Mentors and mentoring, More FRC comments
Mentors and mentoring
Richard G. Weingardt, P.E., has a tendency to write articles in his column, "The View From Here," that discuss exactly what I am thinking about. His articles have been my favorite for years, and when I get my issue of Structural Engineer, I immediately turn to the back page.
I’ve been a structural engineer for 3 years, and I would like a mentor to help me make the right career choices and advise me. I’m at a critical time in my career; I am ready to get my P.E., yet I am not sure I’m working the best place. I need some guidance, someone to talk to. I have been looking for a mentor all week and it is easier said than done. The only thing I found was ASCE’s E-mentors, which I will try out, but I would really like to sit down, face-to-face, with someone local.
I have contacted a few engineers in my field that I have met through various jobs, asking them if they would like to mentor me. Being a young engineer, I don’t know the etiquette for finding a mentor. I don’t know if outright asking someone is considered unprofessional or not. I guess a mentor would teach me that.
Do you have any advice on the best places for a structural engineer to find mentors in his or her area?
Robert J. Naab Jr., E.I.T.
Finding and working with mentors is a very personal thing and everyone needs to craft their own life plan on how to best do it. No one plan fits all because every engineer has different goals and ambitions. Plus, there are several different types of mentors. There are not set rules on how to approach them, but the direct approach is always a good way to start. Just go up to them and tell them what you’re after and ask if they would be so kind as to help you.
One type of mentor is someone who can assist you in best doing your job, advancing in your company, and becoming the best technical engineer possible. These are usually your immediate boss or a colleague at work. You can also get guidance with this by visiting with former college classmates who may be working at some other firm or professors savvy about the real world—and by participating in engineering societies where you can mingle with others in your same situation. The ASCE E-mentors program that you mentioned has good potential and I recommend that you follow up on that. Other engineering groups, such ACEC and NSPE, have similar programs for emerging engineers and committees you can participate on that will expose you to a wide array of knowledgeable mentors and role models.
Mentors that can help you with other aspects of your career and the different directions your life can take don’t necessarily have to be engineers. They can be your parents, your spouse, a favorite uncle or aunt, a minister, or earnest neighbor. Anyone you respect and feel comfortable with, and can openly discuss pros and cons with, will helpful with this part of planning your future.
Richard G. Weingardt, P.E
More FRC comments
I am writing to comment on a recent letter you published concerning the December 2007 article on the use of fibers as temperature and shrinkage crack control reinforcement in composite steel decks. In his letter printed in the March issue of Structural Engineer, Mr. Reiterman expresses two major areas of concern: structural strength and crack control.
Structural strength
The writer asserts that the "strength… will decrease (or not exist at all) up to the instant just prior to concrete cracking…". This statement is not accurate as describing the performance of either welded wire (WWR) or fiber reinforcement (FRC) in concrete.
For WWR and for most, not all, formulations of FRC, the concrete Modulus of rupture (MOR) exceeds the strength provided after cracking has occurred. (Certain higher volume fiber fractions of many types of fiber exhibit increased strength after cracking in what is known as strain hardening behavior). Thus cracking must, and will generally, occur before any reinforcement is engaged in a stress transfer mechanism across a crack and, again for WWR and for certain FRC’s, there will be an accompanying sudden reduction in structural stiffness (increase in compliance) associated with the cracking event.
However, as noted in the paper in Concrete International V13, No.11, which I co-authored with C. D. Hays, either a single layer of WWR, depending on the placement within the depth of the slab, or uniformly distributed fibers, depending on the fiber type and volume fraction, will conduct post cracking flexural tensile stress, often referred to as residual strength (RS), with varying reliability. This fact is readily demonstrated by simply applying a post cracking flexural strength test, such as ASTM C1399 or ASTM C 1609. These tests, which were designed to determine the RS capacities for cracked beams having alternative FRC formulations but can as well apply to cracked WWR samples, provide conservative estimates of post cracking strength and ductility. The test samples are arranged so as to eliminate arching in cracked beam samples and by doing so they directly demonstrate the stress transfer ability of whatever reinforcement, wires or fibers, are bridging the cracks.
It is certain that the resulting RS and stiffness of WWR slabs will depend on the relative location of the WWR within the slab depth and that steel cover requirements would apply. These restrictive requirements do not apply to fiber reinforcement. For FRC there are no cover limitations and the fiber formulation, ie., fiber type and volume fraction, can be specified, assessed and assured, based on the aforementioned ASTM test standards for cracked beam performance evaluation. From an engineering point of view, the reliability of using WWR cannot be assured when compared to that of FRC.
Volume change cracking
The writer correctly points out the differences between plastic and drying shrinkage cracking. The writer also correctly acknowledges the early age benefits of fibers and there is no longer any real debate as to the fact that a random dispersion of fiber in a concrete matrix will act to suppress and control plastic and early age shrinkage cracking in ways in which WWR has no possible contribution. But he does not acknowledge how this translates to performance in the ability of fibers to control long term drying shrinkage cracking. In fact, whether or not either WWR or FRC is effective in preventing or just reducing long term cracking in slabs is not the issue. In either case, WWR or FRC, the issue is whether or not each system can provide the RS in bridging cracks to accommodate shear transfer through aggregate interlock across crack surfaces, both for shear force applied perpendicular to slabs and in-plane as for diaphragms,. This leads to the consideration of several important factors that relate to the effectiveness of aggregate interlock including crack size, crack distribution, and again RS reliability.
In the case of composite decks, aggregate interlock is reasonably assured by the usually corrugated deck system and the debate as to performance of WWR compared to FRC is simply reduced once again to crack control and RS reliability. On this basis there is no real advantage to using WWR in comparison to FRC and the decision to choose one over the other system is reduced to economics and constructability issues.
As to terming either WWR or fibers as "reinforcement", this terminology is used in connection with both systems and is influenced by historical development and tradition more than by anything else. The fact is that neither system is reinforcement in the structural sense. But fibers are reinforcement for the concrete matrix, both in the plastic state during mixing and placement as well as in the hardened state, assuring an enhanced concrete material. More apt terminology would be fiber modified concrete (FMC) in which, by comparison, WWR amounts to a poor distribution of materials from a reliability point of view.
Conclusion
Based on the evidence of performance of fiber reinforcement (FRC) in concrete in slabs—meaning engineered slabs for which QA and QC controls were appropriately applied and not based on anecdotal commentary—there is no rational reason that FRC, depending on the fiber type and volume fraction, should not be qualified a temperature and shrinkage slab reinforcement.
Ronald F. Zollo, Ph.D., P.E., FACI
University of Miami
Richard G. Weingardt, P.E., has a tendency to write articles in his column, "The View From Here," that discuss exactly what I am thinking about. His articles have been my favorite for years, and when I get my issue of Structural Engineer, I immediately turn to the back page.
I’ve been a structural engineer for 3 years, and I would like a mentor to help me make the right career choices and advise me. I’m at a critical time in my career; I am ready to get my P.E., yet I am not sure I’m working the best place. I need some guidance, someone to talk to. I have been looking for a mentor all week and it is easier said than done. The only thing I found was ASCE’s E-mentors, which I will try out, but I would really like to sit down, face-to-face, with someone local.
I have contacted a few engineers in my field that I have met through various jobs, asking them if they would like to mentor me. Being a young engineer, I don’t know the etiquette for finding a mentor. I don’t know if outright asking someone is considered unprofessional or not. I guess a mentor would teach me that.
Do you have any advice on the best places for a structural engineer to find mentors in his or her area?
Robert J. Naab Jr., E.I.T.
Finding and working with mentors is a very personal thing and everyone needs to craft their own life plan on how to best do it. No one plan fits all because every engineer has different goals and ambitions. Plus, there are several different types of mentors. There are not set rules on how to approach them, but the direct approach is always a good way to start. Just go up to them and tell them what you’re after and ask if they would be so kind as to help you.
One type of mentor is someone who can assist you in best doing your job, advancing in your company, and becoming the best technical engineer possible. These are usually your immediate boss or a colleague at work. You can also get guidance with this by visiting with former college classmates who may be working at some other firm or professors savvy about the real world—and by participating in engineering societies where you can mingle with others in your same situation. The ASCE E-mentors program that you mentioned has good potential and I recommend that you follow up on that. Other engineering groups, such ACEC and NSPE, have similar programs for emerging engineers and committees you can participate on that will expose you to a wide array of knowledgeable mentors and role models.
Mentors that can help you with other aspects of your career and the different directions your life can take don’t necessarily have to be engineers. They can be your parents, your spouse, a favorite uncle or aunt, a minister, or earnest neighbor. Anyone you respect and feel comfortable with, and can openly discuss pros and cons with, will helpful with this part of planning your future.
Richard G. Weingardt, P.E
More FRC comments
I am writing to comment on a recent letter you published concerning the December 2007 article on the use of fibers as temperature and shrinkage crack control reinforcement in composite steel decks. In his letter printed in the March issue of Structural Engineer, Mr. Reiterman expresses two major areas of concern: structural strength and crack control.
Structural strength
The writer asserts that the "strength… will decrease (or not exist at all) up to the instant just prior to concrete cracking…". This statement is not accurate as describing the performance of either welded wire (WWR) or fiber reinforcement (FRC) in concrete.
For WWR and for most, not all, formulations of FRC, the concrete Modulus of rupture (MOR) exceeds the strength provided after cracking has occurred. (Certain higher volume fiber fractions of many types of fiber exhibit increased strength after cracking in what is known as strain hardening behavior). Thus cracking must, and will generally, occur before any reinforcement is engaged in a stress transfer mechanism across a crack and, again for WWR and for certain FRC’s, there will be an accompanying sudden reduction in structural stiffness (increase in compliance) associated with the cracking event.
However, as noted in the paper in Concrete International V13, No.11, which I co-authored with C. D. Hays, either a single layer of WWR, depending on the placement within the depth of the slab, or uniformly distributed fibers, depending on the fiber type and volume fraction, will conduct post cracking flexural tensile stress, often referred to as residual strength (RS), with varying reliability. This fact is readily demonstrated by simply applying a post cracking flexural strength test, such as ASTM C1399 or ASTM C 1609. These tests, which were designed to determine the RS capacities for cracked beams having alternative FRC formulations but can as well apply to cracked WWR samples, provide conservative estimates of post cracking strength and ductility. The test samples are arranged so as to eliminate arching in cracked beam samples and by doing so they directly demonstrate the stress transfer ability of whatever reinforcement, wires or fibers, are bridging the cracks.
It is certain that the resulting RS and stiffness of WWR slabs will depend on the relative location of the WWR within the slab depth and that steel cover requirements would apply. These restrictive requirements do not apply to fiber reinforcement. For FRC there are no cover limitations and the fiber formulation, ie., fiber type and volume fraction, can be specified, assessed and assured, based on the aforementioned ASTM test standards for cracked beam performance evaluation. From an engineering point of view, the reliability of using WWR cannot be assured when compared to that of FRC.
Volume change cracking
The writer correctly points out the differences between plastic and drying shrinkage cracking. The writer also correctly acknowledges the early age benefits of fibers and there is no longer any real debate as to the fact that a random dispersion of fiber in a concrete matrix will act to suppress and control plastic and early age shrinkage cracking in ways in which WWR has no possible contribution. But he does not acknowledge how this translates to performance in the ability of fibers to control long term drying shrinkage cracking. In fact, whether or not either WWR or FRC is effective in preventing or just reducing long term cracking in slabs is not the issue. In either case, WWR or FRC, the issue is whether or not each system can provide the RS in bridging cracks to accommodate shear transfer through aggregate interlock across crack surfaces, both for shear force applied perpendicular to slabs and in-plane as for diaphragms,. This leads to the consideration of several important factors that relate to the effectiveness of aggregate interlock including crack size, crack distribution, and again RS reliability.
In the case of composite decks, aggregate interlock is reasonably assured by the usually corrugated deck system and the debate as to performance of WWR compared to FRC is simply reduced once again to crack control and RS reliability. On this basis there is no real advantage to using WWR in comparison to FRC and the decision to choose one over the other system is reduced to economics and constructability issues.
As to terming either WWR or fibers as "reinforcement", this terminology is used in connection with both systems and is influenced by historical development and tradition more than by anything else. The fact is that neither system is reinforcement in the structural sense. But fibers are reinforcement for the concrete matrix, both in the plastic state during mixing and placement as well as in the hardened state, assuring an enhanced concrete material. More apt terminology would be fiber modified concrete (FMC) in which, by comparison, WWR amounts to a poor distribution of materials from a reliability point of view.
Conclusion
Based on the evidence of performance of fiber reinforcement (FRC) in concrete in slabs—meaning engineered slabs for which QA and QC controls were appropriately applied and not based on anecdotal commentary—there is no rational reason that FRC, depending on the fiber type and volume fraction, should not be qualified a temperature and shrinkage slab reinforcement.
Ronald F. Zollo, Ph.D., P.E., FACI
University of Miami