Other Loads for Overall Lateral Stability

8'

FIGURE E4.12

These forces are checked against the usual allowable stresses increased by 50%.

4.4.5.2 Other Loads for Overall Lateral Stability

The overall stability of the superstructure against wind, nosing, and centrifugal forces must also be ensured. The stability of spans and towers should be calculated using a live load, without impact, of 1200 lb/ft.^∗On multiple track bridges this live should be placed on the most leeward track on the bridge. A 50% increase in allowable stress is permissible when determining stresses anchor rods or other members resisting overall instability of the span.

4.4.6 PEDESTRIANLOADS

Typical walkways for steel railway bridges consist of a steel grating or other system with nonslip surfaces. The walkway components are designed for a load of 85 psf and maximum deflection of 1/160 of the walkway span length. Guardrails for pedestrian walkways are typically designed for railing and postloads of 200 lb applied laterally or vertically at the location of maximum effect.

4.4.7 LOAD ANDFORCECOMBINATIONS FORDESIGN OFSTEELRAILWAY

SUPERSTRUCTURES

AREMA does not provide explicit load combinations but incorporates combinations in various design recommendations (Sorgenfrei and Marianos, 2000).Table 4.5 out-lines load combinations that apply to steel superstructure design found in various recommendations of AREMA.

∗This represents a uniform load of empty rail cars.

Loads and Forces on Steel Railway Bridges 145

TABLE 4.5

Load Combinations for Steel Railway Superstructure Design

Load Case Load Combinations Members F_L

A1 DL+ LL + I + CF All members 1.00

D1 SL+ N + CF Members resisting overall instability 1.50

D2 Q Members resisting overall instability 1.50

E1 DL+ EQ All members 1.50

E2 DL+ LL + I + CF + EQ Members in long bridges only 1.50

F W or LV Members loaded by wind only 1.00

G DF Cross frames, diaphragms, anchor rods 1.50

H1 DL Members stressed during lifting or

jacking

1.50

H2 DL Members stressed during erection 1.25

H3 DL+ W Members stressed during erection 1.33

F_L= Allowable stress load factor (multiplier for basic allowable stresses), DL = Dead loads (self weight, superimposed dead loads, erection loads) (seeSection 4.2),LL= Live loads (seeSection 4.3.1),I= Impact (dynamic amplification) (seeSection 4.3.2),CF= Centrifugal force (seeSection 4.3.4),W= Wind forces (on live load and bridge) (seeSection 4.4.1),LF= Longitudinal forces from equipment (braking and locomotive traction) (seeSection 4.3.2.2),N= Lateral forces from equipment (nosing) (seeSection 4.3.2.3),CWR= Forces from CWR (lateral and longitudinal) (seeSection 4.4.3),EQ= Forces from earthquake (combined transverse and longitudinal) (seeSection 4.4.4),DF= Lateral forces from out-of-plane bending and from load distribution effects (seeSection 4.4.5.1),LV= “Notional” lateral vibration load (seeSection 4.4.2), LLT= Live load that creates a total stress increase of 33% over the design stress (computed from load combination A1) in the most highly stressed chord member of the truss. This load ensures that web members attain their safe capacity at about the same increased live load as other truss members due to the observation that in steel railway trusses, the web members reach capacity prior to other members in the truss. This live load, LLT, is based on the requirements discussed in Chapter 5, Section 5.3.2.3.4, SL= Live load on leeward track of 1200 lb/ft without impact, I (seeSection 4.4.5.2),Q= Derailment load, ffat= Allowable stress based on member loaded length and fatigue detail category.

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5 Structural Analysis and