Summary Paper

Feasibility Study of Standard Sections for Segmental Box Girder

Felix Kulka Consulting Engineer (At time study was conducted, Mr. Kulka was President of T. Y. Lin International, San Francisco, )

S. J. Thoman Structural Engineer T. Y Lin International San Francisco, California

egmental prestressed concrete box type, both precast and cast in place, S girder bridges were introduced in were built successfully in the United North America in the late sixties and States and Canada during this time, and early seventies, following their suc- the approximately 70 projects which cessful entry into the European market have been designed to date indicate during the post World War II recon- that the segmental prestressed concrete struction period. Several bridges of this box girder is a very viable alter- native for medium to long span bridge strictures in North America. NOTE: This Summary Paper is a condensation of the results of an investigation commissioned by the Fed- At the same time, it is recognized that eral Highway Administration on the feasibility of using the design and construction of seg- standard sections for segmental prestressed concrete mental bridges still largely follow prac- box girder bridges. The study was initiated in 1980 and completed in July of 1982. The full length report, tices in Europe and that a closer iden- entitled - Feasibility of Standard Sections for Seg- tification with American construction mental Prestressed Concrete Box Girder Bridges" practice is in order. Standardization of {FHWA RO-82;024) by F. Kulka, S. J. Thoman, and T. Y. Lin is available from the National Technical In- certain aspects of segmental box girder formation Service, Springfield, Virginia 22161. bridges appears to he one way to ex-

54 Synopsis Presents the highlights of a study which investigated the feasibility of developing standard sections for segmental prestressed concrete box girder bridges. The report is based on an extensive survey of segmental box girder bridges in the United States and Canada. Recommendations are given for specific items that could be standardized, while also discussing areas which might not be appropriate to standardization.

pand their economical use by instilling specific areas which should be standar- confidence among bridge engineers dized are listed and discussed in the and by producing a cost effectiveness report, as are those which are not cur- through uniformity in design, thus rently subject to standardization and permitting precasters and contractors to those which are questionable. invest in forms and equipment on a broader basis than is done today. Scope of Study This report deals with the feasibility of standardizing segmental prestressed Standardization of highway construc- concrete box girder bridges in the tion elements is a long-standing prac- United States. The study relied heavily tice in the American highway industry. on a survey of bridge engineers in the Development of the AASHTO-PCI I- United States and Canada, which pro- girders is one example; precast con- duced valuable information on all crete culverts, traffic barriers, and piles bridges of this type. Statistical studies are other examples. It is fairly well were conducted to determine correla- agreed that standardization has merits tions and uniformity of significant pa- in cost savings, reduction of construc- rameters, particularly with respect to tion time, and improved product qual- geometry. ity. Analytical design studies, mainly to It was felt that for standardization of determine the economical use of mate- box girder sections to succeed, a uni- rials, were made to augment the statis- form approach should be used in order tical analyses. The results were to permit bridge engineers to design evaluated both qualitatively and quan- such sections with a sufficient degree of titatively, and an advisory technical re- uniformity and to allow precasters and view committee was formed to review contractors to bid and build them as the content of the study and its recom- they would any other advanced type of mendations. structure. The report takes the position that The object of this study, then, was to standardization of segmental pre- consider all the advantages and disad- stressed concrete box girder bridges is vantages of standardization and make possible and should be initiated. The appropriate recommendations for future

PCI JOURNAUSeptember-October 1983 55 development. In doing so, care was State of the Art of taken not to let standardization limit Box Girder Bridges competition, rather, standardization was approached with a view towards Cast-in-place, conventionally formed exploiting all the alternatives, thereby box girder bridges had been used in improving design and increasing com- North America for many years when, in petition. The scope of the study in- the late sixties, segmental box girder cluded: construction was introduced to the 1. An assessment of the state of the continent. This type of structure was a art of segmental bridge construc- European development of the post- tion. World War II era, when the reconstruc- 2. Development of design constraints tion of war-torn European countries as affected by construction limita- demanded methods of construction tion s. which would overcome the scarcity of 3. An analysis of costs and benefits of labor and which would produce many standard sections. structures in the shortest possible time. 4. Development of specific recom- The development of cast-in-place seg- mendations concerning the feasi- mental construction is generally attri- bility of standard sections for seg- buted to Germany, while precast seg- mental prestressed concrete box mental construction is primarily a girder bridges. French innovation. Since the volume of construction was large and there was sufficient invest- Study Approach ment available, the box girder became It was felt essential that the recom- popular even though it is not necessar- mendations concerning possible stan- ily the most economical section for all dardization be based on experiences conditions. The box girder can, how- with existing practices rather than on ever, safely accommodate spans up to arbitrary judgments. 800 ft (244 in) and resist a wide range of Accordingly, a questionnaire con- stresses. Furthermore, its resistance to cerning prestressed concrete segmental torsion made the box girder particularly box girder bridges was sent to bridge suitable for cantilever construction, engineers in all states and territories which proved to be a good method for plus the provinces of Canada. The sur- rapid construction and for achieving vey included bridge site, state of com- long spans without the use of falsework pletion, cross section, design and de- or shoring. tails, construction, costs and other per- The Lievre River Bridge in Quebec tinent information. The response was (completed in 1967) was the first pre- excellent, and the information collected cast prestressed segmental bridge built provided a good sampling for further in North America. This was followed in-depth studies. shortly by the Bear River Bridge near The data obtained were categorized Digby, Nova Scotia. The first major and statistical studies were made to segmental in the evaluate significant parameters, leading United States was the JFK Memorial to a rational assessment of the state of Causeway in Corpus Christi, the art of segmental bridge design and (completed in 1973). construction. Analytical studies were As a result of a fairly active program performed in cases where data were not of promotion, more than 50 segmental available, permitting the establishment bridges have been constructed in North of qualitative and quantitative relation- America since that time. Their record ships. with respect to economy and successful

56 i d

0 ^ C qç/ /

Hawaiian < a_ Islands.------

rte] - _.

I

f I ^

Koror Babelthaup

Puerto Rico

Fig. 1. Distribution of segmental prestressed concrete box girder bridges designed or constructed in the United States or Canada. (Note that bridges in the United States include also those in the Commonwealth of Puerto Rico and the Trust Territories in the South Pacific.) construction was not uniform, owing The conditions surrounding the pres- mostly to a wide variety of site condi- ent state of segmental box girder con- tions, design practices, specifications struction raise the obvious question of and bidding requirements. The 1980 standardizing at least some aspects of dollar cost per square foot of bridge its design and detailing. Ideally, stan- deck of some 37 segmentally con- dardization could bring about cost ben- structed box girder bridges appears to efits by permitting contractors to invest vary widely from $30 to $150 (8323 to in forms, installations and equipment $1615/m 2 ). Nevertheless, sufficient which could be reused more often, thus cases of successful and economical con- reducing the cost of mobilization. De- struction exist to make the segmental tails and joinery could he simplified in box girder a very viable choice in the the process of standardization, and concrete bridge market. overall safety and integrity could he

PCI JOUR NALSeptember- October 1983 57 20 ME

15 15 cn

L m 0 10 0L 7E z

O V] G Lo 0 0 a (0 P- ti CO CO r r r r r

Bid Year Fig. 2. Number of bridges bid in successive years.

added to the structure by making avail- design or start of construction. The able past experience and knowledge to three bridges before 1970 were built in those new in the industry. Canada, which preceded the United The map in Fig. 1 shows the dis- States in segmental bridge construction. tribution of existing segmental bridges Nevertheless, Fig. 2 shows a steady in- in North America (which also includes crease in the use of segmental bridges the Commonwealth of Puerto Rico and and dramatically so in the late seven- the Trust Territories in the South ties. Indeed, it may be concluded that Pacific) in 1981. It can be seen that the this method of bridge construction is vast majority of these bridges lie in the here to stay. eastern part of the United States, which The five major types of constriction is consistent with the distribution of which have been employed are the bal- other bridges as well. Bridges shown on anced cantilever type (precast and cast this map are located in Canada and 18 in place), span-by-span construction, U.S. states and territories. progressive placing, and incremental The histograph in Fig. 2 shows the launching. bridges as they were hid. It can be seen In the balanced cantilever construc- that there is a steady increase in the tion method the segments are cantile- number of bridges from 1970 to the vered out from each side of their sup- eighties. The peaks and valleys are not port, so as to balance the moment too important, since the time at which which is induced in the pier. The seg- the bridge incidence was plotted can ments can either be precast or cast in vary with respect to the completion of place.

58 in progress or completed 35 1- has not been bid or was not selected

30 _Q Q. 3 V

o° 2 5 s ° m U 7 C 61 :s Cl

20 a c C G w a a ro U l a U V > 15 c m I -n to m m m W E U U C z 10 o

Construction Method Fig. 3. Frequency of various construction methods.

In precast construction the segments abutment. Once a segment has reached are manufactured at a factory or at the sufficient strength, it is post-tensioned, project site. The segments are then then vertical and horizontal hydraulic transported to the bridge superstructure jacks are engaged to lift the segments and lifted into their final position and push them out longitudinally from where they are post-tensioned against the abutments. the previously erected segments. Fig. 3 shows the number of segmen- In cast-in-place construction a form tal bridges classified according to con- traveler is employed to carry the forms struction method. Balanced cantilever, into which the segments are cast in both cast in place and precast, com- their final position. After the concrete prises by far the largest percentage of has reached sufficient strength, the bridges. It is interesting to note that in segment is post-tensioned against the reviewing bidding history, more con- already completed superstructure. tractors favored cast in place rather than The span-by-span method features a precast segments when they had a superstructure constructed in one di- choice in the method of construction. rection, one span at a time, incorporat- One reason for this is that contractors ing either precast or cast-in-place seg- are in general more experienced with ments. cast-in-place construction methods. The progressive cantilever method is Of the 33 bridges designed in precast similar to the balanced cantilever segments, 15 were constructed; the cast-in-place construction method, ex- others were not built or changed to a cept that the segments cantilever out- different type. Of the 27 balanced can- ward from only one side of the pier, tilever bridges 24 were constructed as while the sidespan is cast on falsework. designed; the others were either not In the incremental launching method built or changed to a different type. In- the segments are cast near the bridge cremental launching and progressive

PCI JOURNALlSeptember-October 1983 59 ;^ in progress or completed - 4 yw has not been bid or was not selected e 0.09m2 =1.0sf 0

d 0) 3- 0. a) m r c m I

O c2 a 02 ro as ao 0)— } Q U fb _ O c m d N E 0 U U C G IIIII t0 ^C ay 1 v a m M a N . p J C. 0)

Construction Method Fig. 4. Total square footage of bridge deck for various construction methods.

placing represent only one bridge each, equivalent 40-ft (12 m) width, and the hence, cannot necessarily be consid- resulting total bridge length was then ered representative. divided by the number of particular The bridge deck area for the various bridges, thus obtaining an average methods of construction is shown in length of bridge for each construction Fig, 4. It may be seen that span-by-span type. The numbers show that the aver- construction encompasses about 30 age length for span-by-span construc- percent of the total bridge deck area, tion is about 40 percent larger than for but as shown in Fig. 3, represents only balanced cantilever. In other words, it about 10 percent of the total number of bridges. That indicates that this method Table 1. Average bridge lengths for obviously was applied to very large various construction methods. projects. In balanced cantilever con- struction the total bridge deck area is Construction Average length for a about equally divided between cast- method 40-ft (12 m) roadway in-place and precast structures as de- signed, but the proportion of bridges Incremental actually constructed to the total designed launching 1087 ft (331 m) is higher for the cast-in-place structure. Progressive placing The project size greatly influences 1165 if (355 in) the method of construction to he used. Span-by-span 5347 ft (1630 in) Table 1 shows the average length of Balanced segmental bridges surveyed. Here the cantilever total deck area of bridges .for each type (precast) 3133 ft (955 m) (cast in place) 2818 ft (850 rn) of construction was normalized to an

60 ementalin launching

progressive placing v A 0 4A 0.30m=1.Oft AAA pan-byspans 0 • o M• 0 .• •i • • C • • •0"• • U balanced cantilever, PC

■ • ■ ••■■ ■■ ••. ■■ ■ ■■ ME ■ ■ ■ ■ -4^ balanced cantilever, cip !^

100 200 300 400 500 600 700 800 (ft) Span Length Fig. 5. Span ranges of box girder bridges for various construction methods.

200

o a C 150 •• a °

UI • C 0— 100 •• •• U r

o o a Q ••• •^■ e^ ,, tO 50 • m• • c 0.09m2-1.Osf ar ♦ A Em 0.30m=1.Oft a - A

0 a C V

Construction Method

Fig. 6. Cost of segmental prestressed concrete box girder bridges for different construction methods.

PCI JOURNAL/September-October 1983 61

U,a

0 20

a E Z 10

Cross-Sectional Configuration Fig. 7. Frequency of various cross-sectional configurations of box girder bridges.

2.5 0.30m3/m2=1.Ocy/sf 0.30m =1 .Oft

wing section 2.0

1.5 m n C U0 .. w rder 7• 1.0 m V E v 7 O J

0.5

50 100 150 Span Length (ft) Fig. 8. Weight of mild steel reinforcement for various girder types.

62

210 1-cell 2-cells --- 190 0.30m2/m =1.0sf/ft 0.30m= 1.0ft

d 160 /

o // 0 - 130 £( a / A

100 / •• Ar_` W= 70 U

7 0 W - 50• c W - 30^ iitEEEE 40 1ii

10 20 30 40 (ft) Girder Depth Fig. 9. Internal surface forming area for various deck widths and girder depths.

takes a larger project with many short Fig. 6 shows the costs of these spans, as for example a causeway, for bridges. It may be seen that there is no this method to be economical as com- obvious uniformity to be discerned pared to other structural sections. from these cost figures, Partially, the The distribution of construction reason for this is the fact that accurate methods for segmental bridges with re- costs are very difficult to establish. First spect to span length is shown in Fig. 5. of all, cost figures are not readily avail- It can be noted that the balanced can- able; secondly, when they are available tilever method was used primarily for it is not totally clear what the costs spans greater than 200 ft (61 m). The cover. However, costs do vary widely span-by-span method was used for principally as the result of lack in uni- spans between 80 and 180 ft (24 and 55 formity of design and construction m) as were the incremental launching practices. In any event, the figures and progressive placing methods. might demonstrate qualitatively the fact For spans longer than 450 ft (137 m), that costs of the bridges varied consid- only cast-in-place segments were erably within the construction method employed, most likely because of the itself, in addition to differences be- increased weight of precast segments tween the various construction needed for long spans. methods.

PCI JOURNALISeptember-October 1983 63

1 1 -Cell 2 -cells -- -

W=70 10 m / U / 0 // W 5O8

E 5 ^y W=30 o > 0.30 m= 1.Oft 0.09m3/m = 1.Ocy /ft

10 20 30 40 (t t) Girder Depth

Fig. 10. Volume of concrete for various deck widths and girder depths.

100 1 -cell 2-cells -----

$0 4.45N =1.01b 0.30m -1.0ft

c / m 60 of

/ a " 40 J( `o /

20

30 40 (ft) 50 60 70 Width of Deck Fig. 11. Weight of transverse prestressing steel for various deck widths.

64 correlation = 0.85 A cast-in-place = 0.87 • precast = 0.70 0.30m = i.Oft 3.0 W

A T WT 1D A 2.0 I- 3: m..I- r ^' r- m v • a ..A • aL AA 0.003L+0.026 (A) Q • 1.0 L • ... 3: 0.002L + 0.024 (•) .-:;-:r_A

200 300 400 500 600 700 800 (ft) Span Length Fig. 12. Variation of web parameter with span length.

Bridge Cross Sections mild steel, the volume of concrete, and the area of internal forming for various The cross sections of bridges used in girder depths and roadway widths were the United States and Canada con- compared. Figs. 8 through 11 show re- tained single cells, double cells, triple lationships between material quantities cells, and twin single cells. The histo- and girder depths for the various top gram in Fig. 7 shows the number and flange widths of the box sections. shape of cells incorporated into the In Fig. 10 it may be seen that the vol- cross section of the box girders used to ume of concrete in a single cell section date. It is evident that the single cell or is less than that of a double box for a a combination of single cells is the most 30-ft (9 m) width, but as the width in- widely used section, representing about creases the difference diminishes. In a 90 percent of all bridges surveyed. 70-ft (21 m) width the volume of con- In order to establish the cost effec- crete is greater for a single cell than for tiveness of the single cell and double a double cell. This conclusion may also cell cross-sectional configurations, pre- be reached by realizing that a longer liminary designs were made to study span requires the top flange width to be required material quantities. The increased in thickness in order to carry weight of prestressing steel, weight of the heavier traffic loading.

PCI JOURNAUSeptember-October 1983 65 correlation = 0.83 4.0 i W cast-in-place - 0.87 • precast=0.52 0.30m=1.Oft

....:r ST ♦ 3.0 l^ v SW /

(A) 0.005L-0.654

AL

2.0 t (D E A I- AA

• • • ♦ 0 0 1.0 • • ♦0.003L +0.234 (•) A • •

• ♦ N f

200 300 400 500 600 700 Span Length (ft)

Fig. 13. Variation of soffit parameter with span length.

The weight of mild reinforcing steel It can therefore be deduced that the is less for the single cell section than single cell section is more economical for the double cell section for all sec- than the multiple section in all aspects, tion widths. This, again, is reasonable, except for the transverse prestressing since much of the mild steel is nominal steel. This is true up to a width of ap- reinforcing and the loads are carried proximately 70 ft (21 m), at which point largely by the post-tensioning tendons. twin single cells should be considered. The internal surface forming area is considerably less in the single cell sec- tion, which translates into great econ- Statistical Studies of omy for formwork. The elimination of interior webs also produces a more con- Dimension Parameters structable section. The required In order to determine the degree of amount of transverse post-tensioning is, uniformity in dimensions, parameters of course, higher for the single-cell in the transverse and longitudinal di- section than for the double-cell section. rection were studied statistically for the

66 3•1- CL correlation = 0.60 0.30m =1 .Oft

m m 7 > 2.0 • •

• • • 0.064CL + 0.81 4

1.0

0 U O

5 10 15 20 (ft) Length of Cantilever

Fig. 14. Variation of cantilever deck thickness with cantilever length.

bridges surveyed. Linear regression span length is shown in Fig. 12. The curves were fitted through the data correlation coefficient was 0.85 when points using a least square criterion. combined and 0.87 and 0.70 when Correlation coefficients were calcu- studied independently for cast-in-place lated to determine the uniformity be- and precast bridges, respectively. tween the parameters. The parameters These values indicate uniformity, with correlation coefficients greater which suggests the feasibility of stan- than approximately 0.80 were consid- dardization. ered to be related, indicating uni- It is interesting to note that the func- formity. Such uniformity would suggest tion for these precast bridges was that the parameters lend themselves to below and somewhat parallel to cast- standardization. Note that precast and in-place bridges. This indicates that for cast-in-place bridges were considered the same span length the precast seg- together and also independently. ments incorporate thinner webs than To study the web dimensions for a their cast-in-place equivalents, which particular span length, the web area for may be related to weight reduction those bridges surveyed was normalized strived for in plant production. by the bridge width. This accounted for The study of the soffit parameter, the varied number of traffic lanes and shown in Fig. 13, was defined by di- loading conditions. The web parameter viding the soffit cross-sectional area (lo- was defined as the total area of the web cated near the pier) by the bridge divided by the bridge width. The re- width, which normalized the different lationship between web parameter and bridges surveyed. Quantitatively, when

PCI JOURNAUSeptemher-October 1983 67 correlation = 0.95 20 A cast-in-place • precast 0.30m=1.Oft D 7 11

15

m 0 a Z. 0m 0.042L+0.45 i0 10 • slope 1:22 to 1:23 • • ^

• •

200 250 300 350 400 450 (ft) Span Length Fig. 15. Variation of girder depth with span length for balanced cantilever construction.

the structural system is continuous over The low correlation may be attri- a support, the bottom soffit near the buted to the varying amount of trans- support must develop a compressive verse prestressing in the deck, which force to resist the induced moment. was not included in the study. Also, the Since this induced moment is related to deck thickness of the cantilever at its the span length, the bottom soffit area support may he controlled by dimen- must also increase with increasing span sioning requirements to accommodate length. The correlation coefficient con- the longitudinal tendon anchorages, in- sidering both precast and cast-in-place stead of providing the amount of resis- segments was 0.83, indicating good cor- tance to induced forces. relation. A high correlation was found be- In Fig. 14, the deck thickness at the tween span length and girder depth for cantilever base is plotted against the balanced cantilevers with constant length of cantilever. The figure repre- depth sections, as shown in Fig. 15. A sents the results of a study of the deck correlation coefficient of 0.95 was cal- thickness at the transverse cantilever culated for the bridges considered. Re- support as a function of the cantilever sults show that the average span-to- length. AIthough a low correlation depth ratio was between 22 and 23 for coefficient of 0.60 was calculated, the span ranges between 130 and 450 ft (40 deck thickness could intuitively be and 137 m). Also, the majority of con- standardized For a particular bridge stant depth structures are precast as op- width. posed to cast in plaee. correlation 50 r A cast-in-place =0.84 • precast =0.60 0.30m =1.Oft

MIOP L 0

0 4.0 AL s A a W

L ^ Q a 3.0 -0.004L+O.9 (A) c .. a i f • ^ 2

0 2.0 0 a- A 0.003E+1.13 (•) •

300 400 500 600 700 800 (ft) Span Length

Fig. 16. Pier to midspan girder depth ratio for various span lengths for balanced cantilever construction.

The longitudinal haunch ratios, de- segments. Also, the infrequent use of fined as the pier-to-midspan-depth haunched precast concrete segments ratios, were studied for those bridges resulted in insufficient data for statisti- employing balanced cantilever con- caI analysis. struction. The results are shown in Fig. 16. The cast-in-place haunch ratios varied from 1.7 for the shorter spans to Preliminary Designs for 4.3 for the longer spans. A correlation Various Construction coefficient of 0.84 was calculated for cast-in-place bridges, indicating high Methods uniformity. The low correlation coeffi- Preliminary designs were made to cient for precast construction may determine the cost effectiveness of the suggest difficulties or reluctance as- various construction methods. Quan- sociated with using precast haunched tities of materials rather than cost fig-

PCI JOURNAL/September-October 1983 69 5.0

0.30m31m2 =1.Ocf/sf 0.30m = 1.0ft

4.0

incremental r balanced cantilever prismatic, precast/ launching U cast-in-place Cast-in-place O U

3.0

E v/c -precast 0 balanced progressive cantilever non-prismatic ressive placing

span-by-span

100 200 300 400 500 600 700 800 (ft) Span Length

Fig. 17. Volume of concrete for various construction methods.

ures were used, since the latter are too The weight of prestressing steel ver- variable. sus span lengths is shown in Fig. 18. Fig. 17 shows the volume of concrete The relationship is similar to that of plotted against span lengths for the volume of concrete for the various various construction methods. Span- methods ofconstnrction. by-span construction is more efficient From these curves and from other in the lower span ranges, with balanced data presented it may he concluded that cantilever being more efficient in the balanced cantilever is the most preva- higher ranges. Incremental launching is lent method of construction for spans cost-effective up to about 200-ft (61 m) over 150 It (46 m). Up to 300 ft (91 m), spans, but becomes inefficient beyond precast construction is advantageous that point, apparently because of the because such spans permit a constant need to employ concentric prestressing. depth of section. Once a parabolic Progressive placing shows economy of haunch is necessary to accommodate concrete volume up to about 200 ft (61 the span, the cast-in-place section be- m). comes more appropriate. It has been

70 20 incremental launching 0.30m =1 Oft 47.9NJm2=1.Olb/sf 15 / / N ^^ / 5, 1 fl non–prismatic progressive placing-'' o ^! prismatic L 5 ^' a, balanced & progressive cantilever ti- span-by-span

100 200 300 400 500 600 700 800 Span Length (ft) Fig. 18. Weight of prestressing steel for various construction methods.

Table 2. Parameters feasible for standardization. Parameter Yes No Maybe Parameter Yes No Maybe Cross section Span depth ratios ■ dimensions ■

Shape of box • Haunch ratios ■ Number of cells ■ Radius ofcurvature ■ Segment length ■ Construction method ■ Joint details ■ Span limitations ■ Post-tensioning details ■ Roadway details ■ Design guides ■ Reinforcement ■ Design criteria ■ Specifications ■

used for spans up to about 800 ft (244 tion of use, which was based on an ad- m). Span-by-span construction is 're- ditional questionnaire sent to bridge stricted to the shorter spans, perhaps up engineers in the United States and to 150 ft (46 in). Canada, It shows that the projected use, Items suitable for standardization are in their opinion, will feature to a great summarized in Table 2. extent spans between 80 and 120 ft (24 Fig. 19 shows an interesting projec- and 37 m). This indicates that segmen-

PCI JOURNAUSeptember-October 1983 71 95

80

60 m r, m ^ ^ c 0 = o E40

20

o 0 0 0 0 o o MD o 0 Ui 4 W u] N o o 0 0

0 0 0 0 0 0 CD 0 IL) o MD a a m r c) V W m Span Ranges (ft)

Fig. 19. Comparison of projected total square footage of bridge deck within span ranges for steel, reinforced and prestressed concrete structures.

Fig. 20. Configurations of various bridge sections.

72

12 47.9Nlm2=1.Olb/sf 0.30m =1.Oft

10

wing section

8 1-girder C

^ a 6 o ^. box girder z as 4 -T-girder

2

50 100 150 (ft) Span Length

Fig. 21, Volume of concrete for various girder types.

tal box girder construction will have to Fig. 21 plots the volume of concrete compete with other types of construc- versus span length, showing that the tion which have shown economies in volume is lowest for the T-section, these particular span ranges. For that There is a cross-over point at about 90 ft purpose a comparative study was made (27 m) between the box section and the between the box section and other sec- I-section. Comparing the weight of lon- tional forms. gitudinal post-tensioning steel, as shown in Fig. 22, the I-section is low- Comparative Studies of est, but there is a cross-over point at about 110 ft (33 m) with the box section. Bridge Sections Comparing the mild steel required, the An analytical study produced the T-section is again lowest (see Fig. 23). comparison of quantities for the box The I-section and the box section have section, I-girder, T-section, and wing a cross-over point at about 85 ft (26 m). section, as shown in Fig. 20. Assessing the cost, as shown in Fig, 24,

PCI JOUR NAL/September-October 1983 73 6 47.9N/m2= 1.0lbisf 0.30m=1.Oft

5

4 w

N N - m 3 N ^ rder Q) 5- 0 .c 2 a)

50 100 150 (ft) Span Length Fig. 22. Weight of longitudinal prestressing steel for various girder types. the T-girder appears to be the most permit appropriate redesign of impor- economical in the lower spans. The box tant features within the scope of the section becomes most economical specifications. above about 140 ft (43 m). This comparison is not absolute, and all types of construction could be CONCLUSIONS economical under certain conditions. Much depends on the mobilization The survey of segmental prestressed cost, which is a constant cost to be concrete box girder bridges yielded re- added to the individual curves, but one sults which are very encouraging for which is very subjective, and hence potential standardization. A sufficient cannot be accurately determined. It degree of uniformity was found among shows that bridge design should em- the various parameters indicating that phasize the option of the contract and certain aspects of design and construe-

74

1300 1-cell 2 -cells — — —

0.30m=t.Oft / A 1100 14.6N/m =1.01b/ft A/

f

A/

900 Aj A& m u1 W = 70`

m 700 l/ o ^ / s A / W = 50'

500 f/

300

10 20 30 40 (ft) Girder Depth

Fig. 23. Weight of mild reinforcing steel for various deck widths and girder depths.

tion can be standardized. The following 2. The standard sections should recommendations are suggested: specify primary dimensions, as well as 1. Only the single-cell box should be secondary dimensions, defining the standardized. The cross-sectional di- shape, but permitting variable segment mensions to be covered by standardi- lengths for cast-in-place segments and zation should accommodate bridge specifying the segment length for pre- widths between 30 and 70 ft (9 and 23 cast segments. m). Twin cell bridge box sections can 3. Only bridges with constant depth be used to reach roadway widths be- should he considered. Span-to-girder- yond 70 ft (21 m). Multiple—cell sections depth ratios could be specified for con- should be left to individual design. stant depth sections. Both precast and cast-in-place segments 4. Standardization of sections should should he included in the standardiza- be directed to straight bridges with tion. span lengths between 80 and 300 ft (24

PCI JOURNAL/September-October 1983 75

40 0.09$/m2=1.0$/sf

30 r wing section

w N o U ^ 20 box girde I—girder

T—girder 10

50 100 150 (ft) Span Length Fig. 24. Cost comparison for various girder types.

and 91 m). For spans greater than 300 ft considered, especially in conjunction (91 m), standardization does not yet with the transverse prestressing tendon seem practical, but guidelines for de- layout in the deck. sign and construction could be pro- 7. Transverse prestressing design and vided. Similarly, recommendations for tendon layout should be standardized, the accommodation of curved bridges since they can seriously affect top slab should be included. dimensions. 5. Construction methods themselves 8. Vertical prestressing design and should not he standardized, but the tendon layout for the webs should not standardization of sections should con- be standardized, since they are gener- sider the balanced cantilever method ally not needed fbr spans under 250 ft and the span-by-span method, both cast (76 in). The design procedure and de- in place and precast. The progressive tailing may he recommended. placing and incremental launching 9. The possible use of external ten- methods are not as yet sufficiently in dons for shorter spans should be use to be included in standardization of treated, and recommendations fbr dif- sections. ferential localities and environments 6. Longitudinal prestressing design should be made. and tendon layout should not be stan- 10. Joints, both match-cast and wet, dardized, but the magnitude of pre- should be standardized. Single or mul- stressing force and eccentricity re- tiple shear key designs using epoxy quired for the final condition of the between abutting precast segments structure should be indicated. Their could be standardized. The possibility effect on section dimensions should be of eliminating the epoxy between the

76 precast segments should be studied zation could be carried out is not as further. clear in the case of box girder bridges, 11. Typical designs of anchorages and given their complexity, difference in blisters for continuity and cap tendons construction methods, span ranges, and may be suggested, but not standar- other variables. In any event, stan- dized. dardization should be done to enhance 12. The use of bonded mild steel re- the use of segmental bridges by design- inforcement for partial prestressing, ers and contractors, but it should be temperature and shrinkage control, conceived and applied so as not to im- stress concentration, prevention of de- pede new developments which might lamination, and other local problems bring about greater economy, higher may be recommended. safety and better performance. 13. Design of sidewalks, bicycle paths, barriers, and railings should not be standardized. ACKNOWLEDGMENT 14. Deflection control, both during construction and after completion, This investigation was sponsored by should he taken into account in the di- the Office of Research and Develop- mensioning of standard sections. ment, Federal Highway Administration, 15. Location of expansion joints, both U.S. Department of Transportation, temporary and permanent, may affect Washington, D.C., under the direction the design of standard sections, and of Thomas Krylowski and Craig A. Bal- guidelines should be established. linger. 16. Uniformity in design and specifi- The authors wish to thank the cations should be addressed. FHWA's technical review committee for help and guidance throughout the It would be very desirable, indeed, if study and preparation of the report. standardization of segmental pre- Members included Thomas Alberdi, stressed concrete box girder bridges John Breen, Clifford Freyermuth, could be accomplished to the same de- Wayne Henneberger, Jerry Jacques and gree as AASHTO I-girders. The ap- Gordon Ray. The authors also wish to proach could certainly be similar to that express their appreciation to the bridge of the I-girders, inasmuch as dimen- engineers in the United States and sional standards and construction prac- Canada for the information they pro- tices could he made uniform, vided in the bridge survey question- The extent to which such standardi- naire.

NOTE: Discussion of this paper is invited. Please submit your comments to PCI Headquarters by May 1, 1984.

PCI JOURNALlSeptember-October 1983 77