# Chapter 8 - Movable Bridges

## 8.1 General

This chapter contains information and criteria related to the design of movable bridge projects. It sets forth the basic Florida Department of Transportation (FDOT) design criteria that are exceptions or additions to those specified in the AASHTO LRFD-Movable Highway Bridge Design Specifications, First Edition, 2000 and any interim releases thereafter and herein referred to as LRFD-MHBD Specifications, (use soft conversions to convert to English units). Where applicable, other sections of this SDG also apply to the design of movable bridges.

### 8.1.1 Applicability

A. The design criteria of this chapter are applicable for new bridges and the electrical/machinery design for rehabilitation of existing bridges. The requirements for structural rehabilitation will be determined on a bridge-by-bridge basis, based on evaluations during the BDR phase and approval by the Structures Design Office (SDO). Projects for which the criteria are applicable will result in designs that preferably, provide new bascule bridges with a "two leafs per span" configuration.

Commentary: Single leaf bascules are discouraged, but may be considered for small channel openings where navigational and vehicular traffic is low. Assure reliable operation of movable bridges through redundancy features in drive and control systems, for both new and rehabilitation projects.

B. Examine and evaluate alternative bridge configurations offering favorable life cycle cost benefits. Consider improved design or operational characteristics providing advantage to the traveling public. Incorporate design and operational features which are constructible and which can be operated and maintained by the Department's forces. Maintain consistency of configuration, when feasible, for movable bridges throughout the State.

C. The preferred drive system for bascule bridges is electric motors with gears.

### 8.1.2 Redundancy

A. Include recommendations for redundant drives and control systems in the BDR/30% plans submittal. For bridges having low rates of anticipated bridge openings or average daily traffic, application of redundant drive and control systems may not be cost effective. In this event, submit such information in the BDR and provide appropriate recommendations for omission of redundant systems.

Commentary: Redundant drive configurations include:

1.) Hydraulic drive systems operated by multiple hydraulic cylinders. In these systems, a pump drive motor or its hydraulic pump can be isolated and bridge operations can continue while repairs are accomplished.

2.) Gear driven systems that may be driven through one gear train into a single rack of a two-rack bridge.

B. Provide two rack drives actuated by dual motor drive systems either of which will be capable of operating the bridge leaf. Normal operation of this configuration will involve operation of one drive/motor system. Provide an alternator to alternate drives/motors for each opening.

C. Design hydraulic cylinder actuated movable bridges to function in spite of loss of a main pump motor, hydraulic pump, or drive cylinder.

D. Design the system to include all necessary valves, piping, etc., to permit safe and expeditious changeover to the redundant mode. Specify a permanent plaque displayed in a convenient location on the machinery platform describing actions (valve closures and openings, electrical device deactivation, etc.) necessary for operation in the redundant mode.

E. When operating with either a single rack drive functioning or unsymmetrical hydraulic cylinder forces applied to the leaf, design the structure for Movable Bridge - Specific load combinations, strength BV-I and BV-II. Reduce the load factors for strength BV-I to 1.35 [Table 2.4.2.3-1].

### 8.1.3 Trunnion Support Systems

Provide trunnion support systems as follows:

A. Simple, rotating trunnion configuration, with bearing supports, on towers, on both inboard and outboard sides of the trunnion girder.

B. Sleeve bearings should be considered for only small bascule bridges and must be approved by the SDO at the BDR phase. Design constraints and cost justification must be provided.

C. Design trunnion supports on each side of the main girder with similar stiffness vertically and horizontally.

### 8.1.4 Vertical Clearance Requirements

Design bascule leaf for unlimited vertical clearance between the fenders in the full open position. Any encroachment of the leaf into the horizontal clearance zone must receive favorable Coast Guard review prior to approval of the BDR.

### 8.1.5 Horizontal Clearance Requirements (01/06) Jan 2006 - reinstated 110 ft limit per TDB C03-09. July 2005 per AFR deleted specific reference requiring approval for spans over 110 feet.

Design all movable bridges over navigable waterways to provide up to 110 ft. horizontal clearance as required by the United States Coast Guard and the Army Corps of Engineers. Clearances over 110 ft between fenders must be approved by the State Structures Design Engineer.

Commentary:

Since 1967 the exclusive control of navigable waters in the U.S. has been under the direction of the USCG. The USGC is required to consult with other agencies, which may have navigational impacts, before approving USGC permits for bridges over navigable waterways. The USGC was contacted by the Corps of Engineers expressing their needs for a wider channel along the Miami River, due to future dredging operations proposed by the Corps. After consultation between FDOT, USGC and Corps it was agreed that a 110 ft. horizontal channel clearance, between fenders, would be provided on future crossings of the Miami River in locations designated as navigable. This requirement for movable bridges would also apply to other waterways, which might be subject to dredging by the Corps to maintain water depths. The 110 ft. clearance was established as equal to the Corps of Engineers designs for locks along the major rivers in the United States. It is anticipated that where no known dredging operations are required by the Corps, smaller horizontal clearances as established by the USGC and published in the Federal Registry will still be permitted by the USGC. Since the cost of movable bridges vary roughly by the square of the span length, these smaller horizontal clearances should be submitted for approval where dredging is not anticipated. The USGC and Corps of Engineers has committed to working with the FDOT before making the final

decision on required clearances.

### 8.1.6 Tender Parking

Provide two parking spaces for bridge tenders in all new bridge designs, preferably on the tender house side of the bridge.

### 8.1.7 Definitions and Terms

A. Auxiliary Drive: Hand crank, gearmotor with disconnect-type coupling, portable hydraulic pump, drill, etc., that can be used to lower leafs) for vehicular traffic or raise the leafs) for marine traffic if the main drives fail.

B. Creep Speed: Not more than 10% of full speed, final creep speed will be determined by bridge conditions.

C. Emergency Stop: Leaf stops within (3±1) seconds of depressing the EMERGENCY STOP push-button or in the event of a power failure. All other rotating machinery stops instantly.

D. End-of-Travel Function: Contact connection where a closed contact allows operation and an open contact stops operation (i.e., leaf limit switches).

E. Fully Seated: Leaf is at rest on live load shoes, interlock OK to drive span locks.

F. Fully Open: Tip of leaf clears fender of a vertical line as defined by Coast Guard.

G. Hard Open: Leaf opening such that counterweight bumper blocks come in contact with pier bumper blocks.

H. Indicating Function: Contact connection where a closed contact indicates operation and an open contact indicates no operation (i.e., indicating lights.)

I. Interlocks or Safety Interlocks: Ensure events occur in sequence and no out-of-sequence events can occur.

J. Mid-Cycle Stop: Leafs) stop following normal ramping after depressing the STOP push-button when in the middle of an opening or closing cycle.

K. Near Closed: A point 8 to 10 degrees (approximately, final position to be field determined) before FULL CLOSED, drive to creep speed.

L. Near Open: A point 8 to 10 degrees (approximately, final position to be field determined) before FULL OPEN, drive to creep speed.

M. Ramp: Rate of acceleration or deceleration of leaf drive.

### 8.1.8 Movable Bridge Terminology

See Figure 8-2 for standard bridge terminology.