Letters to the Editor
January '08
I read Gerard Feldmann’s article Non-Destructive Testing of Reinforced Concrete, that appeared on page 13 of the January issue with great interest, since for the past few years I have been heavily involved in the development of an instrument to perform that type of testing. The article is extremely informative and very comprehensive.
An additional method could be added to the many mentioned by Mr. Feldmann, ultrasound echo (UE). UE finds delaminated areas of concrete and has been shown to accurately identify delaminations when tested on a concrete slab purposely built with such defects. . It utilizes two ultrasonic probes; one transmits a stress wave field into the concrete specimen, and the other receives a signal corresponding to the natural response of the concrete specimen in the time domain. A Fast Fourier Transform (FFT) is then applied to the received response so it can be examined in the Frequency Domain.
When testing a floor with the UE method, the frequency response of an area of satisfactory quality will be uniquely determined by the floor thickness (thickness mode of vibration). In this situation the response sensed by the receiver is from wave reflections occurring at the floor to subgrade interface and at the floor surface. Testing at a known thickness to calibrate the concrete wave speed is not required, as the wave speed is calibrated by measurement of the surface P-wave. At an area with a shallow delamination, the response will shift to a lower frequency and be determined by the existence of delamination (flexural mode of vibration).
Ed Pristov
Inspection Instruments, Inc.
Author's Response:
Thanks for the compliment on the article.
Yes, I have heard of the test method you mention, but did not include it in the article due to space limitations and my lack of experience with the method. It seems I should look into it a bit more.
Thanks again,
Jerry Feldmann
September '07
Thank you for using dwgs from the HAER collection for the West Baden Springs Hotel article in the September 2007 issue. I was HAER principal architect during the summer of 1973 when the West Baden Springs Hotel was recorded. The Historic Landmarks Foundation of Indiana and the Indiana Historical Society were cosponsors. It was the second of a two-summer project to record a selection of engineering and industrial sites in Indiana.
These were heady and exciting years for HAER (Est. 1969) as we were defining the new field of industrial archeology and engineering heritage by recording engineering phenomenon and industrial processes with measured and interpretive drawings, historical data, and large-format photography. Though I retired nearly four years ago, the program thrives under the capable leadership of Rich O’Connor.
SIA, the Society for Industrial Archeology, is in the process of scanning issues of IA: The Journal of the Society for Industrial Archeology, with The History Cooperative. Unfortunately, Vol. 25, No. 1, which is a theme issue on the first 30yrs of the HAER program, has not been scanned. You can find the journal in any descent university library or it can be ordered form SIA, www.siahq.org. See “HAER: 30 Years of Recording Our Engineering Heritage,” IA: The Journal of the Society for Industrial Archeology, Vol. 25, No.1, 1999.
Eric DeLony
Chief (Retired)
Historic American Engineering Record
National Park Service
pontist@comcast.net
September '07
While I think that Larry Muir's article in the July 2007 issue, 5 Common Myths of Steel Design Debunked, raises excellent points, I must take issue with the net section discussion listed under Myth #1. Many members can be connected with an effective net section much greater than 75%, even when heavy truss chord splices are made with 4 rows of bolts (double gage) across the flanges, by simply using one or two "lead-in" rows of bolts in the flanges before the web bolts are started.
In addition, there are many instances where, due to different member actual depths, significant shimming is required between members and connection material. Take for example a heavy truss tension chord splice where the center segment is larger than the next outboard segment. If the outer member size is dictated by an arbitrary 75% net section capacity, a much larger (and more expensive) section will result. This is especially important in our current market state of high-cost raw steel.
So, while I am 100% in agreement with Larry's general concept that less material weight does not always equate to less cost, this is one instance where I believe that it does. I have witnessed engineers attempting to limit member stresses to accommodate the arbitrary AISC listed values without thinking about how the members may be connected and determining whether the full member capacity could indeed be used. Experienced designers working with competent fabricators need to make this type of decision considering many other criteria specific to the project.
Regarding Myth #5, Larry is 100% right on: Owners should only hire top-notch, experienced fabricators and erectors. Unfortunately, not all such firms have the professional credentials, capabilities and competence of Cives, so the Owners do not always have a choice … too bad for us designers.
Sincerely,
W. Steven Hofmeister, P.E., S.E.
Thornton Tomasetti
Kansas City, Missouri
Author's Response:
Steve brings up a good point. I did not intend that an engineer should always increase the member size to accommodate the reduced area due to bolt holes, but rather that the effects of the connection design should be considered on a case by case basis in the main member design. Least weight does not always equal least cost, but sometimes it can.
If the decision is made to reinforce a member rather than increase the member size, the reinforcing and associated welding should be indicated on the engineer’s drawings so that it can be accounted for in the bid. Having the engineer and fabricator work as a team early in the project often allows these issues to be more effectively addressed in the design phase, and can lead to increased economy for all parties.
Steve’s comment about the conservatism inherent in the 75% reduction reflected in the AISC Manual is also valid, and the Manual Committee is currently working to revise these tables to be more accurate and useful to the designer.
Larry S. Muir, P.E.
Cives Steel Corporation
Roswell, Georgia
August '07
Dear Mr. Weingardt,
I just wanted you to know how much I enjoyed your profile of Timoshenko in the August 2007 STRUCTURE magazine (funny how those back issues pile up). I still refer to Timoshenko's books, and had the pleasure of learning under his co-author Jim Gere, but never knew the turbulent story of Timoshenko's life, or of his wit. Thank you for filling that gap in my technical education!
Leonard Martin Joseph, P.E., S.E.
Senior Vice President
Thornton Tomasetti
Irvine, CA
August '07
Thank you for the article in the August 2007 “Great Achievement” series of STRUCTURE ® about the “father of engineering mechanics,” Stephen P. Tymoshenko.
Among his many contributions to science worth noting was the creation of the Ukrainian Academy of Sciences in Kyiv in 1918, of which he was a founding member. A Ukrainian commemorative postage stamp of 1998 illustrating Mr. Weingardt’s biographical sketch witnesses reverence of his illustrious personality by Ukrainians at home and all over the world.
Roman Wolchuk
Fellow, ASCE
August '07
I would like to comment on the gable end article in your August 2007 issue. Gable end trusses are not allowed in Miami Dade County, and for a good reason. They leave the wall under them basically unbraced, unless the top of the wall under them has a structural element that can span the length of the wall and brace it for out-of-plane wind forces.
For taller walls (ever more present as homes get more and more expensive), there is basically no wood-framed bracing system that can be designed to transfer those forces back in the remaining of the roof structure unless fairly large members and bolted connections are utilized.
A gable end wall should have continuous framing from the foundation all the way to the top of the wall, where roof sheathing and blocking can resist the forces. If the walls are masonry, the cells should be reinforced as such. If they are wood framed, they should have studs continuous from the foundation to the roof sheathing.
Eugenio M. Santiago, P.E.
Chief Building Official
Key Biscayne, Florida
esantiago@keybiscayne.fl.gov
Response
Thank you for your comments regarding the article on Wood Truss Gable End Frames. The scope of this article was to provide basic and additional design considerations for gable end frames (i.e., vertical and lateral load considerations).
Page 55 of the article provides information related to gable end frames under lateral loads acting parallel and perpendicular to their plane. In addition, your comments concerning continuous framing are addressed on page 55 and 56; citing section 2304.3.4 of the FBC with regards to Gable End Wall Bracing. Both Jim and I conducted several claim investigations in Florida related to hurricane and tornado damages, and are familiar with building code requirements in Broward and Dade County.
Figures 6, 7, 8, 10 and 11 (full article) deal with gable end frames resisting lateral loads, stress and/or deflection concentrations and recommended details for gable end walls and/or frames. Figure 11 is taken from SSTD-10-99 (Standard for Hurricane Resistant Residential Construction), which is also provided in AF&PA’s WFCM (Wood Frame Construction Manual).
(The full article can be viewed in STRUCTURE’s online archives.)
Agron Gjinolli, P.E.
WTCA
Agjinolli@qualtim.com
July '07
The article The Case for an Engineer of Record for a Metal Building System in the March 2007 issue of STRUCTURE magazine contains good recommendations for Owners who are considering Metal Building Systems.
One item that is questionable from the Engineer of Record (EOR) perspective is the section on “Inspection Services”. For most other projects, the EOR agrees to “observe” the structure during construction to ascertain substantial compliance with the Contract Documents that the EOR prepared. According to the IBC, “inspections” are performed by the Special Inspector hired by the Owner.
It is an abdication of responsibility that the metal building manufacturer does not observe the metal building system during construction. The location of the manufacturer is not a valid excuse. The metal building steel erector is generally under the same contract as the metal building designer. The design, erection details, drawing quality, construction conformance with the drawings, and construction quality are all the responsibility of the metal building designer working with the erector.
While it is preferable to have the EOR review the Order Document, that is not always done. Metal building manufacturers should adhere to the current industry practice of inspection by the Special Inspector and site observation by the designer, rather than shift such responsibility to the EOR.
Sincerely,
Lawrence R. Chute, P.E.
DESAI/NASR CONSULTING ENGINEERS, INC.
Mr. Chute brings up an interesting point about whether a metal building manufacturer should be responsible for the observation or inspection of a metal building system during construction. We feel that the “special inspector” hired by the building owner should be someone who is independent from the erector/building manufacturer. The metal building manufacturer has a conflict of interest on top of the logistical problems associated with performing the inspection, because the erector is commonly the customer of the manufacturer. Some manufacturers do include onsite inspection as part of their contract, especially with regard to accepting warranty responsibilities on large or complex projects, but if a manufacturer does not include this service, it should not be considered an abdication of responsibility.
W. Lee Shoemaker, P.E., Ph.D.
July '07
Metal Building Systems and the EOR
Dr. Shoemaker's article on metal building system structures in the March 2007 Structural Forum column presents a comprehensive overview of the metal building system world. Metal building systems provide very efficient structures by keeping material costs at a minimum. As a structural engineer who has been responsible for the erection engineering of these structures, I have found that low material cost can require extreme efforts to afford safe erection of a structure. Girders and rigid frames that are adequate, when all intermediate framing members are installed, can require elaborate analysis and special bracing to allow their erection. Anyone entertaining the use of a metal building system structure needs to be aware that the Metal Building Systems Manual excludes structure erection design from the manufacturer's responsibility unless specified otherwise by the purchaser. Metal building systems can provide durable, low-cost structures; their design should include a method for getting them erected, safely.
Alan D. Fisher, P.E.
Manager, Construction Structures Group
Cianbro Corporation
Mr. Fisher is absolutely correct that metal buildings, like other types of construction, require careful planning for the safe sequencing of erection. Metal building erectors are no different from other steel erectors that are typically responsible for determining the best erection sequence and method based on their available equipment and experience. If the metal building erector requires additional special engineering to help determine the erection procedure, such as Mr. Fisher’s expertise, it seems more appropriate that the knowledge of the site-specific conditions be handled by someone other than the metal building manufacturer, because of logistical and practical constraints. Local design and oversight would seem to be the most realistic method to achieve the goal of safe erection.
W. Lee Shoemaker, P.E., Ph.D.
July '07
Communicating with CAD
The author hit the nail squarely on the head with the InFocus column Cad-How It Has Changed the Way We Think (STRUCTURE ®, April 2007). I passed it to several others here, who immediately had the same reaction. Good job!!
Every one of your points were accurate: the lack of x-refs being bound to the transmitted files, the lack of dimensions and the Architect’s attitude – “just scale the cad drawing” and when you do it has some funky fraction at the end. All that stuff is the norm around here and a source of everyday frustration. Thanks for letting us know we are not alone in this gripe.
Stephen M Rudner PE
Robert Darvas Associates PC
440 South Main Street
Ann Arbor, Mi. 48104
(734) 761-8713 ext 12
Fax (734) 761-5236
July '07
NCEES Model Law
I was reading the article titled “Is Four Years Enough?” in the April 2007 issue of the STURCTURE ® magazine and felt that some readers might be lead to a wrongful conclusion based upon a couple of sentences contained in the article.
The articles states “ASCE drafted a model registration law for consideration by the National Council of Examiners for Engineering and Surveying (NCEES). The model law incorporates the above education requirements.” My concern is that action taken last year by NCEES to amend its Model Law to require a Bachelors Degree plus 30 additional credits would be attributed to a similar study that has been under review by ASCE during recent years.
NCEES was fully aware of ASCE’s review of this matter, and the development of a body of knowledge on what one might need to know in order to be qualified for the professional practice of engineering. NCEES has studied this matter separate and apart from ASCE, which included work by two distinct and separate NCEES task forces. After several years of study and review by these task forces, a motion was made and ultimately approved by NCEES to amend its Model Law to require, effective 2015, that candidates for the Principles and Practice exam must have a Bachelors Degree in Engineering plus 30 additional credits. The NCEES Uniform Procedures and Legislative Guidelines (UPLG) Committee has been tasked this year with defining what constitutes 30 additional credits as satisfactory for pursuing licensure.
The point of clarification is that the action taken be NCEES during their 2006 Annual Meeting was not as the result of the ASCE study, but as the result of NCEES’ on study and findings.
Jerry Carter
Associate Executive Director
National Council of Examiners for Engineering and Surveying
May '07
The debate between thermally Restrained versus thermally Unrestrained assemblies, has existed for over 30 years. Industry groups, such as the American Iron and Steel Institute (AISI) have sponsored studies, which lead to the conclusion that all structural steel frames, independent of the level of restraint, can be classified as Restrained assemblies. Unfortunately, the conclusions drawn from these studies do not take into account the effect of the structural frame on the overall fire protection package.
Restrained Assembly beams generally exhibit significant deflections before the required hourly rating is met. A building’s fire protection package, in addition to the sprayed cementitious fireproofing, includes other important features, such as compartmentation, suppression systems and detection systems. While a Restrained structure, in the right situation, may transfer loads without collapse, excessive deflections will likely compromise the compartmentation offered by firestopping and smoke damper systems and jeopardize the effectiveness of sprinkler systems. Non-functioning compartmentation and suppression permit the quick spread of smoke and fire to areas outside of the point of origin.
Actual fire events have proven the effectiveness of Unrestrained classification. The Occidental Tower fire in Los Angeles (Nov 1976), State Office Building fire in Olympia Washington (Oct 1983), First Interstate Bank fire in Los Angeles (May 1988), and Union Bank Building fire in San Francisco are all examples of structures protected with cementitious fireproofing applied to Unrestrained thicknesses. In all cases, the damage to the structural steel was minor and the buildings were open for business shortly after the fire event.
Some failed fire protection packages resulting from Restrained classification or sprinkler tradeoffs are: The McCormack Place fire in Chicago (Jan 1967), One New York Plaza fire (Aug 1970), K Mart Distribution Center fire in Falls Township, PA (Jun 1982), and One Meridian Plaza fire in Philadelphia (Feb 1991). All sustained structural damage beyond repair resulting in major financial losses.
In short, Spray-on Fireproofing works and Unrestrained protection affords the best protection for structural steel framed buildings. It insures full compliance with building codes in all jurisdictions, without assuming the liability associated with designating a building as thermally restrained. The cost difference between fireproofing a building to Unrestrained versus a Restrained classification is generally less than 1% of the overall cost of the building. This amount can prove to be the difference between saving a structure with minimal loss of life and a catastrophic disaster.
Michael Giardinelli
W.R. Grace & Company
Author, Fireproofing Steel Structures (STRUCTURE, February 2007)
May '07
CASE and the members of the CASE Fire Protection Committee found the article, Fireproofing Steel Structures (STRUCTURE®, February 2007), to be potentially misleading. The premise of the article, that structural steel assemblies should be considered to be thermally unrestrained, is not supported by engineering data. There has been significant engineering research over the past 30 years that suggests that structural steel assemblies behave as thermally restrained in almost all instances.
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Restrained Fire Resistance Ratings in Structural Steel Buildings by Gewain and Troup (see reference above) states that most common types of steel-framed construction are classified as thermally restrained.
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Appendix X3 of ASTM E119 lists the few instances where individual steel beams and girders, or steel framed floor and roof assemblies, are classified as unrestrained.
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AISC Design Guide No. 19, Fire Resistance of Structural Steel Framing, clearly indicates that the position suggested in the article is incorrect.
It requires considerably more sprayed fire-resistive material to achieve an unrestrained fire rating, rather than a restrained fire rating. T his appears to be a conservative approach that could present an economic burden to the project. These conditions must be carefully evaluated by the designer.
CASE Executive Committee
Edward W. Pence, Jr., P.E., S.E., F.ASCE, Chair 2006/07
May '07
Much effort has been spent disseminating the research sponsored by the American Iron and Steel Institute (AISI) that confirmed the performance of steel framing and the use of restrained ratings in the selection of fire protection. The conclusion of that research remains valid – steel-framed structures can be considered thermally restrained.
The test data supporting this conclusion are documented in Restrained Fire Resistance Ratings in Structural Steel Buildings (Gewain and Troup; AISC Engineering Journal, 2 nd Quarter 2001). Furthermore, AISC Design Guide No. 19, Fire Resistance of Structural Steel Framing, contradicts the position suggested in Fireproofing Steel Structures (STRUCTURE® magazine, February 2007).
The issue of restrained vs. unrestrained construction is unique to the United States. It has been a source of confusion since the concept’s introduction in 1970. To assist the design professional in determining this parameter, AISC has collected information demonstrating that steel framed construction qualifies for a restrained classification and makes it available so that the provisions of section 703.2.3 of the International Building Code can be satisfied. This being the case, the opinion expressed in the article serves to perpetuate unnecessary questions that have already been answered repeatedly.
John L. Ruddy
Director of Building Design, AISC
March '07
I am writing in response to a portion of Jon Schmidt’s InFocus article in the January 2007 issue of STRUCTURE ®. Jon, stated, “I cannot help but wonder if the ‘commodization’ of engineering services is inevitable if we do not significantly raise the bar of entry into our profession. The moves toward specialty certification and, eventually, separate licensure are certainly steps in the right direction, but may not be enough in the end.” Perhaps I misunderstand the intent, but it seems to me that we are considering professional licensure as a means of job protection. We exist professionally to facilitate the construction of structures — safe structures. If additional licensure or educational requirements are needed to assure safe design, very well. If we begin, however, to consider these methods as some form of trade protection tariff, we are amiss.
Our age is a turbulent one. The ability to communicate globally has altered and is still altering our business fundamentally. This article touched not only on the educational future of engineering, but also on our own fears of our professional future. In a free market, when we perform useful, needed services, we are and will be compensated. If there are quicker, easier or cheaper means of providing those same services, the market will shift the demand to those other sources. If the services performed there are inferior, our services will command more demand and/or money, or at least we hope so!
Perhaps the real goal is not a free market goal. Perhaps the real goal also has an aim to create a union of sorts. I don't know, but I think our intentions ought to be clear at least to ourselves.
Bret Wickham
Contra Costa County, California
I appreciate Mr. Wick's comments and do not necessarily disagree with them. When I talk about “raising the bar”, I do not mean making it more difficult per se, but rather making it more rigorous. Protecting the public is, indeed, the primary intent — not protecting our turf.
Response from Jon Schmidt, P.E., SECB
Feb '07
Mr. Rouis’ December 2006 article, A Better Base, was well written and informative. I offer the attached Pin Wheel Isolation Joint Detail as an alternate to his Figure 7a. The Pin Wheel method of simultaneously isolating the column as a part of the installation of sawn control joints is used very often in warehouse distribution facilities in which the slab is cast after the erection of the steel.
An alternate to Mr. Rouis’ Figure 1 is the Pocket Form Isolator. Information on this pre-manufactured item can be found at www.isolationpocket.com.
D. Matthew Stuart, P.E., S.E., F.ASCE, SECB
Schoor DePalma Engineers and Consultants
mstuart@schoordepalma.com