1、Section 3 LOADS 3.1 NOTATIONS ParA TYPES OF LOADS A = maximum expected acceleration of bedrock at the site a = length of short span of slab (Article 3.24.6) B = buoyancy (Article 3.22) b = width of pier or diameter of pile (Article 3.18.2.2.4) b = length of long span of slab (Article 3.24.6) C = com
2、bined response coefficient C = stiffness parameter = K(W/L) (Article 3.23.4.3) C = centrifugal force in percent of live load (Article 3.10.1) CF = centrifugal force (Article 3.22) C, = coefficient for nose inclination (Article 3.18.2.2.1) CM = steel bending stress coefficient (Article 3.25.1.5) CR =
3、 steel shear stress coefficient (Article 3.25.1.5) D = parameter used in determination of load fraction of wheel load (Article 3.23.4.3) D = degree of curve (Article 3.10.1) D = dead load (Article 3.22) D.F. = fraction of wheel load applied to beam (Article 3.28.1) DL = contributing dead load E = wi
4、dth of slab over which a wheel load is distributed (Article 3.24.3) E = earth pressure (Article 3.22) EQ = equivalent static horizontal force applied at the center of gravity of the structure E, = modulus of elasticity of concrete (Article 3.26.3) E, = modulus of elasticity of steel (Article 3.26.3)
5、 E, = modulus of elasticity of wood (Article 3.26.3) F = horizontal ice force on pier (Article 3.18.2.2.1) Fb = allowable bending stress (Article 3.25.1.3) F, = allowable shear stress (Article 3.25.1.3) g = 32.2 ft./sec.2 I = impact fraction (Article 3.8.2) I = gross flexural moment of inertia of th
6、e precast member (Article 3.23.4.3) ICE = ice pressure (Article 3.22) J = gross Saint-Venant torsional constant of the precast member (Article 3.23.4.3) K = stream flow force constant (Article 3.18.1) K = stiffness constant (Article 3.23.4) K = wheel load distribution constant for timber flooring (A
7、rticle 3.25.1.3) k = live load distribution constant for spread box girders (Article 3.28.1) L = loaded length of span (Article 3.8.2) L = loaded length of sidewalk (Article 3.14.1.1) 17 18 HIGHWAY BRIDGES 3.1 L = live load (Article 3.22) L = span length (Article 3.23.4) LF = longitudinal force from
8、 live load (Article 3.22) MD = moment capacity of dowel (Article 3.25.1.4) M, = primary bending moment (Article 3.25.1.3) M, = total transferred secondary moment (Article 3.25.1.4) NB = number of beams (Article 3.28.1) NL = number of traffic lanes (Article 3.23.4) n = number of dowels (Article 3.25.
9、1.4) P = live load on sidewalk (Article 3.14.1.1) P = stream flow pressure (Article 3.18.1) P = total uniform force required to cause unit horizontal deflection of whole structure P = load on one rear wheel of truck (Article 3.24.3) P = wheel load (Article 3.24.5) P = design wheel load (Articie 325.
10、i.3) PIS = 12,000 pounds (Article 3.24.3) Pm = 16,000 pounds (Article 3.24.3) p = effective ice strength (Article 3.18.2.2.1) p = proportion of load carried by short span (Article 3.24.6.1) R = radius of curve (Article 3.10.1) R = normalized rock response R = rib shortening (Article 3.22) RD = shear
11、 capacity of dowel (Article 3.25.1.4) - R, = primary shear (Article 3.25.1.3) R, = total secondary shear transferred (Article 3.25.1.4) S = design speed (Article 3.10.1) S = soil amplification spectral ratio S = shrinkage (Article 3.22) S = average stringer spacing (Article 3.23.2.3.1) S = spacing o
12、f beams (Article 3.23.3) S = width of precast member (Article 3.23.4.3) S = effective span length (Article 3.24.1) S = span length (Article 3.24.8.2) S = beam spacing (Article 3.28.1) s = effective deck span (Article 3.25.1.3) SF = stream flow (Article 3.22) T = period of vibration T = temperature (
13、Article 3.22) t = thickness of ice (Article 3.18.2.2.4) t = deck thickness (Article 3.25.1.3) V = variable spacing of truck axles (Figure 3.7.7A) V = velocity of water (Article 3.18.1) W = combined weight on the first two axles of a standard HS Truck (Figure 3.7.7A) W = width of sidewalk (Article 3.
14、14.1.1) W = wind load on structure (Article 3.22) W = total dead weight of the structure We = width of exterior girder (Article 3.23.2.3.2) W = overall width of bridge (Article 3.23.4.3) W = roadway width between curbs (Article 3.28.1) WL = wind load on live load (Article 3.22) w = width of pier or
15、diameter of circular-shaft pier at the level of ice action (Article 3.18.2.2.1) X = distance from load to point of support (Article 3.24.5.1) x = subscript denoting direction perpendicular to longitudinal stringers (Article 3.25.1.3) L 3.1 DIVISION I-DESIGN 19 z = reduction for ductility and risk as
16、sessment = (with appropriate script) coefficient applied to actual loads for service load and load factor designs (Article 3.22) y = load factor (Article 3.22) UPL = proportional limit stress perpendicular to grain (Article 3.25.1.4) B = load combination coefficient for buoyancy (Article 3.22.1) c =
17、 load combination coefficient for centrifugal force (Article 3.22.1) D = load combination coefficient for dead load (Article 3.22.1) E = load combination coefficient for earth pressure (Article 3.22.1) EQ = load combination coefficient for earthquake (Article 3.22.1) PICE = load combination coeffici
18、ent for ice (Article 3.22.1) L = load combination coefficient for live load (Article 3.22.1) R = load combination coefficient for rib shortening, shrinkage, and temperature (Article 3.22.1) s = load combination coefficient for stream flow (Article 3.22.1) w = load combination coefficient for wind (A
19、rticle 3.22.1) m = load combination coefficient for wind on live load (Article 3.22.1) = Poissons ratio (Article 3.23.4.3) 3.2 GENERAL 3.2.1 loads and forces: Structures shall be designed to carry the following Dead load. Live load. Impact or dynamic effect of the live load. Wind loads. Other forces
20、, when they exist, as follows: Longitudinal forces; centrifugal force; thermal forces; earth pressure; buoyancy; shrinkage stresses; rib short- ening; erection stresses; ice and current pressure; and earthquake stresses. Provision shall be made for the transfer of forces be- tween the superstructure
21、 and substructure to reflect the ef- fect of friction at expansion bearings or shear resistance at elastomeric bearings. 3.2.2 Members shall be proportioned either with refer- ence to service loads and allowable stresses as provided in Service Load Design (Allowable Stress Design) or, al- ternativel
22、y, with reference to load factors and factored strength as provided in Strength Design (Load Factor De- sign). 3.2.3 When stress sheets are required, a diagram or no- tation of the assumed loads shall be shown and the stresses due to the various loads shall be shown separately. 3.2.4 Where required
23、by design conditions, the concrete placing sequence shall be indicated on the plans or in the special provisions. 3.2.5 with Article 3.22. The loading combinations shall be in accordance 3.2.6 When a bridge is skewed, the loads and forces car- ried by the bridge through the deck system to pin connec
24、- tions and hangers should be resolved into vertical, lateral, and longitudinal force components to be considered in the design. 3.3 DEADLOAD 3.3.1 The dead load shall consist of the weight of the entire structure, including the roadway, sidewalks, car tracks, pipes, conduits, cables, and other publ
25、ic utility services. 3.3.2 The snow and ice load is considered to be offset by an accompanying decrease in live load and impact and shall not be included except under special conditions. 3.3.2.1 If differential settlement is anticipated in a structure, consideration should be given to stresses resul
26、t- ing from this settlement. 3.3.3 If a separate wearing surface is to be placed when the bridge is constructed, or is expected to be placed in the future, adequate allowance shall be made for its weight in the design dead load. Otherwise, provision for a future wearing surface is not required. 3.3.
27、4 Special consideration shall be given to the neces- sity for a separate wearing surface for those regions where the use of chains on tires or studded snow tires can be anticipated. 20 HIGHWAY BRIDGES 3.3.5 3.3.5 Where the abrasion of concrete is not expected, the traffic may bear directly on the co
28、ncrete slab. If con- sidered desirable, YA inch or more may be added to the slab for a wearing surface. 3.3.6 ing the dead load: The following weights are to be used in comput- #/cu.ft. Steel or cast steel 490 Cast iron 450 Timber (treated or untreated) . 50 Concrete, plain or reinforced . 150 Compa
29、cted sand, earth, gravel, or ballast . 120 Loose sand, earth, and gravel 100 Macadam or gravel, rolled 140 Cinder filling . 60 Pavement, other than wood block . 150 Railway rails, guardrails, and fastenings (per linear foot of track) 200 Stone masonry 170 Asphalt plank, 1 in. thick 9 Ib. sq. ft. Alu
30、minum alloys 175 3.4 LIVELOAD The live load shall consist of the weight of the applied moving load of vehicles, cars, and pedestrians. 3.5 OVERLOAD PROVISIONS 3.5.1 For all loadings less than H 20, provision shall be made for an infrequent heavy load by applying Loading Combination IA (see Article 3
31、.22), with the live load as- sumed to be H or HS truck and to occupy a single lane without concurrent loading in any other lane. The over- load shall apply to all parts of the structure affected, ex- cept the roadway deck, or roadway deck plates and stiff- ening ribs in the case of orthotropic bridg
32、e super- structures. 3.5.2 Structures may be analyzed for an overload that is selected by the operating agency in accordance with Loading Combination Group IB in Article 3.22. traffic lanes, spaced across the entire bridge roadway width measured between curbs. 3.6.3 Fractional parts of design lanes
33、shall not be used, but roadway widths from 20 to 24 feet shall have two de- sign lanes each equal to one-half the roadway width. 3.6.4 The traffic lanes shall be placed in such numbers and positions on the roadway, and the loads shall be placed in such positions within their individual traffic lanes
34、, so as to produce the maximum stress in the mem- ber under consideration. 3.7 HIGHWAY LOADS 3.7.1 Standard Truck and Lane Loads* 3.7.1.1 The highway live loadings on the roadways of bridges or incidental structures shall consist of standard trucks or lane loads that are equivalent to truck trains.
35、Two systems of loading are provided, the H loadings and the HS loadings-the HS loadings being heavier than the cor- responding H loadings. 3.7.1.2 Each lane load shall consist of a uniform load per linear foot of traffic lane combined with a single con- centrated load (or two concentrated loads in t
36、he case of continuous spans-see Article 3.11.3), so placed on the span as to produce maximum stress. The concentrated load and uniform load shall be considered as uniformly distributed over a 10-foot width on a line normal to the center line of the lane. 3.7.1.3 For the computation of moments and sh
37、ears, different concentrated loads shall be used as indicated in Figure 3.7.6B. The lighter concentrated loads shall be used when the stresses are primarily bending stresses, and the heavier concentrated loads shall be used when the stresses are primarily shearing stresses. 3.6 TRAFFIC LANES 3.6.1 T
38、he lane loading or standard truck shall be as- sumed to occupy a width of 10 feet. 3.6.2 These loads shall be placed in 12-foot wide design *Note: The system of lane loads defined here (and illustrated in Figure 3.7.6.B) was developed in order to give a simpler method of calculating moments and shea
39、rs than that based on wheel loads of the truck. Appendix B shows the truck train loadings of the 1935 Specifications of AASHO and the corresponding lane loadings. In 1944, the HS series of trucks was developed. These approximate the effect of the corresponding 1935 truck preceded and followed by a t
40、rain of trucks weighing three-fourths as much as the basic truck. 3.7.2 DIVISION I-DESIGN 21 3.7.2 Classes of Loading There are four standard classes of highway loading: H 20, H 15, HS 20, and HS 15. Loading H 15 is 75% of Loading H 20. Loading HS 15 is 75% of Loading HS 20. If loadings other than t
41、hose designated are desired, they shall be obtained by proportionately changing the weights shown for both the standard truck and the corresponding lane loads. 3.7.3 Designation of Loadings The policy of affixing the year to loadings to identify them was instituted with the publication of the 1944 E
42、di- tion in the following manner: H 15 Loading, 1944 Edition shall be designated . H 15-44 3 20 Loading, 1944 Edition shall be designated . H 20-44 H 15-S 12 Loading, 1944 Edition shall be designated . HS 15-44 H 20-S 16 Loading, 1944 Edition shall be designated . HS 20-44 The affix shall remain unc
43、hanged until such time as the loading specification is revised. The same policy for iden- tification shall be applied, for future reference, to loadings previously adopted by AASHTO. gross weight in tons of the tractor truck. The variable axle spacing has been introduced in order that the spacing of
44、 axles may approximate more closely the tractor trailers now in use. The variable spacing also provides a more sat- isfactory loading for continuous spans, in that heavy axle loads may be so placed on adjoining spans as to produce maximum negative moments. 3.8 IMPACT 3.8.1 Application Highway Live L
45、oads shall be increased for those struc- tural elements in Group A, below, to allow for dynamic, vibratory and impact effects. Impact allowances shall not be applied to items in Group B. It is intended that impact be included as part of the loads transferred from super- structure to substructure, bu
46、t shall not be included in loads transferred to footings nor to those parts of piles or columns that are below ground. 3.8.1.1 Group A-Impact shall be included. (1) Superstructure, including legs of rigid frames. (2) Piers, (with or without bearings regardless of type) excluding footings and those p
47、ortions below the ground line. (3) The portions above the ground line of concrete or steel piles that support the superstructure. 3.7.4 Minimum Loading 3.8.1.2 Group B-Impact shall not be included. Bridges supporting Interstate highways or other high- ways which carry, or which may carry, heavy truc
48、k traf- fic, shall be designed for HS 20-44 Loading or an Alter- nate Military Loading of two axles four feet apart with each axle weighing 24,000 pounds, whichever produces the greatest stress. 3.7.5 H Loading The H loadings consist of a two-axle truck or the cor- responding lane loading as illustr
49、ated in Figures 3.7.6A and 3.7.6B. The H loadings are designated H followed by a number indicating the gross weight in tons of the stan- dard truck. 3.7.6 HS Loading The HS loadings consist of a tractor truck with semi- trailer or the corresponding lane load as illustrated in Fig- ures 3.7.7A and 3.7.6B. The HS loadings are designated by the letters HS followed by a number indicating the (1) Abutments, retaining walls, piles except as speci- fied in Article 3.8.1.1 (3). (2) Foundation pressures and footings. (3) Timber structures. (4) Si
