Frequently Asked Questions
WHAT IS THE TOP OF DECK TO THE LOW-STEEL DIMENSION?
The top of roadway wearing surface to the low steel elevation varies depending on the span, roadway width and framing configuration. U.S. Bridge has a chart of preliminary floor beam sizes plotted against span (for beam bridges) or roadway width (for truss bridges) that can be used to approximate the structure depth, given certain design parameters. We often work closely with the Owner’s engineer during the programming or preliminary design phase to determine a structure depth and substructure loads (reactions), upon request.
WHAT ARE REACTIONS?
The bridge’s supporting loads are called reactions. They are the “reacting” forces from a bridge’s own downward gravity force and also from things the bridge carries such as vehicles or pedestrians. Other loads cause corresponding reactions such as those from wind, or streams, or temperature. These are different for each bridge depending on its span, width, truck loading, deck construction and the design code’s requirements.
WHAT VEHICLE LOADS CAN BE DESIGNED FOR?
Bridge engineers use given loads specified by AASHTO in their published bridge specifications to design bridges. The loads discussed below are specified in the AASHTO Standard Specifications for Highway Bridges, 2002, 17 Edition. They are called “”notional”” loads as they represent “”Hypothetical”” trucks. Thus the “”H”” for the basic two-axle truck, and “”HS”” for semi-trucks. They based on a statistical analysis of real-world truck traffic. A brief list of some common designations is given below:
– H15-44 Truck (2 axles) GVW = 30,000 lbs.
– HS15-44 Truck (3 axles) GVW = 54,000 lbs
– H20-44 Truck (2 axles) GVW = 40,000 lbs.
– HS20-44 Truck (3 axles) GVW = 72,000 lbs
The HS25 truck loading is another truck loading specified by many state DOT’s to increase their specified loading to accommodate the fact that many trucks on today’s highways travel with heavier loads than when the H and HS truck series were determined. This allows for the permitting of more overload permit vehicles on the National Highway System (NHS) and other heavily traveled state system routes. The HS25 is 5/20ths heavier (25% heavier) than the HS20-44 truck.
– HS25 Truck (3 axles) GVW = 90,000 lbs.
The HL93 truck loading is an updated truck loading found in AASHTO’s LRFD Bridge Design Specification, the current edition being No. 6. This truck loading is governed by the larger force effect from one of two loadings: 1) an HS20-44 truck coincident with a newly defined lane load of 0.64 kips/ft.; or 2) a tandem vehicle with two 25k axle loads, coincident with the new lane load.
Beyond these commonly specified truck loadings, we routinely design for additional vehicular loads known as Special Design Vehicles. These are, many times, requested by bridge owners with special hauling or access needs such as need in the mining or heavy construction industry. The important thing to know is the vehicle’s loaded gross vehicle weight and the corresponding individual axle loads, and the actual axle spacing(s). These are necessary to properly complete the design.
ARE THERE ANY OPTIONS OTHER THAN WEATHERING OR PAINTED STEEL TO PROTECT AGAINST CORROSION?
Hot-Dip Galvanizing is a very popular method to protect against corrosion. It provides inexpensive, long-lasting, superior corrosion protection.
ARE THE BEARINGS A PART OF THE BRIDGE?
The bearings are usually supplied with the bridge unless excluded within the order. The term “bearings” is more appropriately defined as “bearing device” and usually means everything conveying load between the superstructure and the substructure and includes a “load plate” also known as a “sole plate”, a bearing pad made from reinforced elastomers or resins, and a steel “masonry” or “base” plate. For longer truss bridges the load plate is surfaced with polished stainless steel layer and combined with Teflon coating applied to the bearing pad producing a low-friction bearing that allows for slip under thermal expansion and contraction; thus allowing the substructure to be designed for lighter longitudinal forces.
CAN THE ABUTMENTS BE AT DIFFERING ELEVATIONS?
The maximum roadway slope allowed by the roadway designer will govern the bridge slope. Steep slopes are usually accommodated by beveled load plates or special bearings with extra rotational capacity.
CAN ANCHOR BOLTS BE SET USING EPOXY RESIN OR MUST THEY ALWAYS BE CAST IN PLACE IN THE CONCRETE?
The anchor bolts may be embedded into concrete by securing them to the reinforcing of the concrete abutment seat and casting them into the concrete pour or by drilling and anchoring them into the cured concrete using a two-component, non-shrink, non-sag epoxy or methyl methacrylate adhesive, that is moisture insensitive when cured. Nylon or polyester resins are not acceptable.
IS YOUR BRIDGE CONSIDERED REDUNDANT?
Our two-girder through truss bridges are designed in accordance with AASHTO criteria for non-redundant bridges. The bottom chord of a U.S. Bridge truss is a double-element member, using two channels or WT’s, which provides internal redundancy to the member. This double member, although no longer recognized as redundant, does provide a robust safety factor with regard to connection deterioration, resistance to overloads, and long-term serviceability.
We are in the process of completing a Redundancy Proof Study that we hope will show quantifiable redundancy in alternate load paths such as the floor framing and connections that will allow individual hard-to-inspect members to be declassified as FCM’s.
HOW WILL YOUR BRIDGE HOLD UP UNDER HEAVY SALTING?
Hot-dipped galvanizing is an excellent and proven corrosion protection method. Heavy de-icing chemical application will reduce the overall duration of effectiveness but if proper paving operations and curb and joint details are constructed, any underside deterioration can be kept to a minimum.
HOW LONG DOES GALVANIZING LAST?
Galvanizing is a tremendously long-lasting and effective treatment of steel to minimize or eliminate corrosion. U.S. Bridge specifications require that any steel to be galvanized shall be galvanized according to ASTM A123. If done according to this specification and installed properly in a non-industrial environment, the galvanizing protection will last well beyond 100 years (75 years to 1st maintenance, which is defined as 5% rust).
It is important to remember that U.S. Bridge’s procedure is to fabricate a bridge’s components prior to galvanizing. They are then subsequently bolted together into assemblies achieving galvanizing on both sides of a joined piece. This enhances even further, the corrosion protection afforded by galvanizing.
Another testament to galvanizing’s long term protection is the 35-year Warranty that accompanies this conditioning for primary components of our bridges.
According to the American Galvanizers Association, the corrosion rate of zinc is directly influenced by atmospheric conditions. Certain factors that specifically affect the corrosion of zinc include: temperature, humidity, rainfall, sulfur dioxide (pollution) concentration in the air, and air salinity. None of these factors can be singled out as the main contributor to zinc corrosion, but they all play a role in determining the corrosion protection hot-dip galvanized (zinc) coatings can provide in certain environments.
When galvanizing is exposed to the natural wet and dry cycles of the atmosphere, it develops a series of zinc byproducts on the surface called the patina. The patina is stable and non-reactive unless exposed to aggressive chlorides or sulfides. The patina is a key component in the longevity of the hot-dip galvanized coating in the atmosphere.
For corrosion classification purposes, atmospheres are generally divided into five groups: 1) Industrial Atmospheres; 2) Suburban Atmospheres; 3) Temperate Marine Atmospheres; 4) Tropical Marine Atmospheres; and 5) Rural Atmospheres. Independent and industry testing of galvanized steel samples over decades in industrial, urban, rural, and marine environments, with varying degrees of chlorides, sulfides and other corrosive elements, has yielded performance data for galvanized steel in real world applications.
WHEN IS WEATHERING STEEL NOT RECOMMENDED?
Weathering steel should not be used when the low-steel elevation above the water surface is small; less than 10 ft., and the air flow expected under the bridge is poor. This limit is even less, 8 ft., if the stream is flowing.
Please check out “A Primer on Weathering Steel” by the NSBA/AISC and also “ Uncoated Weathering Steel in Structures” by FHWA.
ARE SUPERELEVATED SURFACES A PROBLEM?
No. Paving details or finished elevation tables are prepared to account for unique or unusual profiles and superelevated deck surfaces.
OUR DOT REQUIRES REMOVABLE FORMS FOR CONCRETE DECK BRIDGES. HOW DO YOU ADDRESS THIS?
It is one of the deck construction methods that we support and detail for.
WHAT GRADE OF STEEL IS USED WHEN DESIGNING A BRIDGE?
Typically ASTM A709 Gr. 50 or Gr50W is used. This is an equivalent specification for ASTM A572 Gr50 and ASTM A588 respectively.
WHAT IS THE HORIZONTAL LOADING FOR THE BRIDGE?
Wind, wind-on-live load, seismic forces, stream loadings and longitudinal (braking) forces all act upon the horizontal (lateral) plane of the bridge and are designed for and accommodated by lateral bracing and/or the deck slab diaphragm.
CAN YOU DESIGN AND SUPPLY THE EXPANSION JOINTS?
Our specifications outline numerous expansion joint and joint armor possibilities. We routinely adapt our bridge framing and detailing to accommodate unique or non-typical joint treatments.
CAN YOU COVER YOUR VEHICULAR BRIDGES?
Yes. From time to time, this request has been entertained. The combination of steel framing and timber siding and roofing elements can result in an attractive covered bridge structure without the costs of heavy timber framing and the speed of steel construction.
CAN YOU PROVIDE A SKEWED BRIDGE?
We routinely design skewed bridges to fit the bridge to the feature it crosses. This usually involves utilizing a planned skew angle, set by a consulting engineer or the bridge owner for a new project. It many times can involve matching the skew of an existing abutment or pier, using field surveyed information. Variations from the conventional framing arrangements involve steel framing that either align the floor beams with their corresponding truss panel points (these are called skewed floor beams) -or- arrangements that are transverse to the trusses, but on a staggered to an adjacent panel or half panel (this being an orthogonal arrangement).
WHAT IS THE TOP OF DECK TO THE LOW-STEEL DIMENSION?
The top of roadway wearing surface to the low steel elevation varies depending on the span, roadway width and framing configuration. U.S. Bridge has a chart of preliminary floor beam sizes plotted against span (for beam bridges) or roadway width (for truss bridges) that can be used to approximate the structure depth, given certain design parameters. We often work closely with the Owner’s engineer during the programming or preliminary design phase to determine a structure depth and substructure loads (reactions), upon request.
WHAT ARE THE LEAD TIMES FOR DRAWINGS AND FABRICATION?
Lead times vary with the particular project and the amount of backlog within our engineering department at that time. Generally speaking, U.S. Bridge can prepare and submit design documentation and engineering plans for a typical beam bridge in 2 weeks, a low (pony) truss bridge in about 4 weeks, and a through truss bridge in about 6 weeks. Additional time will be needed for any of the following complexity factors:
1) Poorly prepared or incomplete scope-of-work
2) Multiple spans of different lengths
3) Vertically curved profile over the bridge
4) Skewed framing
5) Asymmetric railings
6) Supported sidewalks
7) Bridge joint armor and expansion joints
8) Non-Standard Railing Designs
9) SIP formwork
10) Reinforcing for concrete deck slabs
WHAT KIND OF DRAWINGS DO WE PROVIDE?
U.S. Bridge’s typical plan set includes a title sheet, design drawings showing framing, truss geometry (if applicable), transverse sections, abutment sections, railing installations, form or plank layout drawings, reinforcing steel plans, anchor bolt layout, field assembly drawings which include lifting weights. These amount to between 8 and 12 drawings. Following the design drawings you will have a bill of material set (approximately 2 to 3 drawings); and shop assembly, and production details (shop drawings which usually amount to another 10 to 25 drawings, depending on complexity).
These drawings along with accompanying calculations will be sealed by a professional engineer in the state where the project is located.
CAN WE PROVIDE DRAWINGS FOR A FLOOD OR WATERWAY PERMIT?
If a site plan needs to be developed depicting the proposed construction activities and impact limits, a local consulting engineering firm may need to be hired to survey and plan the extent of the project’s disturbance. If the Owner would like this task to be completed as part of U.S. Bridge’s services, please contact one of our salesmen to explore the possibility with us.
WILL YOU DESIGN THE FOUNDATION?
U.S. Bridge provides professional design and engineering services for the bridge superstructure. Foundations, substructure and approach roadway designs are provided by the owner or owner’s representative. If a project needs these design tasks completed as part of a project, U.S. Bridge can contract with consulting engineering firm as a turn-key service. Please contact one of our salesmen to discuss this in more detail.
WHAT LIFTING POINTS SHOULD I PLAN FOR TO ERECT THE BRIDGE?
Detailed installation manuals are available for contractors or agencies performing the installation. The plans submitted for approval and the plans sent with the bridge at the time of shipment have lifting weights of the field segments and assembled units.
CAN OUR BRIDGE BE DESIGNED FOR A STREAM LOADING?
Yes. This should be specified in the scope of design tasks or be shown in the contract documents by declaring or depicting water surface elevations above the low steel elevation. Additionally it is important to list the stream velocities to support the force calculations.
HOW LONG WILL IT TAKE TO SPLICE AND SET THE BRIDGE?
This is highly dependent on the contractor’s or agency’s experience in handling and assembling steel members. It also is dependent on the bridge’s size and complexity.
On average for a medium-span, low (pony) truss bridge, an experienced crew of 4, with the proper tools and equipment can unload and assemble the truss girders for erection in two days. They then can set the girders and erect the floor beams on a separate third day. Stringer beams between the floor beams usually take another day.
For stringer beam bridges, these can usually be erected in a single day.
HOW BIG OF CRANE DO I NEED?
The number of cranes and their lifting capacity is highly dependent on the weight of the heaviest assemblies, the layout of the project site, the staging areas and the proximity of where the crane(s)’s will be set relative to the bridge’s position. Proper planning and review of the erection sequence and the rigging operations is crucial to the safety of the workers erecting the bridge.
A rigging and erection plan should be prepared by the contractor for review by the Owner’s Engineer.
HOW IS THE BRIDGE DELIVERED?
The bridge is shipped in pieces called assemblies. They are usually sized to fit on a standard tractor-trailer (53’ x 8’-6”) and require over-road trucking permits if they exceed these dimensions. Larger span lengths or wider bridge widths will require more truck loads depending on the individual assemblies being shipped.
In general though, we can probably best answer this question using as an example, a 120 ft. span by 24 ft. wide, pony truss where the entire bridge will be delivered on 4 to 5 trucks. The truss girders for this bridges will be shipped in field segments and will be stacked in a logical order on their trailers to maximize the trucking load (usually 2 or 3 girder segments per trailer). The floor framing load (floor beams and stringer beams) will follow and are usually shipped on a separate trailer. Bolts are usually shipped early, with the truss segments to expedite their assembly upon unloading. Smaller pieces and miscellaneous items such as lateral bracing, railing, flooring planks or forms are shipped on another truck.
Also it is important to remember that the weights for all of the pieces and assemblies are listed in the drawings, and they are physically marked to match the labels given in the plans. This of course assists the contractor in joining the pieces or assemblies together.
HOW DO I REPAINT THE BRIDGE?
After many years of service in harsh conditions, it may be necessary to repaint much or all of your bridge. Depending on your site, it may be beneficial to paint on site using a containment system or it may be more economical to remove the structure and have it shop painted and re-erected. Properly preparing steel surfaces and applying coatings is a technical and complex endeavor. It requires specialized experience, equipment, environmental controls and QC procedures; and involves hazardous materials and equipment. To repaint a bridge in the field, it will likely involve hiring a specialty contractor capable of accessing your site and protecting the environment. They will need to demonstrate their experience painting bridges using the coatings and system you have specified.
ARE SUPERELEVATED SURFACES A PROBLEM?
No. Paving details or concrete screed tables are prepared to account for unique or unusual profiles and superelevated deck surfaces.
OUR DOT REQUIRES REMOVABLE FORMS FOR CONCRETE DECK BRIDGES. HOW DO YOU ADDRESS THIS?
It is one of the deck construction methods that we support and detail for.
WHAT IS THE CONCRETE DECK AND REBAR DESIGN?
A typical DOT mix design would be for f’c = 4,500 psi and 60 ksi reinforcing steel. A deck slab of 8 in. (min.) is recommended with two layers of reinforcing steel. The depth of slab is defined to be a continuous thickness above the supporting steel including any stay-in-place formwork. The slab reinforcing is usually made up a two layers of #5 and/or #6 bars in each direction, properly lap-spliced where called for and supported by reinforcing “chairs” and tied together, in accordance with state DOT construction specifications. If a specific state’s design requirements are to be followed, there are provisions for this in our model specifications.
HOW DO YOU MEASURE LENGTH AND WIDTH?
Length can be a somewhat ambiguous term when discussing bridges and a more engineering term is “Span”. Span is defined by the centerline-of-bearing to centerline-of-bearing distance along the centerline of roadway. Overall length is defined as the total length of the steel members but can sometime mean the open dimension between backwalls.
Bridge limits means the distance between approach slabs, if they are proposed, or the distance between the backside of the backwalls, if they are not proposed.
Width also can be a very ambiguous term and is better named “Roadway Width” as it explains more about the intended meaning. Roadway width is the clear or open lateral dimension between two rails or two curbs or between a rail and a curb, depending on the edge details.
The distance between the centerline of the truss from the rail or curb varies depending on the edge detail and/or the railing attachment method.
The dimension between the centerlines of the trusses is the span of the floor beam. The span of the floor beam, like the span of the truss is the dimension used in the structural analysis of the bridge.
DO WE SUPPLY THE ANCHOR BOLTS?
Yes. For beam bridges they usually are ¾” to 1” diameter and sized to provide 12” to 1’-4” embedment into the concrete abutment seats. For truss bridges they are usually 1 ½” in diameter and sized to provide 1’-6” embedment into the concrete abutment seats. Various material properties, fasteners, and surface requirements are met depending on the Owner’s specifications.
WILL YOU HAVE A REPRESENTATIVE AT THE SITE DURING INSTALLATION?
Yes. U.S. Bridge will provide on-site technical assistance for the bridge erection, upon request. Co-ordination of schedules is required. Our Project Managers will work with the Owner’s representative or contractor to understand the planned schedule and arrive on-site ahead of the lifting operations.
THE WEIGHT LISTED IS PER PIECE OR TOTAL?
U.S. Bridge’s price quotations list an assumed number for field segments and corresponding preliminary lifting weights, which are subject to change during final design.
Lifting weights for individual pieces, assemblies, truss segments and complete truss girders are defined and listed within the drawings to support the contractor’s planning. An “as-fabricated” set of plans is shipped with the bridge to assist the contractor during the construction.
WHAT ARE THE FOUNDATION REQUIREMENTS?
Repeating from an earlier question, the foundation recommendations should be made by a geotechnical engineer who has obtained soil borings and studied the conditions. The final foundations should be selected and designed by the engineer responsible for the substructure. Abutments support the bridge superstructure by transmitting the bridge’s resultant loads, called reactions, through load plates and into bearing pads; and then through masonry or sole plates. These plates spread the load out and into the concrete abutment seat. Anchor bolts are formed or installed into the concrete to prevent the bridge from moving in any unintended directions. We provide professional design and engineering services for the superstructure, but the foundations, substructure and approach roadway designs are provided by the owner or owner’s representative. If a foundation and substructure design needs to be prepared, a consulting engineering firm will need to be hired. This can be done through U.S. Bridge as a turn-key service. Please contact your regional salesman to discuss this process.
ARE PREFABRICATED BRIDGES SIMPLE TO INSTALL?
Yes. Especially if you have built one before.
In many cases, a prefabricated steel vehicular bridge is a faster way to complete a bridge construction project. It depends on many things within the goals of the project and what is specified within the scope of work. It is important to remember that steel bridge construction is a technical enterprise requiring experience in construction methods, crane rigging, steel erection practices, planning, scheduling, and managing resources and manpower. For more information to what’s going to be need for your project, please call our Engineering Department to discuss what you are planning – (888) 872-7434.
ARE THE BEARINGS A PART OF THE BRIDGE?
The bearings are usually supplied with the bridge unless excluded within the order. The term “bearings” is more appropriately defined as “bearing device” and usually means everything conveying load between the superstructure and the substructure and includes a “load plate” also known as a “sole plate”, a bearing pad made from reinforced elastomers or resins, and a steel “masonry” or “base” plate. For longer truss bridges the load plate is surfaced with polished stainless steel layer and combined with Teflon coating applied to the bearing pad producing a low-friction bearing that allows for slip under thermal expansion and contraction; thus allowing the substructure to be designed for lighter longitudinal forces.
CAN THE ABUTMENTS BE AT DIFFERING ELEVATIONS?
The maximum roadway slope allowed by the roadway designer will govern the bridge slope. Steep slopes are usually accommodated by beveled load plates or special bearings with extra rotational capacity.
CAN ANCHOR BOLTS BE SET USING EPOXY RESIN OR MUST THEY ALWAYS BE CAST IN PLACE IN THE CONCRETE?
The anchor bolts may be embedded into concrete by securing them to the reinforcing of the concrete abutment seat and casting them into the concrete pour or by drilling and anchoring them into the cured concrete using a two-component, non-shrink, non-sag epoxy or methyl methacrylate adhesive, that is moisture insensitive when cured. Nylon or polyester resins are not acceptable.
CAN YOU DESIGN AND SUPPLY THE EXPANSION JOINTS?
Our specifications outline numerous expansion joint and joint armor possibilities. We routinely adapt our bridge framing and detailing to accommodate unique or non-typical joint treatments.
IS ANY FIELD PAINTING REQUIRED FOR A PAINTED BRIDGE?
Yes. Many of the bridge components will be preassembled and completely painted but where bolted field connections are to be made by the contractor, there will be field painting of the coating layers beyond the prime coat. This is because mated pieces in a bolted joint are considered faying surfaces upon which a certain frictional coefficient has been assumed during design. If these surfaces were painted, the joint’s design will likely be invalid. Our shop prepares the pieces with a stepped back masking of the coating layers. The construction plans and shop drawings detail the portions of the pieces that are masked off and also depict what will need field painting. The contractor should plan for field painting of these connection areas for a painted bridge.
IS TOUCH-UP PAINTING DIFFERENT THAN FIELD PAINTING?
Yes. Touch-up painting is usually surface painting that is required to repair, scuffs, wear marks or evidence of concrete or construction debris. Deep scratches may require touch-up to more than one layer. The contractor is required to complete touch-up painting on all elements identified by the project engineer and must meet their approval.
HOW DO THE PAINTS/ PROTECTIVE COATINGS AVAILABLE TODAY DIFFER FROM THOSE USED IN THE PAST?
Protecting exposed structural steel with paint coatings has always been an area of industrial research and scientific advancement. Certainly changes in surface preparation, primer coat content (zinc vs. red lead), application equipment and testing methods have all advanced over the years but much, much more has gone on into the improvements in this industry than can be explained here.
To read more about this topic and for a better overall understanding, we suggest reading Chapter 23 of Steel Bridge Design Handbook, by the NSBA/AISC or Section II of “Designing with Structural Steel, A Guide for Architects, 2nd Edition”, AISC.
WHAT VEHICLE LOADS CAN BE DESIGNED FOR?
Bridge engineers use given loads specified by AASHTO in their published bridge specifications to design bridges. The loads discussed below are specified in the AASHTO Standard Specifications for Highway Bridges, 2002, 17 Edition. They are called “”notional”” loads as they represent “”Hypothetical”” trucks. Thus the “”H”” for the basic two-axle truck, and “”HS”” for semi-trucks. They based on a statistical analysis of real-world truck traffic. A brief list of some common designations is given below:
– H15-44 Truck (2 axles) GVW = 30,000 lbs.
– HS15-44 Truck (3 axles) GVW = 54,000 lbs
– H20-44 Truck (2 axles) GVW = 40,000 lbs.
– HS20-44 Truck (3 axles) GVW = 72,000 lbs
The HS25 truck loading is another truck loading specified by many state DOT’s to increase their specified loading to accommodate the fact that many trucks on today’s highways travel with heavier loads than when the H and HS truck series were determined. This allows for the permitting of more overload permit vehicles on the National Highway System (NHS) and other heavily traveled state system routes. The HS25 is 5/20ths heavier (25% heavier) than the HS20-44 truck.
– HS25 Truck (3 axles) GVW = 90,000 lbs.
The HL93 truck loading is an updated truck loading found in AASHTO’s LRFD Bridge Design Specification, the current edition being No. 6. This truck loading is governed by the larger force effect from one of two loadings: 1) an HS20-44 truck coincident with a newly defined lane load of 0.64 kips/ft.; or 2) a tandem vehicle with two 25k axle loads, coincident with the new lane load.
Beyond these commonly specified truck loadings, we routinely design for additional vehicular loads known as Special Design Vehicles. These are, many times, requested by bridge owners with special hauling or access needs such as need in the mining or heavy construction industry. The important thing to know is the vehicle’s loaded gross vehicle weight and the corresponding individual axle loads, and the actual axle spacing(s). These are necessary to properly complete the design.
WHAT CODES DO YOU USE?
The Standard Specifications for Highway Bridges, 2002, 17th Edition, or the LRFD Bridge Design Specification, 2010, 6th Edition are currently the latest published codes by AASHTO and are what we expect to use for new projects unless otherwise specified. Canadian and European Bridge Design Codes can be used when applicable.
1) Poorly prepared or incomplete scope-of-work
2) Multiple spans of different lengths
3) Vertically curved profile over the bridge
4) Skewed framing
5) Asymmetric railings
6) Supported sidewalks
7) Bridge joint armor and expansion joints
8) Non-Standard Railing Designs
9) SIP formwork
10) Reinforcing for concrete deck slabs
WHAT WILL THE BRIDGE NEED FOR ABUTMENTS?
Bridges generally sit on concrete abutments supported by shallow or deep foundations depending on the subsurface conditions found at the site. Foundation recommendations should be made by a geotechnical engineer who has obtained soil borings and studied the conditions. The final foundations should be selected and designed by the engineer responsible for the substructure. Abutments support the bridge superstructure by transmitting the bridge’s resultant loads, called reactions, through load plates, into bearing pads; and then through masonry plates also known as sole plates. These plates spread the load out and into the concrete abutment seat. Anchor bolts are formed into the concrete or installed via drilling and epoxy resin, to generally anchor the bridge to the abutment. This prevents the bridge from moving in any unintended directions.
These drawings along with accompanying calculations will be sealed by a professional engineer in the state where the project is located.
WHAT IS THE LIFESPAN OF A BRIDGE?
Bridges are usually designed with an intended design life. This is commonly thought of as 75 years from when the bridge is commissioned into service. The Owner may request a longer design life depending on the function, importance, or sensitivity of the roadway facility; and the bridge’s location. This design life period can include many maintenance tasks and repair/rehabilitation projects that will be needed to maintain its performance level and diminish the effects of environmental deterioration.
DO YOUR BRIDGES MEET ADA?
If requested as part of the scope of work, a bridge’s pedestrian sidewalk can be designed to meet the requirements of the Americans with Disabilities Act (ADA). Several items to keep in mind include:
1. Profile grades of less than 1:20 (5%) are not governed by Section 4.8.2 of the ADA.
2. Profile grades between 1:20 (5%) and 1:12 (8.33%) are permitted ramps but must have a level landing every 30 ft. (ADA 4.8.4) and must also have an ADA approved hand rail with ADA (“loop”) returns (ADA 4.8.5).
3. Bridges must have a toe plate to keep the small wheel of a wheelchair from going off the edge of the deck (ADA 4.8.7).
4. Bridges must have a deck surface that meets the ADA for minimum opening size (ADA 4.5.4).
WHAT ARE THE LEAD TIMES FOR DRAWINGS AND FABRICATION?
Lead times vary with the particular project and the amount of backlog within our engineering department at that time. Generally speaking, U.S. Bridge can prepare and submit design documentation and engineering plans for a typical beam bridge in 2 weeks, a low (pony) truss bridge in about 4 weeks, and a through truss bridge in about 6 weeks. Additional time will be needed for any of the following complexity factors:
- Poorly prepared or incomplete scope-of-work
- Multiple spans of different lengths
- Vertically curved profile over the bridge
- Skewed framing
- Asymmetric railings
- Supported sidewalks
- Bridge joint armor and expansion joints
- Non-Standard Railing Designs
- SIP formwork
- Reinforcing for concrete deck slabs
WHAT COLORS DO THE BRIDGES COME IN?
The top coat of any of our paint system can be custom tinted to almost any color that is desired. We will begin by coordinating a color for the bridge using a color chart published by the FHWA for bridge painting in accordance with Federal Standard 595a. An online version of this color chart is available at: http://www.colorserver.net
WHAT SAFETY RAIL STYLES DO YOU OFFER?
There are two items for consideration when specifying a railing system: the anchoring connection, and the railing members themselves.
The following railing options are able to be specified within our specifications:
- Concrete Safety Shaped Parapet
- Concrete Curb Mounted Post & Rail
- Concrete Slab Top-Mounted Post & Rail
- Concrete Slab Side-Mounted Post & Rail
- Truss-Mounted Single W-Beam Guardrail with Tube Backup
- Truss-Mounted Thrie-Beam Guardrail with Tube Backup
- Stringer Supported Post & Rail
- Stringer Supported Post & Rail with Tube Backup
- Stringer Supported Twin Steel Tubular Rail
- Timber Railing
Crash Tested Railing Systems meeting FHWA’s MASH Standards are part of the rails listed above. DOT standard railing systems are routinely accommodated. If a system is non-standard, the design may be shown to be acceptable through an equivalency calculation using direction given in the AASHTO specifications. This should be specified in the scope of design tasks.
For timber railing, rough sawn timber rails or laminated timber rails may be specified with or without an additional steel back up plate.
Also pedestrian rails can be specified using many different assemblies but usually have a top hand rail and a lower rub rail as well as horizontal or vertical pickets. The pickets must be spaced such that a 6” sphere cannot pass between them. Decorative rails can be accommodated for an aesthetic treatment.
CAN OUR BRIDGE BE DESIGNED FOR A STREAM LOADING?
Yes. This should be specified in the scope of design tasks or be shown in the contract documents by declaring or depicting water surface elevations above the low steel elevation. Additionally it is important to list the stream velocities to support the force calculations.
WILL THE WEATHERING STEEL FINISH STAIN CLOTHING?
Yes and no. A mild “yes” because it possibly could, if left in contact for a long period of time in a wet weather environment, but practically speaking, “no”, not from incidental contact. Generally, it is prudent to paint any railings that pedestrians will come into contact with. This can be accommodated in the specification section under railings.
In general though, we can probably best answer this question using as an example, a 120 ft. span by 24 ft. wide, pony truss where the entire bridge will be delivered on 4 to 5 trucks. The truss girders for this bridges will be shipped in field segments and will be stacked in a logical order on their trailers to maximize the trucking load (usually 2 or 3 girder segments per trailer). The floor framing load (floor beams and stringer beams) will follow and are usually shipped on a separate trailer. Bolts are usually shipped early, with the truss segments to expedite their assembly upon unloading. Smaller pieces and miscellaneous items such as lateral bracing, railing, flooring planks or forms are shipped on another truck.
Also it is important to remember that the weights for all of the pieces and assemblies are listed in the drawings, and they are physically marked to match the labels given in the plans. This of course assists the contractor in joining the pieces or assemblies together.
WHAT ARE THE COSTS AND BENEFITS OF THE DIFFERENT PAINT AND COATING OPTIONS?
In order of increasing costs and corrosion protection:
- Weathering Steel
- Hot Dip Galvanizing (HDG)
- 3-Coat Paint System (Zinc-Epoxy-Urethane)
- 2-Coat Paint System over HDG (Epoxy-Urethane)
- Thermal Spray Metalizing
WHAT ARE THE SPAN RANGES AND ROADWAY WIDTHS THAT YOU COVER?
Typically we design stringer beam bridges with spans between 20 ft. and 75 ft. with no real width limitation.
Low (pony) truss bridges range between 60 ft. and 200 ft. depending on the roadway’s width. Generally they are limited to 3 lanes of width (44 ft. max.) for traffic but may include sidewalks.
High (through) truss bridges range between 160 ft. to 300 ft. depending on the roadway’s width. Through trusses maybe one, two or three lanes but are dependent on other factors such as skew, live loads, sidewalks and floor construction. For a better understanding of bridge spans and widths, please see our Span/Width Matrix.
CAN WE PROVIDE DRAWINGS FOR A FLOOD OR WATERWAY PERMIT?
If a site plan needs to be developed depicting the proposed construction activities and impact limits, a local consulting engineering firm may need to be hired to survey and plan the extent of the project’s disturbance. If the Owner would like this task to be completed as part of U.S. Bridge’s services, please contact one of our salesmen to explore the possibility with us.
IS THE PAINT JOB GUARANTEED?
Yes, as part of our Bridge & Product Warranty, we warrant to the Owner, against defects in material or workmanship for up to 10 years. Contact us to review the full contents of this 10-Year Warranty – (888) 872-7434.
HOW LONG WILL YOUR BRIDGE LAST?
Generally we design to a design life of 75 years which is consistent with many agency life-cycle protocols. It is within the owner’s prerogative to define this differently, at their preference.
To understand this a little further, a 75-year design life is the duration of service expected from the bridge’s superstructure before its complete replacement. Substructures and foundations are expected to last well over 100 years. The costs to maintain the bridge superstructure over this period are called life-cycles costs and include all the inspection, repair and rehabilitation project costs to maintain the bridge is fair condition.
A good resource for bridge owners, with regard to maintenance is: http://www.fhwa.dot.gov/bridge/preservation/guide/guide.pdf
WHAT MUST I DO TO MAINTAIN NEW BRIDGE?
There are maintenance tasks and “projects” that can be undertaken to extend the durability of the bridge and prevent premature deterioration. These are many times, site or location specific and would require a long listing of possible suggestions. Your state DOT may a have a bridge maintenance section within their office that has work item suggestions, or they may be able to meet with you to give you opinions about good maintenance practices. U.S. Bridge also is available to meet and go over possible work items. Also a good resource for bridge owners, with regard to maintenance is:
DOES WEATHERING STEEL STAIN?
Protection from staining is recommended and can be accomplished by any of these three methods:
- Sealing nearby concrete surfaces with an epoxy-urethane coating.
- Painting the weathering steel in the areas above the respective concrete supports (many times this is the 1st ten feet of the span).
- Special details for drip bars and drip pans.
CAN YOU COVER YOUR VEHICULAR BRIDGES?
Yes. From time to time, this request has been entertained. The combination of steel framing and timber siding and roofing elements can result in an attractive covered bridge structure without the costs of heavy timber framing and the speed of steel construction.
HOW LONG WILL A PAINTED BRIDGE LAST BEFORE RE-PAINTING IS REQUIRED?
From AISI’s Myths & Realities of Steel Bridges Publication … “There are cost-effective ways of repainting existing bridges. For new construction, there are modern high-performance coatings which comply with EPA standards and which can provide a minimum service life of 25 years prior to first paint maintenance.”
HOW DO YOU MEASURE LENGTH AND WIDTH?
Length can be a somewhat ambiguous term when discussing bridges and a more engineering term is “Span”. Span is defined by the centerline-of-bearing to centerline-of-bearing distance along the centerline of roadway. Overall length is defined as the total length of the steel members but can sometime mean the open dimension between backwalls.
Bridge limits means the distance between approach slabs, if they are proposed, or the distance between the backside of the backwalls, if they are not proposed.
Width also can be a very ambiguous term and is better named “Roadway Width” as it explains more about the intended meaning. Roadway width is the clear or open lateral dimension between two rails or two curbs or between a rail and a curb, depending on the edge details.
The distance between the centerline of the truss from the rail or curb varies depending on the edge detail and/or the railing attachment method.
The dimension between the centerlines of the trusses is the span of the floor beam. The span of the floor beam, like the span of the truss is the dimension used in the structural analysis of the bridge.
CAN YOU DESIGN TO MEET MY STATE’S DOT REQUIREMENTS?
Yes we can. The scope of your project is dependent on knowing whether your State’s DOT is participating in the funding of the project or not, and whether following their design guidance or their detail standards will be mandatory. If it is, we will want to know what items are important to the DOT and review those requirements before beginning the preliminary design of your bridge.
It will be very important to document this early on in the correspondence and also in the scope of work put forth in your project’s proposal and specifications. This will allow for proper planning of submittals, steel procurement and shop inspections to be performed. This will help to more accurately plan approval and delivery dates.
DO YOUR BRIDGES MEET AASHTO REQUIREMENTS?
Yes. We design for the specified loads, analysis methods, limit states and design criteria given by AASHTO in their published bridge specifications. The two governing specifications at this time are AASHTO’s Standard Specifications for Highway Bridges, 2002, 17 Edition and the AASHTO’s LRFD Bridge Design Specification, the current edition being No. 6.
WILL YOU DESIGN THE FOUNDATION?
U.S. Bridge provides professional design and engineering services for the bridge superstructure. Foundations, substructure and approach roadway designs are provided by the owner or owner’s representative. If a project needs these design tasks completed as part of a project, U.S. Bridge can contract with consulting engineering firm as a turn-key service. Please contact one of our salesmen to discuss this in more detail.
WHAT LIFTING POINTS SHOULD I PLAN FOR TO ERECT THE BRIDGE?
Detailed installation manuals are available for contractors or agencies performing the installation. The plans submitted for approval and the plans sent with the bridge at the time of shipment have lifting weights of the field segments and assembled units.
WILL YOU HAVE A REPRESENTATIVE AT THE SITE DURING INSTALLATION?
Yes. U.S. Bridge will provide on-site technical assistance for the bridge erection, upon request. Co-ordination of schedules is required. Our Project Managers will work with the Owner’s representative or contractor to understand the planned schedule and arrive on-site ahead of the lifting operations.
WHAT KINDS OF PROJECTS ARE PREFABRICATED BRIDGES BEST SUITED FOR?
A prefabricated truss bridge in many cases is the optimal solution for solving challenging bridge replacements over streams and railroads on low volume roads, especially with clear spans of between 120 ft. to 250 ft. It does this by minimizing the profile grade impacts, the number of substructures needed and the crane costs, as compared to multi-span and long-span prestressed girder options.
WHAT ARE THE MOST COMMON BRIDGE STYLES?
U.S. Bridge’s Cambridge and Cambridge Flat are the most common vehicular truss bridge styles.
ARE THERE ANY OPTIONS OTHER THAN WEATHERING OR PAINTED STEEL TO PROTECT AGAINST CORROSION?
Hot-Dip Galvanizing is a very popular method to protect against corrosion. It provides inexpensive, long-lasting, superior corrosion protection.
WHAT IS THE DIFFERENCE BETWEEN AISC AND AASHTO?
AISC stands for the American Institute of Steel Construction. AASHTO is the American Association of State Highway Transportation Officials. AASHTO governs the design of bridges, whether they are wood, steel or concrete. AISC governs the design of steel structures, although most bridges fall outside of AISC’s application because most bridges are funded or specified by a government agency. AISC performs and maintains much of the underlying technical history, research and basis for AASHTO’s steel design code but the AASHTO Code and its technical subcommittees have supplemented this base with their own history and research.
WHAT ARE THE FOUNDATION REQUIREMENTS?
Repeating from an earlier question, the foundation recommendations should be made by a geotechnical engineer who has obtained soil borings and studied the conditions. The final foundations should be selected and designed by the engineer responsible for the substructure. Abutments support the bridge superstructure by transmitting the bridge’s resultant loads, called reactions, through load plates and into bearing pads; and then through masonry or sole plates. These plates spread the load out and into the concrete abutment seat. Anchor bolts are formed or installed into the concrete to prevent the bridge from moving in any unintended directions. We provide professional design and engineering services for the superstructure, but the foundations, substructure and approach roadway designs are provided by the owner or owner’s representative. If a foundation and substructure design needs to be prepared, a consulting engineering firm will need to be hired. This can be done through U.S. Bridge as a turn-key service. Please contact your regional salesman to discuss this process.
ARE PREFABRICATED BRIDGES SIMPLE TO INSTALL?
Yes. Especially if you have built one before.
In many cases, a prefabricated steel vehicular bridge is a faster way to complete a bridge construction project. It depends on many things within the goals of the project and what is specified within the scope of work. It is important to remember that steel bridge construction is a technical enterprise requiring experience in construction methods, crane rigging, steel erection practices, planning, scheduling, and managing resources and manpower. For more information to what’s going to be need for your project, please call our Engineering Department to discuss what you are planning – (888) 872-7434.
To read more about this topic and for a better overall understanding, we suggest reading Chapter 23 of Steel Bridge Design Handbook, by the NSBA/AISC or Section II of “Designing with Structural Steel, A Guide for Architects, 2nd Edition”, AISC.
HOW WILL YOUR BRIDGE HOLD UP UNDER HEAVY SALTING?
Hot-dipped galvanizing is an excellent and proven corrosion protection method. Heavy de-icing chemical application will reduce the overall duration of effectiveness but if proper paving operations and curb and joint details are constructed, any underside deterioration can be kept to a minimum.
HOW DO I REPAINT THE BRIDGE?
After many years of service in harsh conditions, it may be necessary to repaint much or all of your bridge. Depending on your site, it may be beneficial to paint on site using a containment system or it may be more economical to remove the structure and have it shop painted and re-erected. Properly preparing steel surfaces and applying coatings is a technical and complex endeavor. It requires specialized experience, equipment, environmental controls and QC procedures; and involves hazardous materials and equipment. To repaint a bridge in the field, it will likely involve hiring a specialty contractor capable of accessing your site and protecting the environment. They will need to demonstrate their experience painting bridges using the coatings and system you have specified.
HOW LONG DOES GALVANIZING LAST?
Galvanizing is a tremendously long-lasting and effective treatment of steel to minimize or eliminate corrosion. U.S. Bridge specifications require that any steel to be galvanized shall be galvanized according to ASTM A123. If done according to this specification and installed properly in a non-industrial environment, the galvanizing protection will last well beyond 100 years (75 years to 1st maintenance, which is defined as 5% rust).It is important to remember that U.S. Bridge’s procedure is to fabricate a bridge’s components prior to galvanizing. They are then subsequently bolted together into assemblies achieving galvanizing on both sides of a joined piece. This enhances even further, the corrosion protection afforded by galvanizing.
Another testament to galvanizing’s long term protection is the 35-year Warranty that accompanies this conditioning for primary components of our bridges.
According to the American Galvanizers Association, the corrosion rate of zinc is directly influenced by atmospheric conditions. Certain factors that specif-ically affect the corrosion of zinc include: temperature, humidity, rainfall, sulfur dioxide (pollution) concentration in the air, and air salinity. None of these factors can be singled out as the main contributor to zinc corrosion, but they all play a role in determining the corrosion protection hot-dip galvanized (zinc) coatings can provide in certain environments.
When galvanizing is exposed to the natural wet and dry cycles of the atmosphere, it develops a series of zinc byproducts on the surface called the patina. The patina is stable and non-reactive unless exposed to aggressive chlorides or sulfides. The patina is a key component in the longevity of the hot-dip galvanized coating in the atmosphere.
For corrosion classification purposes, atmospheres are generally divided into five groups:
- Industrial Atmospheres;
- Suburban Atmospheres;
- Temperate Marine Atmospheres;
- Tropical Marine Atmospheres;
- Rural Atmospheres.
Independent and industry testing of galvanized steel samples over decades in industrial, urban, rural, and marine environments, with varying degrees of chlorides, sulfides and other corrosive elements, has yielded performance data for galvanized steel in real world applications. For results, view the “Time to First Maintenance” chart (link below).
WHEN IS WEATHERING STEEL NOT RECOMMENDED?
Weathering steel should not be used when the low-steel elevation above the water surface is small; less than 10 ft., and the air flow expected under the bridge is poor. This limit is even less, 8 ft., if the stream is flowing. Please check out “A Primer on Weathering Steel” by the NSBA/AISC and also “Uncoated Weathering Steel in Structures” by FHWA
WILL THE WEATHERING STEEL DRIPPING IN THE CREEK BE AN ENVIRONMENTAL ISSUE?
Corrosion of weathering steel is no different than many other naturally occurring oxidation or weathering processes and is a common occurrence in nature, whether it is naturally occurring elements or man-made materials. To read more about weathering steel, please check out “A Primer on Weathering Steel” by the NSBA/AISC and “Uncoated Weathering Steel in Structures” by FHWA.
HOW DO THE PAINTS/ PROTECTIVE COATINGS AVAILABLE TODAY DIFFER FROM THOSE USED IN THE PAST?
Protecting exposed structural steel with paint coatings has always been an area of industrial research and scientific advancement. Certainly changes in surface preparation, primer coat content (zinc vs. red lead), application equipment and testing methods have all advanced over the years but much, much more has gone on into the improvements in this industry than can be explained here.
To read more about this topic and for a better overall understanding, we suggest reading Chapter 23 of “Steel Bridge Design Handbook”, by the NSBA/AISC or Section II of “Designing with Structural Steel, A Guide for Architects, 2nd Edition”, AISC.
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