Permit/Notional Loads Without Lane Load in Software

Printer EAN 13 in Software Permit/Notional Loads Without Lane Load

1 In addition to HL-93 loads, following special loads need to be considered: Legal, permit, and military loads: Both the superstructure and foundation need to be designed to sustain these loads In earthquakes, all gravity loads including live load on the bridge contribute to seismic forces and moments The length of moving vehicle is also considered to compute design moments and shear forces

2 Permit load for Pennsylvania: minimum length of P-82 permit load is 55 ft with 204 kips weight (Figure 515) AASHTO lengths of permit (notional) loads are 51 ft between the rst and last axles with 199 kips weight 3 Permit load for New Jersey (Figure 516)

1 Live loads for railway bridges are many times higher than even the highest of truck loads requiring a permit A comparison of moving loads is shown in Figure 516 between the latest New Jersey permit load (790 KN or 180 kips total), cooper loads (Figure 517 and 518), 1800 kips or 400 kips for alternate load (Figure 519) 2 It is interesting to note that a bridge is normally designed for single truck occupancy per lane but with simultaneous lane load added Correction factors are applicable to multiple lanes, with live load modi ed by applying a factor Figure 520 shows a comparative study of moments with HS-20 and PA trucks and permit load P-82 3 Application of Impact Factors and Multiple Lane Presence Factors Refer to AASHTO Section 31414: Impact factors are applicable to truck loads and not to lane loads As required in AASHTO standard speci cations, no concentrated load is required in LRFD lane load Unfactored live load analysis is rst carried out without impact factors An impact factor of 13 is applied on truck load elastic analysis In addition, a multiple lane presence factor is applicable

Table 53 Classi cation of live load vehicles Limit State Load Case Strength I Strength II Strength III Strength IV Strength V Service II Fatigue De ection Construction/Uncured Slab Ratings State Permit LRFD Live Load Analysis Live Load Analysis HL-93, H-20, HS-20 H-20, HS-20 N/A N/A HL-93, H-20, HS-20 HL-93, H-20, HS-20 Fatigue vehicle De ection vehicle User de ned HL-93, H-20, HS-20 N/A Permit Load N/A N/A N/A N/A Fatigue vehicle N/A User de ned Permit load State Maximum Legal Live Load Analysis ML-80 ML-80 N/A N/A ML-80 ML-80 Fatigue vehicle N/A User de ned ML-80

Figure 515 Pennsylvania 8-axle permit load (P-82)

(KN)

1 Unit load in uence lines for peak moments have been generated at every beam location for a variety of span lengths At any section of a simple span, peak bending moment Wab/L (a and b are distances from left and right supports) If there was only one axle with full-lane load, peak moment for a single span WL/4 due to axle and w L2/8 due to lane load, both located at midspan For three axle loads on a twospan bridge peak, positive moment will occur at approximately 04L Maximum shear force Maximum reaction at supports 2 For multiple lanes, the multiple-lane reduction factor is applicable Also, for skew slabs, the skew correction factor is applicable

3 Usually, the center of gravity of truck load coincides with the peak moment at the section to give maximum positive bending moment Similarly, maximum negative moment will occur at a support due to different positions of axle loads 4 Using the law of superposition, any section of beam magnitude of bending moment may be added for each axle load to produce the combined bending moment from three or more axles The rst axle is placed at the unit peak value and multiplied by the axle load to give peak moment due to that load Values of other axle loads, say spaced at 14 ft, are multiplied by the corresponding unit values and added For certain positions of truck axles, absolute maximum moment may be obtained by comparison Therefore, many positions of the truck need to be considered 5 For convenience, in uence lines may be divided into regions to extract maximum positive and negative moments Maximum moments so generated will be multiplied by distribution factors and impact factors to give design moment