Precast Segmental Cantilever Bridges

American Segmental Bridge Institute DESIGN AND CONSTRUCTION OF SEGMENTAL CONCRETE BRIDGES Springfield, Illinois April 17-18, 1997

PRECAST SEGMENTAL CANTILEVER BRIDGES

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By: Joseph P. LoBuono LoBuono Armstrong & Associates

"Precast, Segmental, Balanced Cantilever Design" By: Joseph P. LoBuono, P.E. I. INTRODUCTION This portion of the seminar features the design of precast segmental concrete bridges erected by the balanced cantilever method. This presentation will deal with the practical design issues of the method and highlight the impact of the construction methodology on the design. This presentation is divided into the following sections: I. II. III. IV. V.

Introduction Balanced Cantilever Method Cross Section Statical Scheme Construction Methods/Forces

As may already be realized, the segmental concrete bridge form requires an interrelationship of design and construction knowledge. Given the impact of construction methods and construction loads on the basic design of the system, a design for service loads alone is inappropriate. An engineer must take a step back from the project and evaluate the project as a whole before deciding on the method of construction, span arrangement, statical scheme and cross-section to be used. It is the author's belief that a feasible, cost effective and complete construction sequence and methodology should be presented on the design drawings. Maximum leeway for contractor modifications with regard to construction technology should be provided in the specifications, but the basic system should be clearly and concisely presented to facilitate the receipt of accurate and responsive bids. Given the fact that the American system is based on a separation of design and construction, and further considering the relatively short bid preparation time, contractors must be presented with a scheme that works and has no hidden costs that become evident after the project has been awarded. It is not the intent of this presentation to deal with in-depth structural analysis and design issues. As segmental design and construction technology have progressed over the years, several notable text books, papers, articles and handbooks have been published. Additionally, prestressed and reinforced concrete technologies have also been advancing and converging on a unified theory. As these are the basic tools of designing a segmental bridge, the participant is urged to remain current with the latest technological advancements. Particular interest is 1

warranted with regard to shear, torsion and design of areas of disturbance. Advances in compression field theory, truss analogies and strut and tie modelling are now providing the designer with tools to analyze complex problems with a rational feel for the mechanisms involved. Given the inter-relationship of design and construction, the designer is required to determine the dead load stress state upon completion of the construction and at the time of all losses in the future due to creep and shrinkage. The construction method will be a major determinant in the dead load stress state. The goal of the segmental design should be that the structure designed to resist service loads is adequate to resist the forces resulting from the construction process without the addition of permanent strengthening measures. While temporary measures and/or details may be required, for the most part, segmental structures do not require additional permanent strengthening. Should a design require additional strengthening, one should review the choice of construction methodology in order to determine a method or sequence with no impact on the permanent structure. As with any structural system, there is leeway in determining a given project’s parameters. There are not many hard and fast rules, and in most cases engineering and construction judgment play a heavy role. Experience is the best teacher so even though one may be designing one's first bridge, one should avail oneself of sample projects done by others. Knowledge gained by contractors should also be accessed as they generally will provide handson insight into the construction process. It will be virtually impossible to satisfy all possible scenarios in determining the key parameters of a project. The design should be focused on the common denominator with sufficient leeway for someone to propose an innovative, yet safe alternative.

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II. BALANCED CANTILEVER METHOD This presentation will address concrete box girders as this is the most prevalent cross section type used. Balanced cantilever construction generally has a lower span limit based on economics of approximately 180 feet although special considerations may make it economically viable down to spans in the range of 120 feet. On the upper end, the method has been used for spans in the 400 feet range. The weight of the haunched segments near the pier diminish the feasibility of using precast segments in balanced cantilever beyond the 400 feet range. As the name implies, segments are placed symmetrically about a pier so that upon application of the permanent cantilever tendons, the superstructure is balanced. Subsequent pairs are added and stressed to the permanent structure until the cantilever is completed and ready for continuity to be made with an adjacent cantilever or abutment segment (or segments). Figure No. 1 depicts the typical sequence.

FIGURE NO. 1

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A balanced cantilever project starts as any other in that optimum span arrangements need to be determined. First, any underbridge obstructions need to be identified and quantified as to location and dimension. For waterways, the task is sometimes simplified by knowing what the shipping channel and fender requirements are. For other projects, constraints may include roadways, utilities, railroads, streams, secondary shipping channels, etc. Constraints may also be philosophical as in the case of long, open, navigable waterways where a minimization of the number of piers reduces the risk of collision by an errant vessel. Secondly, for projects where there are not many constraints to pier locations, an optimum span study should be conducted for various structural systems. The study should include a fairly accurate substructure evaluation. Cost curves should be generated as a function of span length, and an optimum span should be chosen for each alternative structure type. If the project is not repetitive due to underbridge or other constraints, then a representative portion of the project should be studied to optimize the structural system. If one were evaluating segmental concrete, then one would most likely evaluate span-by-span construction in the 100 to 150 feet range and balanced cantilever construction in the 150 to 400 feet range. A special case arises for projects with a ship impact design criteria. For this case, a minimum foundation strength will be required to resist the ship impact forces. The logical next step would be to select a span length that would utilize this strength for service loads so that ship impact would not be a governing load case. Judgment would, however, be required to evaluate the cost of a more expensive superstructure versus the cost of a foundation system governed by impact forces. The next step in optimizing the segmental alternative is the selection of the erection method. Assuming that a longer span solution is required and that balanced cantilever is the chosen scheme, the choices for erection method are: • • •

Ground/Water Based Cranes Beam and Winch Overhead Gantry

Figure No. 2 depicts schematics of these systems. The systems are listed with regard to cost with cranes normally being the least expensive. However, if the crane needs to be barge mounted and due to the height of the structure and/or the weight of the segments, it also requires a ringer setup, then a beam and winch or even a gantry may be less costly. Each project must be specifically reviewed to determine the appropriate choice of erection equipment.

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FIGURE NO. 2 This is an extremely important step because it will have an impact on the optimization process as well as the method of analysis and design. The designer's choice and his/her assumptions must be clearly shown on the final contract documents. In determining the cost of the erection systems noted above, total costs and duration of need will have to considered. Generally speaking, specially fabricated equipment is amortized on the single project for which it was designed. Therefore, the entire cost of the equipment should be used in determining the project cost. As the use of segmental construction increases, "on the shelf" equipment may be considered, but for the present market the cost of new equipment should be assumed. Some general guidelines for estimating costs at the concept phase are noted below. These are for projects of average complexity and exclude special considerations. •

The cost of special equipment and casting machines should be in the range of 8 to 12% of the estimated bridge cost.

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Cranes should be estimated on a monthly rental basis complete with operator, maintenance and fuel. Duration of usage should be based on an average erection rate of 4 segments (2 symmetrical pairs) per working day. Closure joints will require one week. Cranes needed for other activities such as pier construction are assumed to be included in the unit price for that element of work and need not be considered in the segmental superstructure cost. Crane rental prices can be obtained from local dealers or from larger heavy construction contractors.

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For marine projects, the cost of a barge and ringer setup (if needed) will have to be included. Also, the part time cost of a tug boat should be assigned to the superstructure cost.

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The cost of casting machines for segments of average proportion (weight = 80 tons, length = 12 feet) is in the range of $70,000 to $100,000 per unit. The largest paraMeter affecting this cost is the degree of hydraulics and hydraulic controls utilized in the operational design. This premium cost obviously involves trade-off for the precaster against extra labor for machines which utilize a higher degree of manual adjustment. Pier segment machines are typically 20% cheaper than typical machines, since they generally require less hydraulics and less geometric adjustment provisions than in a typical machine with match casting capabilities.

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Beams for a beam and winch system will cost $50,000 to $80,000 per pair with a pair required at each cantilever tip to lift one segment. A pair of winches can drive two pairs of beams or a winch can be mounted on each machine. The average monthly rental will be approximately $10,000 per winch.

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Gantry costs will vary by the complexity of their function. Key parameters affecting their cost are: -

Presence of horizontal curvature Need to lift pier segments Variability of span lengths Segment weights Maximum span length

As a general guideline based on average complexity: Short Span Gantry (150'-200'): Medium Span Gantry (200'-250'): Long Span Gantry (250'-350'):

$ 750,000 to $1,500,000 $1,500,000 to $2,500,000 $2,500,000 to $4,000,000

As the design progresses from concept thru preliminary to final, the designer will continue to refine the cost estimates. For specialized equipment, the designer must determine the weight of the equipment so as to check the design for construction loads. The author strongly recommends an accurate estimate of the equipment weights and their presentation on 6

the design documents. Figure No. 3 presents an example of such a construction load summary. The designer should perform sufficient preliminary design engineering to know that the weights and distributions used in the design are realistic and achievable.

FIGURE NO. 3 The above point cannot be overstressed. There have been problems in the past where construction loads were underestimated or misrepresented. The result was the required strengthening of the permanent structure which led to additional costs and claims. If one is designing a segmental structure then one must assume responsibility for the design and method of construction used as the assumption for the design Once the span lengths and erection method have been established, the next task is to determine the lengths of the cantilevers. If one is designing a long repetitive viaduct of constant component spans, then the length of cantilever is easily determined as shown in Figure No. 4. However, as is typically the case, span lengths vary due to site constraints. In such cases, it is the designer's responsibility to determine the number of cantilever types (lengths) 7

required. As a general rule there is no penalty for having differing cantilever lengths. However, standardization is at the heart of the segmental process and the number of variations should be minimized.

FIGURE NO. 4 The length of a cantilever is determined by the number of segments required to achieve closure with a neighboring cantilever. The next key decision to make is the weight and therefore the length of the typical segment. Several factors enter into this decision including: Shipping: If the casting yard is remote to the site and segments will be hauled on the road network, then width and weight restrictions will govern the segment size. For onroad hauling, generally the length of the segment will be limited to 9 feet or a weight of 40 to 50 tons. Local requirements must be reviewed in all cases. If water access is available then there is virtually no shipping limitation to the size or weight. However, some projects have shipped by water but then required a transloading to local streets to access the construction site. Erection Equipment: Typical erection equipment for segmental structures is based on handling approximately 80 tons assuming that shipping limitations do not govern. For the case of crane erection, height of lift must be considered to determine crane capacity. For gantry systems, 80 to 100 tons is the norm as well as for beam and winch systems. In general, segment lengths are in the 8 to 12 feet range. Handling Stresses: There are no structural limitations to the length or weight of a segment except that the segment must be lifted several times before its final inclusion into the structure. There are several lifting methods available and most will induce bending and shear stresses into the segment. A large lifting weight will magnify these temporary stresses potentially to the point where the section may have to be thickened or additional reinforcing or prestress added. Handling stresses are generally not a governing consideration but should be investigated. Figure No. 5 depicts several common lifting scenarios.

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FIGURE NO. 5 The term "balanced" cantilever refers to the construction process as sequential pairs are added about the pier. To achieve closure, however, some "unbalance" may be required. It is common for one and sometimes two additional segments to be added to one side of the pier for the purpose of closing a span. Figure No. 6 presents an unbalanced cantilever that was necessary due to constraints on pier locations or the desire to not create an untypical segment length.

FIGURE NO. 6 9

Figure No. 7 depicts a typical situation for a balanced cantilever project. It was decided that the typical lifting weight would be 75 tons based on limitations of the overhead gantry and of the segments to be transported along the deck. Since the haunched segments were heavier in cross-section than typical span segments, their weights were limited to 75 tons by using a shorter segment length. One will note that all haunched segments, irrespective of the span in which they are used, are 5 segments at 7'-0" in length each. This aids in the standardization process. All typical (unhaunched) segments are set at 8'-6" in length. Due to span constraints, there is usually the need to provide "special" typical segments. While this may seem to be an interesting choice of words, special typical segments represent segments with the same crosssection as a typical segment but with a shorter length. They are often necessary to achieve closure without requiring long, cast-in-place closure pours or extremely short precast segments.

FIGURE NO. 7 Due to the need for standardization, it is desirable to minimize the number of special segment lengths required. For the project shown in Figure No. 7, only two special lengths were required. All spans were made up of a series of 5 fixed haunched segments of 7'-0" each, 5 to 12 typical segments of 8'-6" each and one, two or three special segments of either 6'-6" or 7'-0" each. For this project, spans of 196', 203', 222', 237', 250', and 260' were achieved with various combinations of typical and special segments. Having established pier locations and therefore span lengths, it is a highly iterative process to fine tune the cantilever lengths and the number of segments required so as to minimize the number of special length segments. In determining the segment layout, practical casting considerations will generally limit the smallest special length segment to 6'-0". Special segments are formed by having the adjacent mate (the match cast segment) penetrate the typical form for a greater distance than the typical 4" to 6" overlap. One can imagine that if horizontal curvature or vertical camber is prevalent, it would be indeed difficult to secure the forms to the match cast segment. Misalignment may occur in the worst case scenario, and at best an out of tolerance, ill-formed joint may occur. Also to be considered in determining the segment layout, is the length of closure joint. If the joint can be kept to a length equal to the spacing of the transverse reinforcing, then the 10

joint may be unreinforced. This also minimizes the amount of cast-in-place concrete required and therefore the strength of the forming system. Closure joints are thus typically in the 6"-8" range. Some argue for a larger joint and some even require a full segment to be cast-in-place for closure. The argument made is to provide room to transition any geometry differences between the ends of adjacent cantilevers. These differences may be vertical, horizontal or rotational. It is the author's belief that large closure pours are not necessary for geometry control if a project is well inspected. Proper and accurate tracking of the casting and erection process will provide an accurate geometry to the cantilevers. Proper corrective actions during the erection process can achieve a reasonable tolerance at the closure with particular emphasis on control of twisting about the longitudinal axis of the bridge. Given the inherent flexibility of the cantilevers, vertical differences can be eliminated either by the clamping action of the strongback system required for closure or for more pronounced differences, the addition of ballast to the end of the high side cantilever. For cantilevers supported on pot bearings, slight rotations of the cantilevers are also possible to control vertical differences. For horizontal differences, diagonal cables with a come-along are generally sufficient to realign the cantilevers in plan view. Extensive forming, rebar placement and concrete delivery as required by large closure pours, slows down the process and adds heavily to the construction time. A special situation develops at abutments or termination expansion joints. If one had repetitive span lengths, then geometrically it would be ideal for the end span to be one half the interior span. However, since the dead load is supported in cantilever, the only downward dead load reaction at the abutment or expansion joint pier would be the weight of the abutment or expansion joint segment. Since the ratio of the end span to the interior span would be 50%, live load uplift could be a problem. Typically for abutment spans, end segments are supported on scaffolding and the closure joint made at the approximate quarter point. Figure No. 8 depicts such a system. This of course assumes that scaffolding can be used in the end spans and is not restricted by underbridge constraints. In such cases where scaffolding is prohibited, tie downs could be used but are not recommended. Alternatively, the interior of the box girder may be ballasted with lean concrete after closure in order to provide more dead load reaction at the end bearing. Another alternative would be to stiffen the first interior pier so that live load in the first interior span would be resisted by bending in the pier and would not translate to uplift thru the end span. It may then be possible to resist uplift by the weight of the end segment or possibly an additional segment hung in cantilever from the end segment or temporarily supported by the pier shaft.

FIGURE NO. 8

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II. CROSS SECTION The width of cross section at the top slab level is obviously determined by the width of the roadway and sidewalks required to be supported. Once this width is established, other key parameters are determined as shown in Figure No. 9. To date, widths of single cell box girders have been limited to approximately 62 feet based on flexural considerations of the top slab.

FIGURE NO. 9 As shown in Figure No. 10, long slab spans and therefore wide box girders can be accomplished with secondary measures. These include: a) Three Web Box Girder b) Single Cell - Rib Stiffeners c) Single Cell - Strutted In further developing the cross sectional dimensions, consideration must be given to: -

Shear Keys Top Slab Continuity Tendons Bottom Slab Continuity Tendons Temporary Post Tensioning Bars

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FIGURE NO. 10 Figure No. 11 presents a typical bulkhead elevation highlighting the permanent features required of the construction. The shear keys in the slabs are provided for alignment purposes and in the top slab for friction continuity between segments. The web shear keys provide a friction interface for the transfer of the main vertical forces carried by the structure. There is no detailed design required for the web keys except that they should be provided over 80% of the web height between the top and bottom slabs.

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FIGURE NO. 11 Care should be exercised in detailing the area at the intersection of the web and top slab. Figure No. 12 depicts the anchorage area for the top cantilever tendons. Sufficient room should be provided for edge clearances of the anchor plates as well as clearance provided for reinforcing steel.

FIGURE NO. 12 Figure No. 13 presents a typical plan view of both top and bottom slab tendons. Once the cantilever lengths, segment sizes and distribution have been determined, the number of cantilever tendons can be determined and the bulkhead pattern for the top slab arranged. In general, one cantilever tendon is installed per web per segment pair. The anchorage position alternates between the outboard and inboard position for subsequent segments until the last segment is reached. The last segment is generally not supported by a permanent cantilever 14

tendon due to obstructions from the last segment of the adjacent cantilever. Since the top slab will be in compression for future loads (wearing surface, barriers, live loads) and from creep and shrinkage redistribution, a permanent tendon is not generally required. However, for safety, it is prudent to provide prestressing across the top of the closure joint. For the layout shown, this is accomplished by using the temporary bar locations for permanent anchorages.

FIGURE NO. 13 Knowing the number of cantilever tendons and the segment layout, one next must determine the size of the tendon. In general, it is preferable to use one size for the entire project so that only one jack size will be used for all cantilever tendons. Depending on segment weights and the length of cantilever arm, cantilever tendons are typically 12, 15 or 19 strand tendons composed of 0.6" diameter strands or an equivalent number of 1/2" diameter strands. The cantilever tendons provide not only the resistance to the free cantilever negative moments but also provide the resistance to future negative moments arising from superimposed dead loads, live loads, creep and shrinkage redistribution, and thermal gradients. While providing sufficient resistance for future applied negative moments, one must observe bottom fiber stresses during the cantilever construction process. Tension at the bottom surface must be checked as there exists the potential to provide too much cantilever prestressing particularly at the first joint behind the lead edge of construction. In general, providing sufficient prestress to resist dead and construction loads during cantilever construction will provide sufficient primary force and moment resistance for future loadings. Additional top fiber 15

stress over the pier will be provided when the positive moment continuity tendons are stressed after mid-span closure. Recent code developments have presented the concept of negative thermal gradients, i.e., the top slab cooler than the bottom slab. This "new" load case will generate negative moments over the piers and may require additional top tendons which are generally added after closure. For wide cross sections (W/D > 5), it is recommended to add edge continuity tendons in the top slab. These are generally small tendons placed near the wing tips. Their purpose is to protect against shear lag effects which may decompress the joints at the extremity of the cross section. While sophisticated finite element analysis may provide a quantification of this prestress, the effort may not be justified except in special cases of large W/D ratios. Generally, the addition of two - 4 strand tendons in each wing tip should be sufficient. Bottom slab continuity tendons are installed and stressed as part of the span closure operation. These tendons are anchored in blisters at the web/bottom slab interface as shown in Figure No. 14. These tendons are located in the bottom slab as shown in the bulkhead elevation. Generally, the full compliment of tendons is used for approximately 50% of the span symmetrical about the closure joint. Thereafter, the tendons are anchored in successive segments. The terminus of the tendons in relation to the pier segment is established by the number of tendons to be anchored as well as by considering bottom fiber tension arising from thermal gradients. As a general rule, the bottom slab tendons will cover 75-80% of the entire span.

FIGURE NO. 14 For closure sequence design, a strongback system should be provided as schematically shown in Figure No. 15a. The strongback is used to resist differential vertical movement between the ends of adjacent cantilevers. It may also be used to provide small adjustments in the vertical alignments as it can transfer a vertical shear which results in the raising of a low side cantilever while lowering a high side cantilever. For larger adjustments of vertical 16

misalignments, one can provide ballast to the high side cantilever. The use of ballast for adjustment must be checked by the designer during construction for its temporary and permanent effects. Generally the use of ballast is not a problem as the long term effects will redistribute through the system.

FIGURE NO. 15 The strongback system must have freedom to move somewhat in the longitudinal direction as there will be longitudinal movement when stressing the continuity tendons. The closure process starts with the installation of the strongback system and performance of whatever corrective procedures are required to achieve vertical and horizontal alignment. Next, quick-set cast-in-place blocks or precast concrete blocks are grouted into place as shown in Figure No. 15b. The first continuity tendon is installed and stressed to approximately 20% of its final force. This will lock the cantilevers together to prevent displacement during the concreting operation. Forms are then installed and the concrete for the closure joint placed and consolidated. After achieving approximately 2,500 psi of strength, installation and final stressing of the continuity tendons may commence. 17

Another detail affecting the proportioning of the cross section is the location and method of providing temporary post-tensioning. Temporary P/T is provided in the form of bars and is used to squeeze out excess epoxy from the joint as well as to attach the segment to the structure prior to installation and stressing of the permanent, strand cantilever tendons. For a single cell box girder, generally four bars are located in the cross section as shown in Figure 16. The object of the temporary bar design is to provide an approximately uniform stress across the cross section with a minimum stress of 40 psi. Prestress and dead load bending moments are to be considered at the erected joint. The author recommends the use of bottom bars although top bars plus dead load bending stresses may indicate a theoretical uniform stress distribution across the cross section. The potential for improper mating along the bottom slab may lead to an unintentional angle break which could have serious implications for geometry control of the cantilever. It is therefore recommended to use bars across the full cross section to guarantee a positive, mechanical mating of the match cast interface. As shown in the figure, there are two choices for the location of the bars, i.e., internal and external. Placing the bars within the body of the concrete requires coupling and coupling pockets at each joint. Bars must be continuously coupled for the entire cantilever and upon completion, one can imagine the difficulty of uncoupling and removing the bars. Additionally, the ducts may have to be grouted and the blockouts filled. Given this effort, the designer may wish to consider making the bars permanent and leaving them in place. The more common approach is to use external anchorages. These anchorages are located inside the girder at the slab/web interfaces. For the top bars, the blisters are specifically added for the sole purpose of anchoring the temporary bars as shown in Figure 12. On the bottom, the continuity blocks may be used as presented in Figure No. 14. Where continuity anchorages are not present, blocks need to be added exclusively for the temporary bars. By providing two anchorages per block, both top and bottom, bars can be overlapped so that an additional safety can be provided and coupling avoided. If two anchorages per block are not provided, then the bars must be coupled. In either case, one can see that one never requires more than enough bars to support two segments so that once completed, bars for the earlier erected segments may be destressed and used for subsequent segments.

FIGURE NO. 16 18

Another method for external bars is the use of steel bolsters that anchor in holes in the top and bottom slabs (Figure No. 16a). The negative aspect of this scheme is the need to patch the holes as well as the need to move around heavy bolsters. It is however a viable alternative, and the contractor should be permitted to substitute its use. In determining the method of anchoring the temporary bars, attention needs to be given to horizontal curvature. The designer must compute the angle break required and determine if it is within the acceptable limits of the bar anchorage and coupling. If there is a significant curvature, it may not be possible to couple the bars thereby precluding the use of internal bars. Bar anchorages provide more of an allowable angle break than bar couplers so the designer may be forced to use external anchorages. It is general practice to reuse the bars, and therefore the temporary stress in the bar should be limited to 50% of ultimate. If the designer elects to use an internal system and leave it in place, then the one time usage would permit normal, permanent stressing forces. The final topic of this section is the depth of structure. In determining the depth, the predominant considerations are structural optimization, aesthetics and vertical clearance constraints. As a general rule, constant depth girders are used for spans up to the 240' range. Thereafter, a haunch is provided at the pier and either a parabolic or linear variation is utilized in transitioning to a constant depth section. Figure No. 17 presents span to depth ratios for both constant depth and haunched girders.

FIGURE NO. 17 19

For haunched girders, the haunch transition is generally accomplished over one quarter of the span. Should a linear haunch be chosen, then a component vertical force arising from an angle break in the bottom slab must be evaluated. Figure No. 18 depicts such a situation. Depending on the angular change and the magnitude of bottom slab stress, the bending of the bottom slab arising from the vertical component may be managed by additional transverse reinforcing or post-tensioning in the bottom slab. For more severe cases of geometry, a stiffening rib or a full depth diaphragm may be employed as a secondary pour to resist the bottom slab bending.

FIGURE NO. 18 Depths of structure based on structural optimization and maximum efficiency of prestress would generally have a span to depth (S/D) ratio of 18 to 20. For aesthetic considerations, the S/D ratio may be increased to 25 provided one realizes that additional prestress will be required and perhaps a thickening of the webs to accommodate shear requirements. Increased prestress will increase both initial elastic and creep displacements thereby potentially influencing the substructure design. S/D ratios greater than 25 are generally not recommended except under special circumstances and/or requirements. A minimum structure depth is generally established at eight feet based on headroom clearances for workers. This height will permit workers to traverse internally at full standing height. This also permits inspectors adequate headroom for future inspection of the structure. This minimum is therefore based on convenience and efficiency of work area. While several projects have been built with shallower box girders, these have been for span by span construction. Minimum structure depth would generally only come into play for haunched girders where the constant depth section near midspan is based on an VD ratio equal to 50. As an example, a span of 300 feet would indicate a constant depth section of 6 feet and a haunch depth of 15 feet. In a practical application, consideration should be given to a transition from 15 feet to 8 feet. Due to the cantilever method, large compressive forces are generated in the bottom slab over and near the pier. As a result, the bottom slab generally requires thickening not only for 20

cantilever dead load moments but also negative moments generated by superimposed loads and creep and shrinkage redistribution. Figure No. 19 presents a typical situation in both elevation and cross section. The bottom slab thickness at the pier is a function of the width of the bottom slab, the depth of the box, and the maximum cantilever length. It is generally determined by a check of both service load stresses and ultimate strength considerations. For haunched girders, the thickness is generally transitioned over the length of the haunch. For constant depth sections, the slab is transitioned over 15-20% of the span length.

FIGURE NO. 19

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IV. STATICAL SCHEME This section discusses concepts for establishing the statical scheme of a precast, balanced cantilever bridge. Statical scheme means the length of continuity to be achieved, the location of expansion devices and the connectivity of the superstructure to the substructure. Starting with superstructure expansion devices, as shown in Figure No. 20 there are 3 choices for locations: 1) At centerline of pier 2) At adjacent cantilever ends 3) At inflection points

FIGURE NO. 20 One does assume that there will be expansion joints at the bridge ends at abutments, and this discussion therefore is directed at interior expansion joints. Expansion joints at the centerline of pier require that the two segment halves be temporarily locked together as shown in Figure No. 21. This is required so that cantilever construction may proceed from an expansion pier. After closure with adjacent cantilevers, the cantilever tendons must be released and removed in order to provide free movement of the superstructure. Additionally, during the 22

cantilever process, stability must be provided to the pier segments since they will be supported on bearings. Stability methods are discussed in Section V. Upon release of the expansion joint segments by cutting of the temporary cantilever tendons, stress built into the locked halves will be released into the structure. For this reason, at least 2 spans ahead of the expansion joint pier are constructed and closed prior to release of the expansion segment halves.

FIGURE NO. 21 Hinges may be used at or near inflection points in the span, and in a similar manner to joints at a pier, the hinge segments must be temporarily locked to permit cantilevering past the hinge. This is schematically presented in Figure No. 22. After closure of subsequent spans, the cantilever tendons must be released and removed to free the hinge.

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FIGURE NO. 22 A third choice is to provide the expansion joint at the interface of the two cantilevers comprising a span. Instead of providing a concrete closure joint at this point, one can provide a shear transfer device as shown in Figure No. 23. This interface would appear to be the logical and natural location at which to install a joint provided deflection and rotation criteria can be met. This location does not require the temporary locking and subsequent release of expansion joint segments. Accordingly, it is less disruptive to the construction process.

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FIGURE NO. 23 Connectivity of the superstructure to substructure will become a matter of personal choice as there are positives and negatives to creating a fixed, pinned or roller connection. If there are extraordinary loads present in a design such as ship impact or earthquake forces or if there are piers of significant height then a fixed connection becomes desirable to reduce the moments in the piers and to provide improved bracing characteristics through reverse curvature bending. Another positive to a fixed connection is that temporary stabilizers will not be required during construction and the elimination of bearings will reduce future potential maintenance costs. On the negative side of fixity, a stiffer structure is created longitudinally thereby increasing the effects of creep, shrinkage and temperature on the substructure design and also the restraint effect for continuity tendons. From a maintenance perspective, it is preferable to maximize the length of continuity to eliminate expansion joints. It is further preferable, from the same maintenance perspective, to eliminate bearings. One of the key parameters affecting this choice is the height and therefore the flexibility of the piers. To maximize length of continuity for a project with short piers, bearings will be required so as not to introduce large moments in the substructure due to creep, shrinkage and temperature. For piers of medium height, a hybrid of fixed and roller 25

connections may be the most efficient to maximize the length of continuity. For tall piers, priority consideration should be given to fixed connections for their inherent bracing and stability characteristics. When considering fixity of the superstructure to the substructure, details of fixity must be given serious consideration. Precast pier segments are inherently complicated due to high densities of reinforcing steel and post-tensioning. The introduction of vertical post-tensioning for future connectivity to the pier will present a further complication to the precasting operation. The author believes that consideration should be given to the use of cast-in-place pier segments. These segments may be cast at the time of pier shaft construction and the connectivity details constructed with reinforcing steel arising from the shaft. The advantages of this concept are: 1) Elimination of pier segments from the casting yard operation. 2) Elimination of the need for temporary stabilizing systems 3) For sizing of erection equipment (gantry, crane, etc.), elimination of a major design consideration - erection of the pier segment On the negative side, closure joints will be required either side of the pier segment to provide an interface between the cast-in-place and precast construction.

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V. CONSTRUCTION METHODS/FORCES Once the decision has been made to use the balanced cantilever method to erect segments on the bridge, a suitable method of transporting segments and lifting them into position on the structure must be chosen. There are three basic categories of segment placement methods in common use with limitless variations in their detailed application (Figure No. 2). The three categories are: 1) Independent Lifting Apparatus (Land or Barge-Mounted Cranes) 2) Deck-Mounted Lifting Equipment (Beam and Winch) 3) Overhead Gantry The selection of a segment transport and placement method is based primarily on three considerations specific to the project: 1) Site Conditions/Structure Characteristics 2) Economics of Scale of the Project 3) Schedule Constraints Often the factors dictating the use of the balanced cantilever method of construction of bringing the segments into position and lifting them into place. The project location and characteristics of the site may dictate or eliminate certain methods of construction. This may include access limitations underneath the bridge when positioning the segment for lifting or for supporting the erection equipment during lifting and placement operations. Such restrictions may be imposed when constructing bridges over water too shallow to accommodate barges, in mountainous or environmentally sensitive terrain, or in congested urban environments. Other site factors or structure characteristics which may influence the choice of segment placement methods is the geometric configuration of the structure. This may involve consideration of the maximum span length, the range and uniformity of span lengths, the height of the piers, or the degree of horizontal curvature and maximum grade of the alignment. Economics of scale refer to the size of the project and length of structure to be built. On smaller projects, it may be necessary to resort to relatively simple equipment and more time-consuming methods to erect segments in order to keep equipment costs commensurate with the overall cost of the project. On larger projects where repetition is involved or where significant cost can be saved through increased speed of erection, it may be prudent to invest in more sophisticated equipment. There is a trade-off between speed and increased equipment cost on projects of significant size to absorb the additional capital expenditure. Closely related to the scale of the project is the scheduling or time constraints imposed on the project. Fast-track construction may necessitate the use of multiple erection headings which require more than one set of erection equipment. The cost of multiple beam and winch setups for simultaneous construction of several cantilevers may or may not trade off favorably with the cost of a launching gantry which can quickly erect cantilevers sequentially from one end of the project to the other. Construction contracts which reward early completion often promote increased expenditures on the equipment necessary to meet these ends.

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Other factors which may influence the choice of segment placement methods include the past experience of the contractor, the availability of equipment used on a previous job, or the possibility for future work utilizing the same equipment. A brief description of the three basic methods of segment placement follows along with a discussion of their application including advantages and disadvantages. The methods will be presented in increasing order of cost and complexity. By virtue of its sophistication over the other two methods, the discussion concerning launching gantries will predominate. Independent Lifting Apparatus Independent lifting apparatus refers to cranes or towers whose contact with or support from the primary bridge structure is minimal. They generally represent the least investment in equipment and are very cost effective when conditions allow their use. Figure No. 2a shows a barge-mounted crane erecting typical segments. In addition to the relatively small investment in equipment, this method of erection is particularly favorable since its impact on the structure is minimal. It results in virtually no additional loads on the structure during construction so its implementation is relatively simple. A refinement of the method of cantilever erection by barge-mounted or groundbased cranes involves the use of an overhead stabilizing girder as shown in Figure No. 24. The girder serves to brace the cantilever under construction against the previous cantilever during the initial or out-of-balance phase of placement of a segment pair. When equipped with a trolley and lifting apparatus, the girder can be used to erect segments on the trailing cantilever. This is particularly useful in placing segments near the closure where the proximity to the previous cantilever makes placement by a crane difficult.

FIGURE NO. 24 Use of independent lifting apparatus requires accessibility for both mounting/removing the lifting equipment and in positioning the segment prior to lifting 28

(i.e., transporting the segment by truck or barge to a location beneath the cantilever). Some instances where this method may not be feasible include urban structures over existing roadways, bridges in mountainous or environmentally sensitive terrain, bridges in extremely shallow water or marshlands, or on extremely high structures where the boom requirements of the crane become excessive. Deck-Mounted Lifting Equipment This method employs either a stationary lifting beam or a girder equipped with a trolley positioned at the tip of the cantilever. Figure No. 2b shows a beam and winch system for placing segments. Erection is simplified if the segments can be brought directly under their final position in the structure. A deck-mounted system is relatively simple and inexpensive. However, it is not entirely independent of the deck system. Because it is supported by the cantilever it will impose additional loads on the structure during construction. Though generally not excessive, they should be investigated for both general effects and local effects associated with their connection to the structure. As with segment placement utilizing independent lifting apparatus, it may be necessary to stabilize the cantilever during out-of-balance phases with an overhead girder. It is also necessary to resort to alternate methods for pier segment placement in establishing a starting point for mounting the erection equipment. This may be accomplished by in-situ casting of the pier segment or by an auxiliary mast fixed to a face of the pier column. The use of this segment placement method seems to be limited only by the ability to position a segment for lifting and possibly by the speed of the erection cycle. Successive cycles require placement of a segment pair followed by advancement of the beams to the next erection position. Overhead Gantry Where erection is to proceed sequentially from one end of the project to the other, a launching gantry may be an attractive alternate. It offers the advantage of being able to supply precast segments for erection from the ground or water level below the bridge or across the completed structure. This latter restriction may be imposed for bridges built over water where barge access is prohibited or in a congested urban environment where site conditions prevent the use of ground-based cranes or the ground delivery of precast segments. Site factors which may negatively influence the decision concerning gantry utilization are the degree of horizontal curvature and the maximum grade which the gantry will be required to accommodate. Because the gantries in use today have evolved to a rather high level of sophistication and are generally customized for a particular project, they represent a major investment in equipment. Their successful utilization is governed by the job's capacity to absorb the up-front cost in achieving a greater benefit. This may be through increased speed of erection or in overcoming access restrictions as discussed previously. It is apparent that the cost impact of the gantry is reduced as the overall cost of the project becomes larger. 29

The impact of schedule may also be a deciding factor in utilizing a launching gantry. Where the structure is relatively long and there is a reasonable repetitiveness of spans, a launching gantry can be a fast and efficient means of erecting cantilevers. There are usually three distinct operations which govern the type selection and design of a gantry: 1) Typical Segment Erection 2) Launching 3) Pier Segment Placement Typical segment erection denotes the procedure followed during construction of an individual cantilever at a pier. After the pier segment is in place, either by setting it if precast or by casting it in place, the launching gantry is positioned as shown in Figure No. 25 to begin typical segment erection. The gantry rests on two supports and is, therefore, statically determinate. The center support is anchored on the pier segment where the cantilever is being erected while the rear support is tied down at some point on the previous cantilever. The segments to be erected are either transported from the rear of the gantry along the completed structure or lifted directly from below depending on the conditions of ground or barge access at the site. In the former case, the gantry supports must be transversely spaced such that a segment, when rotated 90° to the bridge, may pass between them. A trolley which generally rides on the lower chords of the launching girder is used to transport the segment and to position it for erection.

FIGURE NO. 25 After erection at a particular cantilever is completed and any span closures are made as required, the gantry is launched to begin cantilever erection at the following pier. The exact sequence involved depends largely on the gantry being used and the span configuration of the bridge under construction. If the gantry is of sufficient size relative to the forward span length, it may be possible to place the pier segment for the next 30

cantilever without changing the longitudinal positioning of the gantry. If the nose section of the gantry is shorter than the forward span length as shown in Figure No. 26a, some sort of two-stage launch or temporary, intermediate pier bent must be used. Figure Nos. 26a through 26c show a typical sequence for a two-stage launch. In the initial configuration cantilever erection has just been completed with the gantry supports positioned as shown in Figure No. 26a. The back span has been closed in preparation for the launch to the next pier. Because the front tip of the gantry falls short of the forward pier, it is necessary to somehow advance the launching girder to allow a temporary front prop to be fixed to the forward pier segment or pier column. One way of accomplishing this is to place a temporary movable support ahead of the central support as shown in Figure No. 26b. The gantry can then be jacked up off the central support and translated forward until the nose is at the required location. The intermediate step shown in Figure 26b is also the normal support configuration for placement of the pier segment. For precast pier segment erection the temporary front support would be attached to the front of the pier column itself in order to allow placement of the segment on top of the pier table. After the pier segment is in position, the gantry can be advanced until the central support is located over the pier segment (Figure No. 26c). The support is then anchored to the pier to begin the next cycle of typical segment erection on the new cantilever. The mechanism by which the launching girder advances relative to the bridge depends on the overall design of the gantry and the particular construction application. Once the gantry is supported on the temporary nose and rear supports, the gantry is prepared to launch by freeing the center support. The gantry rolls over the nose support by means of a trolley fixed to the support which is attached to the ahead pier or pier segment. With the rear support anchored to the deck, the girder can roll over a similar trolley at this location as well. Alternately, the rear support may be attached to the launching girder itself. In this instance the support would actually roll along the deck with the aid of rubber tires or some type of rollers travelling over the completed deck.

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FIGURE NO. 26 Figure No. 27 illustrates a typical gantry configuration for erecting a pier segment. The launching gantry rests on three supports - a center support located on the leading end of the newly erected cantilever, a rear support positioned on the back span and a temporary front prop attached to the front edge of the next pier. The pier segment is delivered to the gantry either by transport along the completed structure or by picking it up directly from ground or water level. 32

FIGURE NO. 27 Pier segment placement is often the most critical operation during the erection cycle for several reasons. Because the gantry is supported at three points, it is subject to positive bending in the nose as the pier segment moves along the gantry to its final position at the lead pier. Typically, the gantry will be supported at only the center and rear locations during all other segment lifting operations resulting in strictly negative bending on the nose section. The pier segment is usually the heaviest segment due to the weight of its diaphragm. This, coupled with the requirement for an eccentric connection of the nose support to the pier column, mean special consideration must be given to the design of the gantry for strength and stability during pier segment placement operations. With the gantry in position to place the pier segment, the center support rests at approximately the same location on the previous cantilever as the rear support will be during erection of the typical segments. This is potentially the largest reaction imposed on the cantilever at the location of the center support. To prevent this loading condition from governing the design of the cantilever, it may be desirable to redistribute the gantry reactions to ensure that the reaction at the central support is less than or equal to that which will be produced by the rear support during the next construction stage. This is possible since the gantry support system is statically indeterminate at this stage. It may be accomplished by adjusting the reaction at the nose support to achieve the desired redistribution. Once the decision has been made to utilize a launching girder based on the constraints of the project, the type of gantry must be chosen. The total contract bid price, the availability of existing gantries, and the potential future use of the gantry on other projects are all factors that influence the ultimate design of the gantry. Of particular importance is the length of the gantry with respect to the typical span length. The earliest gantries were designed to be slightly longer than the span length of the bridge on which they were being used. A typical gantry of this classification is shown in Figure No, 28a. The length was sufficient to span between the previous cantilever and the present cantilever being erected while supported on the rear and center supports. By minimizing the distance between gantry support points, it resulted in a leastweight girder design. The disadvantages to this type of launching girder, however, involve the position of the rear support on the previous cantilever during typical segment erection and the increased complexity of pier segment placement and girder launching operations. Figure 33

No. 28a shows the gantry in position during typical segment erection. The proximity of the rear gantry support to the tip of the cantilever may result in a worst load case scenario for the cantilever or the pier. This would require additional temporary post-tensioning for moment or shear consideration in the cantilever or bending reinforcement in the pier above the service level requirements. During launching operations, this type of gantry requires an intermediate step to position the temporary nose support over the pier to begin pier segment placement operations. Alternately, a temporary pier bent may be required to support the nose where a span is exceptionally greater in length than the typical span. The evolution of launching gantries has recently been towards girders whose total lengths are slightly greater than twice the typical span length (Figure No. 28b). This has been made possible largely due to the more efficient use of materials than the firstgeneration girders. The modern gantries often rely on exterior forces supplied by active cable stays and post-tensioning to resist stresses in the girder. As is demonstrated in the schematics in Figure No. 28b, the longer girders offer the advantage of ensuring that loads transmitted to the superstructure remain over the piers or at least in the very near vicinity. Pier segment placement and girder advancement operations are simplified by allowing the simultaneous placement of the typical segments of a cantilever and the pier segment at the following cantilever pier location.

FIGURE NO. 28

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As a practical matter, the tendency is to size a gantry based on twice the "typical" span length. This keeps the gantry to a reasonable length to facilitate erection and launching operations for the majority of spans. For span lengths in excess of the typical span special, though not necessarily complex, procedures will need to be carried out to allow the gantry to be launched from one pier segment to another as was demonstrated in Figure 26. Regardless of the equipment and method chosen to erect the cantilevers, consideration must be given to the stability and loads on the structure during all stages of construction. As noted previously, it is imperative that the loads during construction assumed in design and the limitations of the structure be stated clearly on the contract drawings. As the name implies, the nature of balanced cantilever construction is to assemble the segments at each pier in pairs, thereby maintaining a balanced condition at the completion of each cycle. However, the process of placing the segments of a pair noncoincidentally results in an unbalanced condition which may exist anywhere from a matter of seconds to an indefinite amount of time For example, it is evident how successively lifting a segment at one end of a cantilever and securing it with temporary post-tensioning bars followed by a similar procedure at the other end would result in a period of load imbalance. But it is also true that the "simultaneous" casting of paired segments in an in-situ balanced cantilever or the "simultaneous" lifting at either end of precast segments with two sets of erection equipment cannot be assumed to occur at precisely the same time. It is therefore necessary to design for this temporary condition of load imbalance. Aside from the obvious case of imbalance due to noncoincidental segment placing, other more subtle loading conditions such as differential cantilever weights due to casting inaccuracies and roadway geometry, the random placement of miscellaneous equipment or the effect of wind loads can cause overturning moments on the free cantilever pier system during construction. These factors will be addressed later in more detail, but first it is necessary to describe the three common support conditions at the superstructure/pier interface. They are: (1) superstructure rigidly and permanently fixed to the pier (complete moment transfer capability), (2) deck simply-supported on a single row of neoprene bearing pads or pot bearings (no moment transfer capability), and (3) deck supported on a double row of bearings (i.e., limited moment transfer capability). These three conditions are illustrated in Figure No. 29.

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FIGURE NO. 29 It is fairly evident that conditions 2 and 3 above require some sort of temporary measures to stabilize the cantilever system during construction. But even condition 1, where the piers are relatively slender and designed mainly to transfer vertical loads during service, may require some sort of bracing to guarantee stability. In structures supported on bearings in the permanent condition, it is necessary to create temporary fixity between the deck and the substructure when erecting by the balanced cantilever method. This is normally accomplished by "packing" the gap between the deck and pier cap with some sort of compression wedge and vertically post-tensioning the pier segment to the pier (Figure No. 30). Once continuity has been made between adjacent cantilevers and the overall structure is stable, the vertical bars are destressed and the deck is jacked up to remove the temporary blocking and transfer load to the permanent bearings.

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FIGURE NO. 30 The need for providing additional stability measures to a cantilever pier system increases as the length of the cantilever and/or weight of the individual segments increase. The pier column and foundations must be able to resist a moment of P x a induced by the out-of-balance segment (plus ideally any additional loads alluded to previously), or alternate balancing measures must be used to relieve the moment transferred to the pier. There are numerous means by which the cantilever system may be braced. The three most common ones are shown in Figure No. 31. Figure No. 31a shows an asymmetric pier segment or "half" segment. The idea is to eliminate the situation of having a full segment out-of-balance pair. This is achieved by maintaining a half segment out-of-balance at all times, first on one side and then on the other. In reality, this is not even a balanced cantilever since the cantilever never reaches a balanced condition. While this method may be plausible in certain instances, it may not be effective for longer cantilevers. It may also considerably complicate the layout of the cantilever tendons. The overturning cantilever moment in the pier may also be reduced with the aid of a temporary support. As Figure No. 31b indicates, the support may act in tension by utilizing eccentric vertical post-tensioning or in compression in the form of a prop. It is also possible to use a pair of temporary supports with tension or compression capability at either end of the 37

cantilever. The additional cost, however, is normally not justified in that the main advantage is merely to allow construction to begin from either end of the cantilever. It is becoming more commonplace in situations where a launching gantry is used to provide stability for the cantilevers by means of a stabilizing arm on the gantry. The deck is then rigidly linked to the gantry by a vertical frame member able to resist either tension or compression (Figure No. 31c).

FIGURE NO. 31 As mentioned previously, there are other conditions of loading besides segment imbalance which cause an overturning moment in the substructure during construction. These different conditions are illustrated in Figure No. 32 and are designated as they appear in the 1989 "AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges". The loadings can be grouped into four basic categories as follows:

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1) Dead Load - This consists of the self-weight of the structure (DL) including any diaphragms, anchor blocks, or any other deviations from the typical cross section. It is common to use 155 pcf for the unit weight of concrete to take into account the large proportion of reinforcing in the segment and other items such as post-tensioning anchors or external post-tensioning bars. A differential load (DIFF) of 2% of the dead load is applied to one cantilever to account for differences in weights between the two cantilevers because of casting inaccuracies (i.e., differences in concrete unit weights, bulging of the forms, etc.) or minor geometrical differences due to grade and camber considerations. Finally, the placement of the initial segment of a balanced pair will cause an unbalanced load equal to the segment weight (U) on one side. 2) Equipment Loads - Equipment loads include both quantified loads (CE) representing the primary equipment assumed for deck construction and random loads (CLL) placed in the most disadvantageous manner on the deck surface. The loads assumed in design for CE should have a rational basis and should be clearly spelled out in the Contract Drawings. Figure 32b indicates a possible loading situation for beam and winch erection. A different loading situation would occur if a launching gantry had been assumed by the designer (Figure 3). Here it would be necessary to consider both the lifting and launching cases to produce the most critical load effects on the structure. The launching and segment transport cases would also produce a longitudinal construction equipment load (CLE) which is recognized by the AASHTO Guide Specifications. For the random construction live load the Guide Specifications require a CLL of 10 psf on one cantilever and 5 psf on the other. They also recommended an impact load (IE) of 10% of the equipment load. This is based on the case of gradual segment lifting involving small dynamic effects. Engineering judgment may dictate this value be increased in situations not meeting this description. 3) Wind Loads - An uplift load on one cantilever (WUP) of 5 psf is required by the AASHTO Guide Specifications. Additionally, the general cases of wind (W) and wind on erection equipment (WE - analogous to wind on live load, WLL) should be checked where critical. 4) Accidental Segment Load - This is described by the AASHTO Guide Specifications as an "accidental release or application of a precast segment load or other sudden impact from an otherwise static load of A". The code requires that this be used with a 100% impact load, or 2A. There has been some debate as to whether the assumption of the 100% impact is realistic. Some feel it is a best guess at a difficult to quantify value while others feel it is too conservative and unnecessarily penalizes the designer. Consideration of the accidental segment release load is a catastrophic loading condition, and the designer is only required to consider it under load factor, or strength design, conditions.

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FIGURE NO. 32

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