Loads and Load Combinations by AASHTO.pptx

This is a genuine effort to understand load combinations as per AASHTO codes, though not all load cases are covered.
An additional note: The content is for study purposes only; I do not claim ownership or authorship, as it is compiled from publicly available sources.

1. Loads & Load Combinations to AASHTO Presentation by Hariyali March 21, 2025

2. Flow of Presentation Philosophy of LRFD Load Categories in AASHTO LRFD Load factors & Combinations Load Calculation

3. Figure 1.1 Graphical Representation of LRFD ≥   1. LRFD Philosophy The primary goal of structural design is to size members and components while ensuring the structure is safe, efficient, and cost-effective . This aligns perfectly with the LRFD approach .   = R-DL-LL     …1.3.2.1-1 (AASHTO LRFD) = resistance factor =nominal resistance = load modifier = load factor =force effect/nominal load  

4. LRFD (Load and Resistance Factor Design) ASD (Allowable/working Stress Design) It applies safety factors directly to both the loads and material Strengths. It results in a design that balances both factors for more efficient and consistent safety levels. The traditional method, which applies safety factors only to the material Strength, tends to be more conservative and less optimized. Provides a statistically optimized safety margin . Provides a fixed safety margin . More efficient by optimizing safety and cost. Often over-conservative , leading to higher material usage. Figure 1.1 Graphical Representation f LRFD Figure 1.2 Graphical Representation of ASD

5. Resistance factor   For all other limit states: = 1.0   For the strength limit state     = 0.90 (tension-controlled reinforcement concrete section) = 1.0 (tension-controlled prestressed concrete sections with bonded strands) = 0.90 (tension-controlled prestressed concrete sections with unbonded strands) (shear and torsion reinforcement concrete section) (shear and torsion in monolithic prestressed concrete and CIP prestressed ) (compression-controlled concrete section with spiral or ties) = 0.90 = 0.85 = 0.75 (bearing on concrete) = 0.75

6. 2. Load Categories in AASHTO LRFD

7. 3. Load Factors and Combinations:

8. HS20-FTG Design Truck footprint for Fatigue Design

9. 4. Load Calculation Dead Loads (DC and DW) Dead Load (DC): Dead loads refer to permanent, static loads from the structure itself and barriers. They are calculated using the following steps: Load Determination : Dead Load = Material Density × Volume of the Structural Element Wearing Surface Dead Load (DW) : This includes the weight of the road surface and other fixtures that are not part of the primary structural system. Asphalt or concrete wearing surfaces are calculated similarly to structural dead loads.

10. Vehicular Live Loads (LL) AASHTO uses a standardized truckload model called HL93 for design. This represents a typical truck weight and configurations, including axle loads and spacing. Design for Truck Design Truck HL-93 Design Tandem HL-93 Design Lane Load HL-93 Fig. 1.3 Permit Vehicle (Federal Highway Administration) Fig. 1.2 Lane and Truck Loading Combination Diagram 3.6.1.2.2-1—Characteristics of the Design Truck (AASHTO) Fig. 1.2 Design Lane Load (AASHTO) HL-93TRK HL-93TDM Fig. 1.1 Design Tandem (AASHTO)

11. Number of Design Lanes Standard design lane width = 12.0 ft , unless specified otherwise. Number of design lanes = integer part of (clear roadway width / 12.0 ft) . If traffic lanes are narrower than 12.0 ft , use actual traffic lane width , and number of lanes equals actual number of traffic lanes . For roadway widths between 20 and 24 ft , always use two design lanes , each half of the roadway width . Multiple Presence Factor MPF ensures a realistic assessment of bridge loads by considering that not all lanes are likely to experience their full design loads simultaneously under normal traffic conditions.

12. Dynamic Load Allowance (IM) The impact factor accounts for the dynamic effects of vehicles moving over the bridge. According to AASHTO: 1.75 for deck joints 1.15 for fatigue limit states 1.33 for all other limit states Centrifugal Forces (CE) The centrifugal effect on live load shall be taken as the product of the axle weights of the design truck or tandem and the factor C where, Centrifugal forces shall be applied horizontally at a distance 6.0 ft above the roadway surface.   ………………………..….(3.6.3-1)

13. A pedestrian load of 0.075 kilo-pound per square foot shall be applied to all sidewalks wider than 2.0 ft and considered simultaneously with the vehicular design live load in the vehicle lane. Pedestrian Loads (PL) Braking Loads (BR) The braking force shall be taken as the greater of: 25 percent of the axle weights of the design truck or design tandem, or Five percent of the design truck plus lane load or five percent of the design tandem plus lane load This braking force shall be placed in all design lanes irrespective of traffic direction. These forces shall be assumed to act horizontally at a distance of 6.0 ft above the roadway The multiple presence factors shall apply

14. Vehicular Collision Force (CT) To protect structures from collisions, we have two options : Provide Structural Resistance , or Redirect/Absorb Collision Load using Barriers . Option 1: Structural Resistance If we choose to make the structure resist the collision , the pier or abutment should be designed to resist a 600 kip static force , This force is applied: Horizontally in a direction ranging from 0 to 15 degrees from the edge of pavement , At a height between 2 to 5 feet above ground Option 2: Redirect or Absorb Using Barrier Use a 42-inch high MASH TL-5 crash-tested rigid concrete barrier. Place the barrier at least 3.25 feet away from the face of the pier.

15. Water Loads (WA) Static Water Pressure , Buoyancy , and Stream Pressure — which includes both longitudinal and lateral components 1. Static Water Pressure   2. Buoyancy   = Unit weight of water = height of water above base   = Unit weight of water = Volume of submerged part of the structure  

16. Water Loads (WA)   3. Stream Pressure (Flowing Water Pressure ) L ongitudinal stream Pressure L ateral Stream Pressure  

17. Wind loads are calculated based on wind velocity, exposure category, and the area exposed to wind. Calculation: WS = Wind Pressure × Exposed Area Wind pressure on a bridge is a function of the wind speed, air density, and the area of the structure exposed to wind. The general equation for wind pressure acting on a surface is given by: Where: = Design wind pressure ( ksf ) = Design 3-second gust wind speed specified (mph); (Table 3.8.1.1.2-1) = Pressure exposure and elevation coefficient, G = Gust effect factor, accounting for fluctuations in wind speed over time (Table 3.8.1.2.1-1) C D =  Drag coefficient (Table 3.8.1.2.1-2)   Wind Loads (WS & WL)  

18. Figure 3.8.1.1.2-1 Design Wind Speed , in mph (m/s)    

19.   Factors Affecting Wind Pressure Based on Location: Wind Loads (WL & WS) Cont. Ground Surface Roughness Wind Exposure Distance Conditions Urban, suburban, wooded areas (Category B) B >1,500 ft for mean height ≤33 ft, or >2,600 ft or 20H for height >33 ft Open terrain, scattered obstructions (Category C) C All cases where B or D do not apply Large bodies of water, flat unobstructed areas (Category D) D >5,000 ft or 20H, or Structure is within 600 ft or 20H from Category D surface = 0.71   = 1.00   = 1.15   for z = 33 feet

20.   Wind Loads (WL & WS) Cont.

21. Wind Load on Live Load: For typical girder and slab bridges (span ≤ 150 ft and height ≤ 33 ft) Transverse component : 0.10 klf transverse Longitudinal component : 0.04 klf longitudinal Vertical Wind Pressure: Vertical wind pressure x Width of deck (including parapets and sidewalks) 0.020 ksf for Strength III load combination 0.010 ksf for Service IV load combination Vertical wind load is considered only for Strength III ( does not include wind on live load, WL) and Service IV load combinations. Wind Loads (WL & WS) Cont.

22. Thermal Load There are two thermal effects which potentially induce stresses in bridges, these are : 1. Uniform Temperature: 2. Gradient Temperature: Uniform Temperature: Procedure A Temperature Ranges ………………………………………………3.12.2.3-1 Coefficient of thermal expansion (e.g., 6.5×10 −6 / F for steel) = Length of member  

23. Procedure B Temperature Ranges Determination of Design Temperatures: For concrete girder bridges with concrete decks : =To be obtained from Figure 3.12.2.2-1 . =To be obtained from Figure 3.12.2.2-2 . For steel girder bridges with concrete decks : = To be obtained from Figure 3.12.2.2-3 . = To be obtained from Figure 3.12.2.2-4 .  

24. Temperature Gradient : Zone T 1 F T 2 F 1 -16.2 -4.2 2 -13.8 -3.6 3 -12.3 -3.3 4 -11.4 -2.7 Zone 1 -16.2 -4.2 2 -13.8 -3.6 3 -12.3 -3.3 4 -11.4 -2.7 Decks With an Asphalt Overlay Zone T 1 F T 2 F 1 -10.8 -2.8 2 -9.2 -2.4 3 -8.2 -2.2 4 -7.6 -1.8 Zone 1 -10.8 -2.8 2 -9.2 -2.4 3 -8.2 -2.2 4 -7.6 -1.8 Plain Concrete Decks Positive Gradient

25. Thank You ! Any Questions…