Loads in Structures.

Define Loads: The amount of weight carried by a structure, as per Cambridge Dictionary. Loads are what we design the structures for. So, it is imperative to work on it carefully. Defining loads is usually a one-time thing which is sorted out at the start of a project, but there could be changes. And as an Engineer, it's always important to understand the know-how. 

In the Construction Industry, there are certain specified load patterns. These are Dead, Live, Wind, Snow, Earthquake and Special Loads. Now I won't go around explaining the different kinds of loads in this blog. It is quite vast. What I would be writing about is the way to incorporate these loads in the software. 

First you have to define the Load Patterns. It looks as below.

Load Patterns

Here, you can define the types of loads. You can see there are lots. One should know the different load types while defining one. For example, Dead denotes the self-weight of the elements of a structure while Super Dead load considers the non-structural elements.
Self-Weight Multiplier is necessary too. It controls whether or not the structural self-weight is included in the load pattern. Typically, it is 1 only for the pattern of Dead Load. 
The specific codes are also implemented here, every country has their own specific requirements. If you select Seismic, you will get the option of selecting a particular code to consider. IS-1893 is the Indian Code for Earthquake Loads.

Code Specific Loads


Then comes the Load Cases. It defines how the defined load patterns are applied, how the structure responds and how the analysis is performed. 

Load Cases

This is where the magic happens. A lot can be commanded here. The load applied can be either static or dynamic, the response either linear or non-linear and there are different analysis methods like Modal, Response Spectrum, Time History etc. 
For each analysis to be performed, a load case is defined. A load case can constitute a single load pattern or a combination of load patterns. After an analysis had run, load-case results can be derived for all or any of the run load cases. 
I do believe one has to understand the working of the methods before jumping into the software. It gets easier to decipher the output results for methods like response spectrum, time history etc. 

Another typical loading technique is the Shell Uniform Load Sets. You create a combination of load patterns for a specific area, like the load for an Electric room, a Cafe, storage room etc. 

A28_Electrical Room



While we are at it, let's discuss load combinations too. 

Load Combination

One can find different combination types. The Linear Add is the most popular, it just does an algebraic sum by multiplying with the scale factor. Envelope is an important one and a bit tricky-at-first kind of combination. It creates an envelope of the load patterns and gives you the extreme outputs for those loads. Do remember: Envelope combinations must never be used for Design. 
Others, you can just figure out through their name: Absolute, SRSS and Range Add combination.

These can get quite complicated and messy if you decide to change it in the later phases of a project. I am not saying it's impossible, but it can be a bit messy. So, it's better to decide on it at the initial of a project and make minor adjustments as the project progresses. 

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