- Introduction to structural loads-LRFD and ASD.
- Introduction to LRFD design
- Another definition for the limit state.
Introduction to structural loads-LRFD and ASD.
The video I used in the illustration.
The video gives an introduction to structural loads. The permitted values of dead and live loads based on the table by The ASCE are listed. The other various types of loading, for instance, wind, and earthquake are also included. The LRFD and ASD design principles are presented herewith. The video has a subtitle and a closed caption in English.
If you wish to review the pdf data used in the illustration, please continue reading.
You can click on any picture to enlarge, then press the small arrow at the right to review all the other images as a slide show.
Content of lecture.
The items that we will discuss will be the dead load& live load and the relating tables that include the values of permitted dead loads and live loads. Then how to make a load combination.
We will use two methods; the first one will be the LRFD coefficients. LRFD is the abbreviation for the load factor resistance design.
In this design method, we will increase the dead loads and live loads, but simultaneously. We will reduce the strength of the element in question. We will have reduction factors for strength based on the types of the nature of the acting forces and moments, as we will see later on
The ASD is the abbreviation for the allowable strength design. This method is a modification of the factor of safety or working stress design method. In that working design method, a safety factor was applied to the yield stress to get the allowable stress.
So a modification has been made to (WSDM), or the working stress design method.
This is a brief content of the post.
Dead load as a structural load.
In this slide, there is table 17-12 for densities of the common materials, which we use to calculate the value of the dead loads. We have another item, for instance, the aluminum, cast, hammered, The weight ranges from 159 lb/ft3 to 171 lb/ft3.
There are varieties of materials, for example, Copper, Iron, and Lead. We have a steel deck that has a comment from the manufacturer.
The stone structural lightweight, Suspended system, for which the weight is given in lb/square foot.
We have other items like partition and drywall partitions, that contain studs and sheets.
There is a category for walls, whether made from brick or CMU block.
The 4-inch CMU blocks weigh 29 lb/square ft. The different finishes category is also given, Terazzo 1 inch weight 13 lb/ft2
Live load as a structural load.
The next table source is ASCE-02. The title is given as the Minimum uniformly distributed, Live loads, L0, and minimum concentrated live loads. The Access floor system office’s uniform loads are estimated as 50lb/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=60.00. The corresponding weight in KN/m2=2.87. For balconies (exterior), The uniform lb/ft2=100.00.
The Si units’ corresponding uniform load is 4.79 KN/m2. For corridors weight as uniform load=100 lb/ft2.
We continue checking table-4-1. For hospitals, its operating room weight as uniform load =60lb/ft2, which is equivalent to 2.87 KN/m2. Private rooms weight as uniform load =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, weight as uniform load =100 lb/ft2, equivalent to 4.97 KN/m2.
Introduction to LRFD design
The next subject is the Load and Resistance Factor Design. 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. The Limit state is a condition in which a structure becomes unfit.
Structural members can have several limit states. Strength limits state concerns 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, which is 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, And movable equipment and partitions.
In LRFD, the notation L refers to floor live loads and Lr, live roof loads.
R. denotes rain loads. The snow loads are indicated by (S). E. denotes Earthquake loads. W represents wind loads.
What are the environmental loads? Environmental loads are 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.
The earthquake loads, it is necessary to consider seismic forces in design for all types of structures.
Another definition for the limit state.
Another definition for the limit state is used to describe 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 the presence Of both 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, 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).
On the right-hand side are the abbreviations for all the loads used in the different combinations.
Symbols are γ load factor,φ resistance factor, and Ω safety factor.
D &L&W are not necessary to be loads only but could be shear forces or moments applied to a structure.
Torsional moments may also apply. For instance, a given moment due to a dead load case is given plus am moment due to live load is given, then 1.2D+1.6L means multiplying the dead moment value by 1.20 and 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, which means that 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. So for moments, we estimate the ultimate moments as 1.2Md+1.6Ml
The previous load factors were 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, but it differs based on the condition of load, tension members, compression members, beams under flexure, and fasteners. Φ=0.90 for tension members (yielding state), and Φ=0.75 is used for tension members (fracture state).
Φ=0.90, for compression members Φ=0.90.
ASD load parameters.
For the ASD Design method, the D&L&W&E are left without multiplying in 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: ΣγQ=D. The second formula ΣγQ=D+L+H.
The third formula is represented by ΣγQ=D+(Lr or S or R)+H. The fourth formula is given ΣγQ=D+0.75L+H +0.75(Lr or S or R). If we draw a graph for the relationship between displacement and the load. Rn is the nominal strength.
In the case of LRFD, we multiply by φ so that we will 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 starting from Zero to yield stress. The ASD value =Rn/Ω.The relation of the Ω value and Φ as 1.50/Φ=Ω.
If we Φ =0.90 in bending, then 1.50/0.9=1.67 is the bending reduction factor in The ASD design if the Live load is equal to 3 times the Dead Load value.
This is a link for the pdf data used in this post.
In the next post, We will solve an example by using the different load factors for LRFD and ASD designs.
A very good reference, A Beginner’s Guide to Structural Engineering.