Etabs Manual Pdf Free Download

[Namuncura Mckoy]

Inmanual calculation we do 2D analysis of structures using different method of analysis than stiffness method. So somewhat difference will always be there while you compare results of manual vs Etabs in Systems.. For individual elements, they are 100% same.... More over, be sure to make modifiers of slab to 0.0001 to nullify slab stiffness because in manual analysis, might be u are assuming only rectangular beam and applying line load to this beam. but in etabs you are modeling slab with beam making system a bit stiffer due to slab stiffness. If you will reduce slab stiffness to negligible value, than results will be near to each other. Still there will be some difference because we might be using force methods idealizing 2D structure while etabs performs 3D analysis using direct stiffness method.

The first thing you need to check is if you have done a modelling mistake or not. ETABS and Manual results shouldn't be off by more than 2%. Here are a few items that you should check and then get back to us.

The easiest way to do it is that to check vertical reaction (gravity only) at a column. If the manual and ETABS column reactions for gravity load are same at different locations in your model, then you can assume that your loading input is correct. As a rule of thumb, you centre columns with biggest tributary areas should have higher reaction than corner and edge column assuming constant grid framing.

The next step is to check the load combination and that if the loads have been correctly assigned to each load category. The best way to do that is to print your load combinations and check them on paper. Similarly, load cases assignments can be checked as well. Pay attention to signs of +ve and -ve.

I have checked a lot of models where everything was correct except for material properties. Sometimes, the design engineer enters wrong unit weight and it messes all the reactions and design. Be sure to verify material properties.

You need to make sure that your model is stable. If your model is unstable your results would mean nothing. I don't know what the latest version of ETABS does in-terms of instability warnings but SAP2000 would never give you instability warning. You will have to go in the analysis log and check if the model is stable or unstable. A lot of junior engineer just get their results from unstable models which is dangerous. Check the analysis log and check model for all error including meshing.

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It's extremely important for structural engineers to know how to deal with structural software inputs to obtain the optimum results. A structural software is nothing but a tool to obtain numbers. However, understanding these numbers is important to make judgments and thus designing the structure.

In this post I will explain an important input called "Property Modifiers". Understanding property modifiers helps you to get very similar software-results to theoretical results which are the basis of any structural software.

But how can we decide to use property modifiers or not? and how does it effect the results? to answer that question, five (5) different models with a 4m x 4m single span and one way ribbed slab system were modeled and analyzed using (Etabs v19) as the following: (Note that all the beams are 20x60cm, and all the modifiers are as per ACI 318 / CSI Knowledge Base)

From the bending moment diagrams (1.2DL+1.6LL load combination) in figures from 1 to 5, Model 5 gives very similar results compared to the manual calculation for the ultimate moment on the internal ribs with a value of 1.1 ton.m, and a bending moment value of around 4.6 ton.m which is also very similar to the manual calculation.

In conclusion, using property modifiers is not considered as an optimum solution with higher factor of safety as shown in models 1,2,4 especially in the deign of certain types of slabs such as the ribbed (waffle) slabs. However, this modeling technique gives also accurate results for the load transfer from solid slabs to beams.

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