us informed [regarding "The need for adequate field experience," by Richard Weingardt, P.E., December 2007, page 50].
Mohan Gadhia


The future of civil engineering

I am writing in response to Richard Weingardt’s column in the February 2008 issue of Structural Engineer, which is focused on defining our profession for the future. As you might already know, ASCE has a long-term effort underway to define the role of civil engineering in the future. It is called, "The Vision for Civil Engineering in 2025". This effort started with a Summit on the Future of Civil Engineering in June 2006. The report from that event is available as a free download at: http://content.asce.org/files/pdf/TheVisionforCivilEngineeringin2025_ASCE.pdf
Stan R. Caldwell, P.E., F.ASCE, F.AEI
Chair, ASCE Technical Region Board of Governors
Member, ASCE Executive Committee & Board of Direction