CODE UPDATES

code developments and announcements The American Concrete Institute (ACI) published the Building Code Requirements for Structural Concrete (ACI 318-14) and Commentary (ACI 318R-14) in the Fall of 2014. ACI 318-14 has been adopted by reference into the 2015 International Building Code (IBC). There are very significant organizational as well as technical changes between ACI 318-11 and ACI 318-14. A two-part article on the changes was published in the April and May 2016 issues of STRUCTURE magazine. A follow-up article on one of the most significant technical changes – the seismic design provisions for special (meaning specially detailed) shear walls – was published in the July 2016 issue. This is the last follow-up article on another critical change in the requirements for the confinement of columns in special moment frames of reinforced concrete.

Introduction to the Changes

The ability of the concrete core of a reinforced concrete Confinement of Special column to sustain compressive strains tends to increase Reinforced Concrete with confinement pressure. Moment Frame Columns Compressive strains caused by lateral deformation are additive to the strains caused by the axial load. It follows Requirements of that confinement reinforcement should be ACI 318-14 increased with the axial load to ensure consistent lateral deformation capacity. The dependence of By S. K. Ghosh, Ph.D. the amount of required confinement on the magnitude of the axial load imposed on a column has been recognized by some codes from other countries (such as Canada’s CSA A23.3-14 and New Zealand’s NZS 3101-06) but was not reflected in ACI 318 through its 2011 edition. S. K. Ghosh (skghoshinc@ The ability of confining steel to maintain core congmail.com) is President, S. K. crete integrity and increase deformation capacity Ghosh Associates Inc., Palatine, IL and Aliso Viejo, CA. He is a long-standing member of ACI Committee 318, Structural Concrete Building Code, and its Subcommittee H, Seismic Provisions. is also related to the layout of the transverse and longitudinal reinforcement. Longitudinal reinforcement that is well distributed and laterally supported around the perimeter of a column core provides more effective confinement than a cage with larger, widely spaced longitudinal bars. Confinement effectiveness is a key parameter in determining the behavior of confined concrete (Mander et al. 1988) and has been incorporated in the CSA A23.3-14 equation for column confinement. ACI 318, through its 2011 edition, did not explicitly account for confinement effectiveness in determining the required amount of confinement. It instead assumed the same confinement effectiveness independent of how the reinforcement is distributed. Given the above, confinement requirements for columns of special moment frames (Section 18.7.5, Figure 1), with high axial load (Pu > 0.3Ag f'c) or high concrete compressive strength (f'c > 10,000 psi) are significantly different in ACI 318-14. The following excerpt from Sheikh et al. explains why high-strength concrete columns are grouped with highly axially loaded columns:

“For the same amount of tie steel, the flexural ductility of HSC [High Strength Concrete] columns was significantly less than that of comparable NSC [Normal Strength Concrete] specimens tested under similar P/f'c Ag values.

For the same percentage of the confining steel required by the ACI Building Code, NSC columns displayed better ductility than comparable HSC columns tested under similar

P/f'c Ag. However, for the same level of axial load measured as a fraction of Po (the ultimate axial load capacity), HSC and NSC columns behaved similarly in terms of energy-absorption characteristics when the amount of tie steel in the columns was in proportion to the unconfined concrete strength. Conversely, the amount of confining steel required for a

The online version of this article includes a key to notations and detailed references. Please visit www.STRUCTUREmag.org.

Figure 2. Confinement of high-strength or highlyaxially-loaded rectangular column of special moment frame.

certain column performance appears to be proportional to the concrete strength as long as the applied axial load is measured in terms of Po rather than P/f'c Ag.” The discussion below is about confinement over the length lo, the region of potential plastic hinging. One important new requirement is as follows: 18.7.5.2 – Transverse reinforcement shall be in accordance with (a) through (f) Where Pu > 0.3Ag f'c or f'c > 10,000 psi in columns with rectilinear hoops, every longitudinal bar or bundle of bars around the perimeter of the column core shall have lateral support provided by the corner of a hoop or by a seismic hook, and the value of hx shall not exceed 8 in. (Figure 2). Pu shall be the largest value in compression consistent with factored load combinations including E. The change from prior practice is that instead of every other longitudinal bar having to be supported by a corner of a tie or a crosstie, every longitudinal bar will have to be supported when either the axial load on a column is high, or the compressive strength of the column concrete is high. Also, the hooks at both ends of a crosstie need to be 135-deg. As importantly or perhaps more importantly, the center-to-center spacing between laterally supported bars is restricted to a short 8 inches. In the absence of high-strength concrete or high axial loading, the maximum spacing goes up to 14 inches. In ACI 318-11 and prior editions, the 14-inch limitation used to apply to the center-to-center spacing between legs of hoops and crossties. The other new requirement is in the following section: 18.7.5.4 – Amount of transverse reinforcement shall be in accordance with

Table 18.7.5.4 (reproduced here as

Table 1). The concrete strength factor, kf , and confinement effectiveness factor, kn, are calculated by (a) and (b). kf = f'c 25,000 + 0.6 ≥ 1.0 (18.7.5.4a)

kn =

nl nl – 2 (18.7.5.4b)

Where nl is the number of longitudinal bars or bar bundles around the perimeter of a column core with rectilinear hoops that are laterally supported by the corner of hoops or by seismic hooks. See Tables 2 and 3 for values of kf and kn, respectively, calculated by the above formulas.

Impact of Changed Confinement Requirements

As is seen above, for columns that are made of concrete with specified compressive strength, f'c, exceeding 10,000 psi and/or are subject to factored axial force, Pu, exceeding 0.3Ag f'c (Ag = gross cross-sectional area), the required confinement over regions of potential plastic hinging (typically at the two ends) is now a function of the axial force. The impact of the changed requirements is assessed in Table 4 (page 28). Bars larger than No. 6 in size are not very practical for use as transverse reinforcement. Also, the ensemble of one hoop and crossties in two orthogonal directions has a thickness of 2¼ inches for No. 6 bar size, which translates into a 1¾-inch clear spacing for a 4-inch center-to-center spacing. Thus, Table 4 shows the limitations on sustainable axial load as the specified compressive strength goes beyond 6 ksi. The limitations have become significantly more severe under ACI 318-14. It should be noted that ACI 318 does not allow Pu to exceed 0.8 (accidental eccentricity factor) x 0.65 (φ for columns

Table 2. Values of concrete strength factor, kf .

Specified compressive strength of concrete, f'c

10,000

Concrete strength factor, kf

1.0

12,500 1.1

15,000 1.2

17,500 20,000 1.3 1.4

22,500 25,000 1.5 1.6

Table 3. Values of confinement effectiveness factor, kn .

No. of laterally supported longitudinal perimeter bars, nl Confinement effectiveness factor, k n

4 2.00

6

8

10

12

14

16 1.50

1.33

1.25

1.20

1.17 1.14

18 1.13

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with discrete transverse reinforcement) x Po = 0.525 Po, where Po = Ag f'c + Ast (fy – f'c) So, 0.5 f'c Ag is an extremely high axial load level, which is unlikely to be encountered in special moment frame columns. Also, if one needs to go beyond the range of factored axial loads and concrete strengths that can be accommodated with No. 6 transverse reinforcement at a reasonable spacing, the most eff ective solution is to switch to transverse reinforcement with yield strength, fyt, higher than 60 ksi. ACI 318 allows fyt to be up to 100 ksi.

Conclusions

Th is article discusses the modifi ed ACI 318-14 confi nement requirements for columns of special moment frames. It is shown that the modifi ed requirements have a signifi cant impact on columns that are highly axially loaded (Pu > 0.3Ag f'c) or made of highstrength concrete (f'c > 10,000 psi) or both.▪

Acknowledgments

Grateful acknowledgments are due to Pro Dasgupta and Ali Hajihashemi of S. K. Ghosh Associates Inc. for their considerable help with the paper.

Portions of this article were originally published in the PCI Journal (March/ April 2016), and this extended version is reprinted with permission.

Historic structures

significant structures of the past

Brooklyn Bridge

Part 1

By Frank Griggs, Jr., Dist. M.ASCE, D.Eng., P.E., P.L.S.

Dr. Griggs specializes in the restoration of historic bridges, having restored many 19th Century cast and wrought iron bridges. He was formerly Director of Historic Bridge Programs for Clough, Harbour & Associates LLP in Albany, NY, and is now an Independent Consulting Engineer. Dr. Griggs can be reached at fgriggsjr@verizon.net.

John A. Roebling’s vision of the Brooklyn Bridge 1867.

The Brooklyn Bridge, across the East River in New York City, is perhaps the most recognized bridge in the United States. As a result of David McCullough’s book The Great Bridge and Ken Burns’ American Stories – Brooklyn Bridge series, many engineers know some of the backstories of the bridge. Don Sayenga’s book, Washington Roebling’s Father, also clarified which Roebling, John or Washington, built the Bridge. Part 1 starts with the earliest plans for a bridge and runs up to John Roebling’s death in 1869 before the onset of construction. In the 19th century, Manhattan and Brooklyn were the first and third largest cities in the United States, separated only by the East River, which varied in width along its length. Some of the earliest proposed bridge locations were across Blackwell’s Island, well above the centers of population where the island separated the river into two channels. Proposals were made as early as 1804 to cross the river at that point. Graves, in 1837, and John A. Roebling, in 1856, made following proposals. Farther to the south, closer to the centers of population, Thomas Pope proposed his Flying Pendant wooden bridge in 1809, followed by a suspension bridge by Julius Adams in 1864 and another suspension bridge by John A. Roebling in 1867. Roebling had been looking at a bridge at the Fulton Ferry for many years, starting in 1852 when, as the story goes, he and his son Washington were stuck on a ferry in an ice jam. He wrote a letter to Abram Hewitt, a New York City leader, about his plan for a bridge. Hewitt forwarded the letter to the Journal of Commerce for publication. His plan was for a 1,600-foot span with a vertical clearance of 130 feet with ornate towers. In March 1860, he had completed his Niagara Railroad Bridge and started his 1,057foot span Covington and Cincinnati Bridge (STRUCTURE, May 2016) across the Ohio River. It was then that the Architects and Mechanics Journal wrote an article questioning if a long suspension bridge was even possible in Brooklyn. Roebling responded with a lengthy letter supporting his proposal, entitled Bridging the East River. He wrote, “A few years ago I was requested by some prominent citizens of New York and Brooklyn, to investigate this project and to state my views in a general way. Those views were published in the Journal of Commerce. They have been undergoing, since, a further review and scrutiny.” He made three main points. The first being that only a suspension bridge or a tunnel would keep the harbor free for ship traffic. The second that trains of cars propelled by wire-operated ropes and stationary engines were necessary to move a half million people daily across the bridge. Moreover, finally, “the merits of the enterprise as a good first rate investment must be undoubted, else no private capital can be enlisted. As to the corporations of Brooklyn and New York undertaking the job, no such hope need be entertained in our time. Nor is it desirable to add to the complication and corruption of the governmental machinery of these cities. There would be no objections to a subscription by either corporation, but the enterprise to be successful must be conducted by individuals.” By 1864, while still working on the Covington & Cincinnati Bridge, he wrote an article for Engineering Magazine in London. It was under the heading Proposed American Suspension Bridge:

I propose to start in the vicinity of the Park of the city of New York, at an elevation of about 80 feet above tide, thence ascending about 125 feet, to the centre of the East River (having a clear elevation of 180 feet), thence descending towards the heights of Brooklyn, and landing within sight of the City Hall…

The superstructure of this magnificent bridge would thus form an arch about two miles long, clearing the water of the east River in one sweep of 1,600 feet to 1,800 feet span, and extending over the houses of both cities….My plan provides two floors similar to the Niagara

Bridge, the upper floor for railway conveyance, the lower one for promiscuous travel on foot, horseback or carriage. The entrance of the upper or railway floor will be next to the City Halls of New York and Brooklyn and may be kept independent of the entrance to the lower floors, which may be located nearer to the river.

There will be sidewalks on both floors, and these will become favorite resorts for those who want take exercise in the open air. The great majority of passengers will of course, use the cars on the upper floor. My experience and long familiarity with the working of inclines enables me to devise such plans as will render this portion of the structure and its operation, perfectly successful. The materials of construction will be principally granite and iron, the latter placed so that it can be readily preserved by painting. The rigidity of the superstructure will be as great as that of a tubular bridge. Iron trusses of great depth, connecting both floors, together with effective over-floor stays, and the great weight of the structure itself and inherent rigidity of the cables will provide ample stiffness. He kept working on different deck layouts, starting with the Niagara double deck plan as well as a single level deck with two tracks down the middle flanked by roadways. In late 1866, William Kingsley, a well-known Brooklyn contractor, was convinced that a bridge was possible and decided to visit the home of Henry Murphy, a New York State Senator, about supporting legislation to authorize the creation of a corporation to build and operate a toll bridge across the river. On January 25, 1867, Murphy, good to his word, submitted the proposal to the legislature. The act was approved on April 16, 1867, as Chapter 399 of the 19th Session. It was entitled “An Act to incorporate the New York Bridge Company, for the purpose of constructing and maintaining a bridge over the East River, between the cities of New York and Brooklyn.” The capital stock was fixed at $5,000,000 in shares of $100 each. The bridge was to be completed on or before June 1870 and was to provide a clearance of 130 feet over the East River so as not to obstruct river traffic. One month after the Company was formed, The Brooklyn Daily Eagle, May 24, 1883 wrote, “The managers came to the point where it was necessary to appoint a chief engineer to complete the plans and build the bridge. Among those earliest suggested for the position was Julius W. Adams, who was regarded as a brilliant and talented member of the profession. He was strongly pressed for the place by incorporators and outsiders. Objection was made, however, that he had never built a large structure of this kind, and it was deemed advisable to secure, if possible, the services of someone experienced in the construction of great suspension bridges, particularly as the public were beginning to doubt the possibility of building the bridge in consequence of the natural and mechanical difficulties to be overcome. It was known to some of the incorporators that John A. Roebling, then residing in Trenton, N. J. has had large experience in works of the character of the proposed to be built.” Kingsley approached Roebling in Trenton, and Roebling agreed to take on the role of Chief Engineer provided he was also appointed to supervise the construction of the bridge and not just prepare the plans. He was awarded a contract dated May 23, 1867, for $8,000 per year and told to begin work immediately, with the understanding that Kingsley would pick up the initial costs while financing of the bridge was initiated. On or about the time he was approached by Kingsley, he modified his deck design once again to provide for a central walkway by separating the two center cables, with the walkway or promenade just above the adjacent railway and carriageway. Later in May, as he was beginning to put together his report, he maintained the pedestrian way in the middle of the bridge but raised it, evidently to give the pedestrians a better view of the river. His six (6) lines of equal depth trussing were apparently fixed in his thinking at this time, but he widened his outer carriageways. He knew the success of his project was having a firm foundation for his masonry towers and anchorages. He sent Washington to Europe on a belated honeymoon and also to study what the Europeans were using for bridge foundations, especially pneumatic caissons and their use of steel in their bridges. Washington graduated from Rensselaer Polytechnic Institute in Troy, New York in 1857. He then worked on Roebling’s Allegheny River Bridge in Pittsburg, after which he enlisted in the Union Army in which he served for over three years, resigning with the rank of Lieutenant Colonel. He had married Emily Warren, the sister of General G. K. Warren, his commanding officer, and worked on finishing the Covington & Cincinnati Bridge that opened on January 1, 1867, staying on for another few months finishing up the project. While Washington was away, John worked on the preliminary plans for the bridge with Wilhelm Hildenbrand and Griefenberg, both

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German immigrants. By September 1, 1867, he had completed his preliminary plans and written a 48-page report. On September 7, he gave an oral report to the Board. The Report, like all of Roebling’s writings, was well written and thorough. He ended with,

As a great work of art, and as a successful specimen of advanced Bridge engineering, this structure will forever testify to the energy, enterprise and wealth of that community, which shall secure its erection. He still hadn’t decided on a foundation as Washington had not returned from Europe with his suggestions. On September 10, the Brooklyn Eagle published an extensive summary of the report in which Roebling gave an estimated cost of $6,675,357. It said:

We devote a very considerable portion of our space today to the report of Mr. Roebling, the engineer of the proposed bridge across the East River designed to secure to these two great centres of population ample and uninterrupted communication. The report will attract great interest, for it may be accepted as the first practical step towards the realization of one of the most remarkable enterprises of our time, and inaugurating a new era in the history of

Brooklyn…. Mr. Roebling discusses in his report seven questions. The people of

Brooklyn and New York are mainly interested in three of them

Is a Bridge necessary?

Can it be built?

Will it pay? They agreed with Roebling that the answer to all three questions was yes, quoting from his report and adding thoughts of their own. They concluded:

But the project will pay? As an investment, it will receive encouragement from capitalist everywhere. Brooklyn herself can afford to build. Nothing is more certain than that she cannot afford not to. We refer our readers to Mr. Roebling’s report with great pleasure. The Committee on Plans and Surveys approved the Report, with the understanding that some things were still not fixed but that they could be addressed during the next phase of design. Later in October, the full Board met and accepted the recommendation of the committee for the “immediate commencement of the work.” This would be the last meeting for over a year as the Board was to have great difficulty in interesting many people in the value of its stock. Funding was a problem. Much of 1868 was taken up in getting the Cities of New York and Brooklyn to take stock in a private bridge building company. The infamous Boss Tweed and his colleagues in Tammany Hall of New York City had their of 1,600 feet span, 135 feet elevation, across the East River, in accordance with the plans proposed by Mr. Roebling, and that such structure will have all the strength, stability, safety and durability that should attend the permanent connection by a bridge of the cities of New York and Brooklyn. With this expression of 1867 plan with elevated walkway and two cross beam designs. our professional judgment we could, and perhaps should, close this report. hands out. Brooklyn approved the purchasing It was not until June 21, 1869, that General of 30,000 shares on December 22, 1868, and A. A. Humphries of the Corps of Engineers

New York City purchased 15,000 shares two informed Murphy and the Bridge Company days later. Private individuals, including Tweed’s of the findings of his Board of Engineers. 560 shares for which he paid nothing, held the He had four conditions, the first of which remaining 5,000 shares. had the most impact on the design of the

In early January 1869, Roebling, sensing that bridge. It was “… the centre of the main span momentum was growing for the bridge, sug- shall, under no conditions of temperature or gested to the Board that they appoint a Board load, be less than one hundred and thirty-five of Consulting Engineers to review his plans. feet in the clear above mean high water of

The Board approved of this, believing that con- spring tides, as established by the United State firmation and approval by some of the leading Coast Survey.” The other conditions dealt engineers of the country would bolster public with the sizes of structural members, stating confidence in the bridge. The Board consisted that no member shall be reduced below the of seven (7) prominent engineers: Horatio Allen, sizes given, no part of the foundations of

Chairman, Benjamin H. Latrobe, William J. the piers shall project beyond the existing

McAlpine, John Serrell, James Kirkwood, J. pier lines, and “no guy or stays shall ever be

Dutton Steel and Julius Adams. The fledgling attached to the main span of the bridge which

American Society of Civil Engineers had been shall hang below the bottom chords thereof.” reconstituted in 1868 after a period of inactiv- Humphries wanted the higher clearance as ity of 10 years, and four of its early presidents the busy Brooklyn Navy Yard was just north were Kirkwood, McAlpine, Allen, and Adams. of the bridge site. This clearance was greater

The committee started meeting in March than Roebling thought necessary, but the 1869, with both John and Washington in Company was forced to submit to it. attendance, reviewing the plans and listening Things were looking up for Roebling and the to John’s description of his report and design. Company and, in June, Washington was at

To give a better understanding of the plans, the bridge performing the survey that would especially as this bridge would have almost a be used during construction of the bridge. 40% increase in span length and be a much Using triangulation methods, they measured more heavily loaded bridge with an 85-foot a base line and the required angles to arrive wide deck, Roebling suggested they make a at the final centerline of the bridge from City grand tour of his bridges in Pittsburg, Niagara, Hall, New York to City Hall, Brooklyn. When and Cincinnati. In Pittsburg, on April 15, he these spikes were set, and the lines marked on showed them his Smithfield Street Bridge over buildings, it was the first actual work on the the Monongahela River built in 1845-46 and ground since the borings were taken in 1867. his Allegheny Bridge constructed in 1860. Later that month, John came up from Trenton

They then moved to Cincinnati on April 17 to either help in the survey, or to check the to view his 1,057-foot long bridge with a deck work Washington and his crew had done. On width of 35 feet, then the longest suspension Monday, June 28, John was standing on some bridge in the world. This was followed, several guide piling, called the rack, leading to the days later, by a trip to Niagara to view his Fulton Ferry slot when a ferry hit against the 820-foot span double deck railroad/carriage/ piling and a string piece moved causing his foot pedestrian bridge (STRUCTURE, June 2016) to be crushed between the timbers or piling. over the Niagara Gorge. This bridge was fin- The doctors recommended amputating his ished in 1855 and, after 12 years of service, crushed toes. Roebling consented but insisted was performing at a high level. that it be done without anesthesia. After that,

The Board of Engineers submitted their Roebling followed his own advice and used a report in June 1869, fully supporting water drip therapy. This was unsuccessful. After

Roebling’s plans writing, two weeks tetanus set in followed by lockjaw,

That it is beyond doubt entirely practi- and he died on July 22nd. The Chief Engineer cable to erect a steel wire suspension bridge was dead. What would happen to the Bridge?▪