Last Updated on September 11, 2024 by Maged kamel

Introduction to structural loads-LRFD and ASD.

Content of lecture.

We’ll discuss the dead load and live load and the associated tables with the allowed live load and dead load values. Next, we’ll learn how to create a load combination.

Two approaches will be employed. The first will use the LRFD coefficients. LRFD stands for load factor resistance design.

In this design strategy, we shall simultaneously increase the dead loads and live loads. We are going to lessen the element’s strength. As we shall see later, strength reduction factors will depend on the kinds of acting forces and moments.

The acceptable strength design is shortened to ASD. This design approach is a variation of the working stress or factor of safety. To determine the allowed stress in that working design method, the yield stress was multiplied by a safety factor.

Thus, the working stress design approach, or WSDM, has been modified.

Dead load as a structural load.

Table 17-12 on this slide contains the usual material densities, which we utilize to determine the dead load value. We also offer cast and hammered aluminum, which weighs between 159 and 171 pounds per square foot.

Different materials exist, such as lead, copper, and iron. Our steel deck has a manufacturer’s comment attached.

The stone is structurally lightweight and suspended; the weight is given in lb/square foot.

Other items like partitions and drywall partitions contain studs and sheets.

There is a wall category, whether made from brick or CMU block. The 4-inch CMU blocks weigh 29 lb/square ft. The different finishes category is also given, such as Terrazzo 1 inch weight 13 lb/ft2.

Live load as a structural load.

The next table source is ASCE-02. The title is the minimum uniformly distributed live loads, L0, and minimum concentrated live loads. The Access floor system office’s uniform loads are estimated at 50 lb/ft2. The corresponding KN/m2 is 50.0.

For the assembly areas and theaters, fixed seats are fastened to the floor). The uniform weight in lb/ft2 is 60.00. The corresponding weight in KN/m2 is 2.87. For balconies (exterior). The uniform lb/ft2 is 100.00. The Si units’ corresponding uniform load is 4.79 KN/m2. For corridors, the weight as a uniform load is 100 lb/ft2.

We continue checking table-4-1. For hospitals, the operating room weight as uniform load is 60lb/ft2, equivalent to 2.87 KN/m2. Private rooms have a weight as uniform load of 40 lb/ft2, equivalent to 1.92 KN/m2. For Office building live loads, file and computer rooms shall be designed based on anticipated occupancy. For Lobbies, the weight as uniform load is 100 lb/ft2, equivalent to 4.97 KN/m2.

Introduction to LRFD design

The next subject is Load and Resistance Factor Design (LRFD). LRFD is a method for designing structures so that no applicable limit state is exceeded when a structure is subjected to all appropriate combinations of factored loads. A limit state is a condition in which a structure becomes unfit.

Structural members can have several limit states. Strength limits state concerns about safety and relates to the maximum load-carrying capacity (e.g., Plastic hinge and buckling).

Servacibilty limit states related to performance under normal service load (e.g., excessive Deformation and vibration). The formula may summarize the LRFD method as applied to each limit state.

Σγi Qi <=φRn. γi are factors that are multiplied by loads. Qs are loads, whether dead load, live loads, wind, earthquake, etc. Φ is a reduction factor for the strength. While R strength capacity of the member. Φ is a reduction factor for the strength. While R strength capacity of the member. The raised loads, ultimate loads, should be less or equal to the modified strength.

Types of structural loads and related symbols.

In the next slide, we will check the types of loads and symbols. We start with the dead load, denoted as D. The next load is the live load, denoted by L, the gravity load due to the intended usage and occupancy, including the weight of people, furniture, movable equipment, and partitions.

In LRFD, the notation L refers to floor live loads, and Lr refers to live roof loads. R. denotes rain loads. The snow loads are indicated by S. E., which denotes earthquake loads. W represents wind loads.

Environmental loads.

What are the environmental loads? Environmental loads are loads due to the environment.

Snow. In the colder states, snow loads are often quite important. One inch of snow is equivalent to a load of approximately 0.5 PSF. But it may be higher at lower elevations when snow is denser. For roof designs, roof design snow loads varying from 10 to 40 psf are commonly used. The snow loads are increased due to bad drainage systems due to accumulation.

Rain as an environmental Load.

The second environmental load is Rain. Though snow loads are a more severe problem than rain loads for the usual roof, the situation may be reversed for flat roofs, particularly those in warmer climates. if water on a flat roof accumulates faster than it runs off, the result is ponding or accumulation of rainwater. The ponding causes the roof to deflect in a dish shape that can hold more water.

Wind as an environmental Load.

The third environmental load is the wind load. A survey of engineering literature for the past 150 years reveals more references to structural failures caused by the wind. The most infamous of these have been bridge failures, such as those of the Tay bridge in Scotland in 1879, which caused the death of 75 persons.

Earthquake as an Environmental Load.

For earthquake loads, it is necessary to consider seismic forces in the design of all types of structures.

Another definition for the limit state.

Another definition for the limit state describes a condition at which a structure or part of a structure ceases to perform its intended function. Strength limit stats define load-carrying capacity, including excessive yielding, buckling, fatigue, and gross body motion. Serviceability Limits state fine performance, including deflection, cracking, slipping, vibration, and deterioration. All limit states Must be prevented.

LRFD load parameters.

We will start to talk about the parameters of γi. The first formula is 1.4D. This formula is applied when there is only a dead load. No other loads exist, which is the case during construction.

In case of dead and live load in a structure, the formula is 1.2D+1.6L and add load acting on the roof, 0.50(Lr or snow or rain). The biggest value is shown. The third formula is(1.2D+1.6(Lr or S, or R) +(0.5L or 0.80W).

The loads considered are D& w&L&Lr and roof loads. The bigger value of 0.50L or 0.80 W is to be considered. In the fourth formula, the wind direction is in the direction of the Dead load. The formula includes 1.2D+1.3W+0.5L+0.5(lr or S or R).

The earthquake is acting in the direction of the Dead load. We have 1.2D+-1.0E+0.5L+0.2S plus if the E is in the direction and minus if the E is opposite to D.

We have an exception: The load factor on l in combinations. A4-3, A4-4, and A4-5 shall equal 1.0 for garages, areas occupied as places of public assembly, and all areas where the live loads are greater than 100 psf (pound per square foot).

The abbreviations for all the loads used in the different combinations are on the right-hand side.

Symbols are γ load factor,φ resistance factor, and Ω safety factor.

D &L&W are not necessarily to be loads only but could be shear forces or moments applied to a structure.

Torsional moments may also apply. For instance, if a given moment due to a dead load case is given plus a moment due to a live load is given, then 1.2D+1.6L means multiplying the dead moment value by 1.20 and the live load Moment by 1.60.

Sometimes the shear is given as Ql as alive load shear force acting.

For the live load case, then multiply by 1.60. Sometimes, factored dead is given, meaning the load is already multiplied by the load factor.

This is explained in the next slide. Note that D, L, W, S, etc. Are loads in a general sense, which includes bending moment, shear, axial force, and torsional moment. Sometimes, these internal forces are called load effects.

Thus, the symbol D means dead load, load moment, load shear, and axial force. For moments, we estimate the ultimate moments as 1.2Md+1.6Ml. The previous load factors considered as 1.4D+1.7L can be found in earlier versions of books.

LRFD Resistance Factors

The reduction factor Φ is not a constant value; it differs based on the condition of load, tension members, compression members, beams under flexure, and fasteners. Φ=0.90 is used for tension members (yielding state), and Φ=0.75 is used for tension members (fracture state).Φ=0.90 is used for compression members.

ASD load parameters.

For the ASD Design method, the loads D&L&W&E are left without multiplying in the load factor. For the strength factor, instead of multiplying by Φ, the design is divided by Ω, which is the safety factor, as shown. The first formula is ΣγQ=D. The second formula is ΣγQ=D+L+H.

The third formula is ΣγQ=D+(Lr or S or R)+H. The fourth formula is ΣγQ=D+0.75L+H +0.75(Lr or S or R). If we draw a graph showing the relationship between displacement and load, Rn is the nominal strength.

In the case of LRFD, we multiply by φ to have φ*Rn.  In the case of ASD, we divide by Ω, which is 1.67 for the moment. The values of Ω are shown in the slide, Ω =1.67 in the case of compression. Ω =1.67 in the case of tensile yielding.

The initial slope of the curve starts as a line from zero to yield stress. The ASD value =Rn/Ω.The relation between the Ω value and Φ is 1.50/Φ=Ω. If we Φ =0.90 in bending, 1.50/0.9=1.67 is the bending reduction factor in the ASD design if the Live load equals 3 times the Dead Load value.

In the next post, We will solve an example using the different load factors for LRFD and ASD designs.

If you want to see the major changes between CM #14 and CM#15 regarding the different grades of steel and their uses for steel elements and bars, please refer to post 1A.

A very good reference is A Beginner’s Guide to Structural Engineering.