B-42 Design Criteria for Bridges

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The following guidelines are provided to clarify design criteria for bridges. The following are addressed: new construction, replacement, and repair.

Section 13-33 of the Sonoma County Code Fire Safe Standards (FSS) and Section 503.2.6 of the 2016 California Fire Code (CFC) notes that bridges shall be constructed and maintained in accordance with the AASHTO Standard Specifications for Highway Bridges (HB-17).

For existing bridges, Section 13-17 of the Sonoma County Code Fire Safe Standards amends the CFC to require that existing bridges are evaluated per AASHTO Manual for Bridge Evaluation, 2nd Edition. All existing bridges require examination for deterioration and damage by California licensed civil or structural engineers. The extent of deterioration and damage shall be recorded in a Bridge Foundation Report to be submitted with structural drawings. The Bridge Foundation Report shall include results of visual observations on extent and severity of damage or deterioration, results of any non-destructive or destructive testing performed, and recommendations on design criteria and repairs. See section on Bridge Repairs.

Per the Contractor’s License Board, only contractors with Class A licenses are permitted to build/repair bridges.

Effective date: October 2017

Technical Details

Bridges shall meet the design, construction, and inspection requirements of HB-17 and CBC 2016. Per CBC Chapter 17 Structural Observation is required for all bridges. Special Inspection is required where noted in CBC Chapter 17: e.g. reinforcement, concrete, steel, welding, bolting, geotechnical, etc. Special Inspection Forms CNI-033 and CNI-033A should be submitted by the engineer of record and signed by the engineer and owner.

Plan submittals shall include: all architectural and structural details, site plan, vicinity map, and bridge cross section(s).

New Construction and Complete Replacement Bridges

New bridges shall be designed for HS20-44 vehicle live load with load distribution per HB-17 Chapter 3. The vehicle live load shall reflect heavier loads if other vehicles are intended (e.g. vineyard equipment, construction vehicles, etc.). A geotechnical report for foundation recommendations is required. Lateral Load Design shall be per ASCE7-13 or HB-17.

Designs with replacement of longitudinal span members shall meet the requirements for new construction. If foundations remain, they shall be justified for HS20-44 loading. Foundation retrofit would be required if the existing foundation is not sufficient for the loading criteria.

More Information:
AASHTO Standard Specifications for Highway Bridges 17th Edition, Section 3: Loads(PDF: 4.25 MB)
HS20-44 Table of Maximum Moments, Shears, and Reactions- Simple Spans, One Lane, HB-17 (PDF: 213 kB)

Bridge Repairs

Repairs include repair or replacement of some structural members of the bridge where primary structural members are intended to remain. Foundation retrofit would be required if the existing foundation is not sufficient for the loading criteria.

Bridge repairs should be designed for HS20-44 vehicle live load with load distribution per HB-17. Justification of the load carrying capacity of existing structural members and connections shall be provided (e.g. decking, girders, foundation, abutments, steel, welding, bolting, reinforcement, etc.) The effects of weathering and damage shall be addressed. The justification shall include written engineering observations, exploratory field observations, methodology to determine the integrity of members/connections, and material testing. For fire damaged structures, visual observation alone is not sufficient to justify integrity of remaining structural members. Assumed engineering properties for existing structural members used in design calculations shall also be justified.

Where the engineer cannot justify HS20-44 loading due to existing remaining structural members (e.g. girders, foundation, etc.) a reduced loading shall be permitted with approval from the Fire Code Official per Sonoma County Fire Safety Ordinance based on the table below. The vehicle live load shall reflect heavier loads if other vehicles are intended (e.g. vineyard equipment, construction vehicles, etc.).

The vehicle information below was obtained from local fire districts. Engineers may also use the most conservative load from the table for the bridge design without contacting the fire code official. The most conservative load is dependent upon the bridge length. GVW=Gross Vehicle Weight. All listed weights are in pounds. 

 Unit # GVWFront Rear Wheel Base Vehicle Type
7580 4400020000 20000 170"   14.17' Type 1
75814380019500 24000 182"   15.17'Type 1
759041660 1460027000200"   16.67' Water Tender
7291406001354027060 186"   15.50' Water Tender
759239000 12000 27000 190"   15.83'Water Tender
7281 365001750019000 172"   14.33'Type 1
WT21363401172024620 186"   15.50' Water Tender
7564360001200024000 184"   15.33' Type 3 4WD
72823540015600 19800 172"   14.33' Type 1
7535330001200021000 204"   17.00' Rescue
726624240 880015400 154"   12.83' Type 3 2WD

The engineer of record should also provide observations on protection and/or susceptibility of foundation from erosion, scour, etc. If foundation retrofit work is needed then a geotechnical report is required. If rip rap or other stream bank protection measures are required then a Grading permit and/or Storm Water permit is required.

Temporary Bridges

The required vehicle live load for temporary bridges is HS20-44 loading.

The following apply to new construction and repairs:

Vehicle Barriers

Vehicle barriers shall be designed with dimensions and loads per HB-17 or CBC Chapter 16 except where bridges qualify as low-volume roads. Most bridges will qualify as low-volume roads. Low-volume road bridges may have curbs designed per Design and Evaluation of Two Bridge Railings for Low-Volume Roads (USDA 1995). Low-volume roads serve residential parcels and are generally single lane with a 15 mph limit.

More Information:
Design and Evaluation of Two Bridge Railings for Low-Volume Roads (PDF)

Pedestrian Guards

Bridges with separate walkways for pedestrian traffic shall have Guards designed per California Building/Residential Code.

Waterway Freeboard

Bridges are required to have minimum 1 foot freeboard above the 100-year water surface elevation (Sonoma County Water Agency, Flood Control Design Criteria). A hydrology and hydraulic calculation will need to be prepared by a civil engineer. The Storm Water Section of Permit Sonoma will review any deviations from the minimum 1 foot requirement. If the bridge is expected to be submerged in 100-year flood event, the bridge shall be design for hydrostatic, hydrodynamic, and debris impact loads.

Existing bridges where the abutments remain and bridge deck elevation does not change will be permitted to remain below the 100-year flood level with the bridge designed for hydrostatic, hydrodynamic, and debris impact loads. For existing bridges, anchor bolts of bridge decking to girders and embedded anchorage of girders into concrete may be considered by the engineer of record to resist these loads.

Floor Surface

Bridges shall be provided with a skid resistant wearing surface (HB-17 Sec. 1.8). Wood non-structural longitudinal runners suffice for wood bridges.


Signage shall be provided to identify the design load of the bridge and certification date of the load. The proposed text shall be provided to the Fire Section for review prior to permit issuance. See sign template, last page.

505.2.6 Signs identifying traffic access limitations. (CFC SoCo Addition)

A sign identifying traffic access or flow limitations, including, but not limited to weight or vertical clearance limitations, dead-end road, one-way road (or single land conditions) shall be placed: (a) at the intersection preceding the traffic access limitation, and (b) no more than one hundred feet (100’) before such traffic access limitation.

505.2.1 Size of letters, numbers and symbols for street and road signs. (CFC SoCo Addition)

Size of letters, numbers, and symbols for street and road signs shall be a minimum of three inches (3”) for letter height with a three-eighths inch (3/8”) stroke, reflectorized, and contrasting with the background color of the sign. 

Sec. 13-43. Orientation and height of road signs. (FSS)

All road signs shall have an orientation and height in conformance with county road standards.

Additional Reviews

Review by other Divisions of Permit Sonoma

If a parcel is zoned as Riparian Corridor or Biotic Resource, work adjacent to or in waterways will require a zoning permit from the Planning Division. Work within a waterway will require clearance from the Engineering and Water Resources Division and may require a Grading permit and/or Storm Water permit. Contact those divisions for additional information.

Review by other Agencies

Work adjacent to or in waterways may require approval from other agencies such as the Regional Water Board, California Department of Fish and Wildlife, and the US Army Corps of Engineers. It is advised that applicants contact the other regulatory agencies regarding the construction/permitting requirements of those agencies. If a zoning permit is not required (i.e. ministerial permit), issuance of the Permit Sonoma building permit will not be contingent upon clearances from agencies outside of Permit Sonoma.


Weight Limit Sign

Weight Limits

A – 24
B – 36
C – .375
D – .625

E – 3.25
F – 3.5 E
H – 3.5 D
J – 9.465

K – 6.403
L – 10.375
M – 7

P – 11
Q – 12.813
R – 3.5
S – 1.5


Legend – Black

Background – White (retro-reflective)