The ETABS model is now complete but does it make sense?
One way to strengthen the structural gut feel as discussed in my previous post, is to observe trends. And one way to do this is to keep records of previous results or table extracts, compare, list other notable behaviours, and take action. I know that this is a very vague statement so I am going to list and elaborate one by one the importance of the records/tables that I extract after revving up the ETABS model.
- Wind Load Definition
Changes come along the course of the design process and there is nothing unchangeable when it comes to wind loading. For example, in the ASCE code the gust factor can drastically change the design wind loads (as in our experience, the change in the tensile load on the wall skyrockets the rho value with a mere 0.10 difference!).
The same is true with the other parameters such as the eccentricities, windward and leeward coefficients, and whether the parapet wall is included.
A quick view on this table assures you that intended changes are implemented and unintended edits are checked and modified.
- Seismic Load Definition
Aside from the seismic loading parameters such as importance factor I and the R factor, this table when compared with the previous runs, can tell whether something changed in the framing or weight. For example let’s say the base shear increased. With respect to the overall building weight, the base shear is a function of the seismic mass consisting of dead loads and a percentage of live loads as defined in the codes.
With a constant building stiffness, an increase in the seismic mass will result in a lower building period and thus a higher base shear (since earthquake is an inertial force, the heavier the mass, the greater the force required to get it in motion. That makes sense?) The same mass however when supported by a stiff structural system will require a larger force to move it and thus a larger base shear.
Now with a constant seismic mass, if the period increased it may mean that the building stiffness “softened”. Just note that there is a limit to the minimum and maximum base shear based on the code be it ASCE or UBC.
- Modal Participating Mass Ratios
This is the actual computed fundamental period of the structure based on the structural properties and deformational characteristics of resisting elements as per Section 12.8.2 of ASCE 7-05.
According to good practice, the first two modes should be translation (that is in the x and y directions or in directions orthogonal to each other) and the third should be torsion. I have yet to find a good reference for this but if you do know where and in what reference will I find it, kindly post it in the comments section.
Why? The explanation is this:
A true period along X displaces the largest fraction of the seismic mass along X only. The same is true on the Y direction. If a considerable fraction of the weight is displaced on both directions for the first and second mode of vibration, then it may require rotating the whole model depending on the animation of the modal vibration or the modal direction factors. More of that in a while. And in case you don’t know, ETABS chooses the period of, say along X direction the largest mass percentage displaced along X as it is with the Y direction. Are the periods in the table below similar to the seismic load definition table above?
Note also that you can check the model for disjointed elements that need to be meshed depending on the said table. And one tip, as a rule of thumb the fundamental period of a building is roughly the number of stories divided by 10. So if you have a 2 story building and the software gives you a 20 second period (equivalent to a height of a 200 story building!), something is very wrong and it ought to be investigated and corrected immediately.
- Modal Direction Factors
Speaking of directions, the modal direction factors show the trajectory or the direction of vibration. Ideally, if for example the first mode of vibration is along X, the direction factor along X should be 1.0 and that of Y = 0.
So if the first two modes of vibrations are translations, then the direction of the first mode should be orthogonal (at 90 degrees) to that of mode 2. However, you cannot perfect the modal direction factor of 1.0 for let’s say UX and UY = 0. There’s a tolerance depending on the designer. Like the building I am in charge of designing. There is a 5 degree discrepancy from the horizontal and yet the design manager finds it acceptable.
And if there is a need to rotate the model, just be sure to view the animation of the modes so that you’ll know at what angle you should adjust, be it clockwise or counter-clockwise. It would either vibrate along quadrants I & III or at quadrants II and IV.
- Base Reactions
While all the sensibility checks I presented earlier are important, one of the most important is the base reaction table because it tells you whether all the forces in your building makes sense and that “nothing’s lost” nor anything’s insanely over. Say you have a typical two story residential building and it registers a 650 MN of self-weight reaction. Does it make sense? The building whose global reaction is shown below is a 4 story building with dimensions of roughly 100m x 100m and has a self-weight base reaction of ≈ 645,000.00 kN or 645 MN. Maybe the problem in the said 2 story building is a wrong property definition or an erroneously applied loading value. You will never know immediately if you have any problem with these if you will not check the base reactions.
And of course there are the lateral loadings.
How does the wind load along X compare to that with the Y direction. Have you wondered why the base reaction along X is greater than Y when you defined it as Wind Y for example? Or say if the wind base shear along X is greater but the building area (the larger the area, the greater the wind force it has to resist) along Y is greater, then we may have a problem.
One cannot make a conclusion that will be universally true to all projects. Every project is unique and it will pose its own unique challenge no matter how similar they are to previous ones.
But generally, the digits should always make sense. And if you’re considering modal and response spectrum analysis in earthquake, the base reaction is your tool to know whether the static and dynamic load case is balanced, i.e. base shear due to dynamic loads should be greater than or at least equal to the static earthquake load case.
These are very straightforward methods. And it may come to the point of being boring and monotonous but these sensibility checks are very handy and very easy to perform in order to spot the odd.
But then again, it will take a lot of constant practice to strengthen that structural gut feel to feel that something’s not right in the model. Especially if there really is something very wrong, or a specific check was not performed and you don’t know it yet.
“It’s no trick to get the right answer when you have all the data. The real creative trick is to get the right answer when you have only half the data and half of it is wrong and you don’t know which half is wrong.
The reason these are called sensibility checks is to ensure that the building makes sense. Full reliance to the software without verifying the results is like touring the nuclear power plant with blinders on, which may lead to catastrophic results. To quote my professor, which I believe is quoted by many – “GARBAGE IN, GARBAGE OUT.” This is to assure us that the inputs aren’t crappy so as not to produce crappy results.
Always remember that we as structural engineers or aspiring structural engineers have very little tolerance for errors because the safety of one if not thousands of lives depends on our diligence to ensure that our calculations are correct and that they make sense.
Does that make sense? What about your structural model?