Unimap Module

Loads
*Highway Bridge loads
- Primary live loads are those due to traffic
- Specifications for truck loadings are reported in AASHTO
- For 2-axle truck, these loads are designated with H followed by the weight of
truck in tons and another no. gives the year of the specifications that the load
was reported

*Railway Bridge loads
- Loadings are specified in AREA
- A modern train having a 320kN (72k) loading on the driving axle of the engine is
designated as an E-72 loading
- The entire E-72 loading for design is distributed as shown in Fig 1.11




*Impact loads
- Due to moving vehicles
- The % increase of the live loads due to impact is called the impact factor, I
- From AASHTO



*Wind loads
- Kinetic energy of the wind is converted into potential energy of pressure when
structures block the flow of wind
- Effects of wind depends on density & flow of air, angle of incidence, shape &
stiffness of the structure & roughness of surface
- For design, wind loadings can be treated as static or dynamic approach

*Wind loads
- For static approach




*Wind loads
- Once qz is obtained, the design pressure can be obtained from a list of relevant
eqns



*Wind loads


*Wind loads
- Applications of eqn 1.3 will involve calculations of wind pressures from each
side of the building with due considerations for the possibility of either
positive or negative pressures acting on the building’s interior
- For high-rise building or those having as shape or location that makes them wind
sensitive, it is recommended that a dynamic approach is used

Example
*The enclosed building shown in Fig 1.4a is used for agricultural purposes and is
located outside of Chicago, Illinois on flat terrain
*When the wind is directed as shown, determine the design wind pressure acting on
the roof and sides of the building using the ASCE 7-02 specifications

*Fig 1.14






solution








Fig 1.14(c)


*Wind loads
- If the structure represents an above-ground sign, the wind will produce a
resultant force on the face of the sign which is determined from:


*Wind loads


*Snow loads
- Design loadings depend on building’s general shape & roof geometry, wind
exposure,location and its importance
- Snow loads are determined from a zone map reporting 50-year recurrence interval
- For flat roof (slope < 5%):




Example
*The unheated storage facility shown in Fig 1.15 is located on flat open terrain
near Cario, Illinois where the ground snow load is 0.72kN/m2
*Determine the design snow load on the roof

Fig 1.15




*Earthquake loads
- Earthquake produce loadings through its interaction with the ground & its
response characteristics
- Their magnitude depends on amount & type of ground accel, mass & stiffness of
structure
- To illustrate, consider Fig 1.16
- This model may represent a single-story building
- The top block is the lumped mass of the roof
- The middle block is the lumped stiffness of all the building’s columns
- During earthquake, the ground vibrates both horizontally & vertically
- Horizontal accel -> shear forces in the column
- If the column is stiff & the block has a small mass, the period of vibration of
the block will be short, the block will accel with the same motion as the ground
& undergo slight relative displacements
- If the column is very flexible & the block has a large mass, induced motion will
cause small accelerations of the block & large relative displacement
- Fig 1.16

- The effects of a structure’s response can be determined & represented as an
earthquake response spectrum
-For small structure, static analysis is satisfactory




*Hydrostatic & Soil Pressure
- The pressure developed by these loadings when the structures are used to retain
water or soil or granular materials
- E.g. tanks, dams, ships, bulkheads & retaining walls

*Other natural loads
- Effect of blast
- Temperature changes
- Differential settlement of foundation

Structural Design
*Material uncertainties occur due to
- variability in material properties
- residual stress in materials
- intended measurements being different from fabricated sizes
- material corrosion or decay

*Many types of loads discussed previously can occur simultaneously on a structure
*However, it is unlikely that the max of all these loads will occur at the same time
*In working-stress design, the computed elastic stress in the material must not
exceed the allowable stress along with the following typical load combinations as
specified by the ASCE 7-02 Standard
- Dead load
- 0.6 (dead load) + wind load
- 0.6 (dead load) + 0.7(earthquake load)
*Ultimate strength design is based on designing the ultimate strength of critical
sections
*This method uses load factors to the loads or combination of loads
- 1.4 (Dead load)
- 1.2 (dead load) + 1.6 (live load) + 0.5 (snow load)
- 1.2 (dead load) + 1.5(earthquake load)+ 0.5 (live load)