Manual and Etabs result don't match

[farooqbro]

i am doing a project of 5 story in peshawar zone for seismic factor 0.2

data

beam (18*30)inch

tributary area (18*26) ft

dead and live live load (150 and 100 psf )

manual moment =470k-ft

etab result =232 k-ft

for case (1.2*dead+1.6*live )

why ?

please guide me 

thank you 

[WR1]

Please provide a sketch.

[farooqbro]

 

 

etab file

chamkni ground story.EDB

[WR1]

Farooq bro, I meant the sketch for the the loads and bending moment diagram from ETABS. I am afraid we cant open and check your ETABS model.

[farooqbro]

[Waqas Haider]

In manual 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.

[Ayesha]

On 2017-05-20 at 3:47 AM, farooqbro said:

i am doing a project of 5 story in peshawar zone for seismic factor 0.2

data

beam (18*30)inch

tributary area (18*26) ft

dead and live live load (150 and 100 psf )

manual moment =470k-ft

etab result =232 k-ft

for case (1.2*dead+1.6*live )

why ?

please guide me 

thank you 

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.

1. Check Total Gravity Load

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.

2. Check Load Combinations

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.

3. Check Units

When checking assigned load cases, you need to make your that you have applied the loads in the correct units. It is very common to juggle the units while multi-tasking. Verify all units.

4. Check Material Properties

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.

5. Check Stiffness Modifiers

If you are assigning stiffness modifiers, please check them to ensure they are correct.

6. Check Slab Thickness

Make sure your slab thickness is correct and not in wrong units.

7. Check member sizes

Make sure that the members are correctly modelled.

8. Check Model for Errors

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.

After you have completed all these steps, you will have more confidence in stating the discrepancy between ETABS and Manual Calcs.

[farooqbro]

thanks alot