factor of safety - a factor  applied to the failure stress assumed to exist in a structure to provide a conservative margin in the strength of a structure compensating for irregularities existing in structural materials and workmanship, uncertainties involved in mathematical analysis and stress distribution, service deterioration and other unevaluated conditions 

failure - a condition at which a structure reaches a limit state such as cracking or deflection; usually does not involve fracture since failing structures are deemed unsafe, therefore unusable, before they collapse 

falsework - a temporary wooden or metal framework built to support the weight of a structure during the period of its construction and until it becomes self-supporting 

fascia - an outside, covering member designed on the basis of architectural effect rather than strength and rigidity although its function may involve both 

fascia girder - an exposed outermost girder of a span sometimes treated architecturally or otherwise to provide an attractive appearance 

fatigue -  the tendency of a member to fail at a lower stress when subjected to cyclical loading than when subjected to static loading 

fatigue crack -  any crack caused by repeated cycle loading 

fatigue damage - member damage (crack formation) due to cyclic loading 

fatigue life  - the length of service of a member 

fender - a structure that acts as a buffer to protect the portions of a bridge exposed to floating debris and water-borne traffic from collision damage; sometimes called an ice guard in regions with ice flaws 

fender pier - a pier-like structure which performs the same service as a fender but is generally more substantially built; see GUARD PIER 

field coat - a coat of paint applied after the structure is assembled and its joints completely connected; quite commonly a part of the field erection procedure and is termed field painting 

fill - material, usually earth, used to change the surface contour of an area, or to construct an embankment 

filler - a piece used primarily to fill a space beneath a batten, splice plate, gusset, connection angle, stiffener or other element; also known as filler plate 

filler metal - metal prepared in wire, rod, electrode or other adaptable form to be fused with the structure metal in the formation of a weld 

filler plate - see FILLER 

fillet -  a curved portion forming a junction of two surfaces which would otherwise intersect at an angle

fillet weld - a weld of triangular or fillet shaped cross-section between two pieces at right angles 

filling - see FILL 

fine aggregate - sand or grit for concrete which passes a sieve mesh of 5 mm square 

finger dam - expansion joint in which the opening is spanned by meshing steel fingers or teeth 

fish belly - a term applied to a girder or a truss having its bottom flange or its bottom chord constructed either haunched or bow-shaped with the convexity downward; see LENTICULAR TRUSS 

fixed beam - a beam with a fixed end 

fixed bearing - a bearing which does not allow any longitudinal movement; see BEARING 

fixed bridge - a bridge having its superstructure spans fixed in position except that provision may be made in their construction for expansion and contraction movements resulting from temperature changes, loadings, or other forces 

fixed end - movement is restrained 

fixed-ended arch - see VOUSSOIR ARCH 

fixed span - a superstructure span having its position practically immovable, as compared to a movable span 

flange - the horizontal parts of a rolled I-shaped beam or of a built-up girder extending transversely across the top and bottom of the web 

flange angle - an angle used to form a flange element of a built-up girder, column, strut or similar member 

floating bridge - see PONTOON BRIDGE 

floating foundation - used to describe a soil-supported raft or mat foundation with low bearing pressures; sometimes applied to a "foundation raft" or "foundation grillage"

flood frequency -  the average time interval in years in which a flow of a given magnitude will recur 

flood plain - area adjacent to a stream or river subject to flooding 

floor - see DECK 

floorbeam - a horizontal member located transversely to the general bridge alignment  

floor system - the complete framework of members supporting the bridge floor and the traffic loading  

flow capacity - maximum flow rate that a channel, conduit, or culvert structure is hydraulically capable of carrying 

flux - a material which protects the weld from oxidation during the fusion process 

footbridge - a bridge designed and constructed to provide means of traverse for pedestrian traffic only; also known as pedestrian bridge 

footing - the enlarged, lower portion of a substructure, which distributes the structure load either to the earth or to supporting piles; the most common footing is the concrete slab; footer is a local term for footing 

foot wall - see TOE WALL 

force - an influence that tends to accelerate a body or to change its movement 

forms - the constructions that hold concrete in place while it is hardening; also known as form work, shuttering; see LAGGING, STAY-IN-PLACE FORMS 

form work - see FORMS 

foundation - the supporting material upon which the substructure portion of a bridge is placed

foundation excavation - the excavation made to accommodate a footing for a structure; also known as foundation pit 

foundation failure - failure of a foundation by differential settlement or by shear failure of the soil 

foundation grillage - a construction consisting of steel, timber, or concrete members placed in layers; each layer is normal to those above and below it and the members within a layer are generally parallel, producing a crib or grid-like effect. Grillages are usually placed under very heavy concentrated loads 

foundation load - the load resulting from traffic, superstructure, substructure, approach embankment, approach causeway, or other incidental load increment imposed upon a given foundation area 

foundation pile - see PILE 

foundation pit - see FOUNDATION EXCAVATION