How to Model Friction in ETABS and Its Application in Thermal Analysis

“Where there’s friction, there’s fun” our professor in Physics 11 way back in college told us.

We all gave him knowing smiles. Sensing something else, he proceeded to explain the mathematical explanation that relates ‘fun’ to friction.

friction.jpg
You remember this, right?

Combining Newton’s 1st and 3rd law of motion, the block won’t move given that force F does not exceed μn. So putting both forces in equilibrium, F = µN.

Fun right? It is indeed. Brace yourself though, this is going to be weird stuff.

In the Middle East where there are significant temperature fluctuations all year round, a structure should be provided with the necessary reinforcement to inhibit the formation of cracks during casting. And that means you’ve got to have information on the construction methodology and the sequence of pouring before you can do your calculations. That is the short term effect of temperature.

On the other hand, the implication of fluctuating temperatures on the structure during its lifetime is called the long term effect.

I will not discuss about the short term effects, CIRIA already took care of that. I will discuss however how we considered the long term effects in ETABS. But first, let us have some flashback on temperature, cracking and its effects on the structure. Time to set the imagination mode on.

rigid-bar
The rigid bar. You thought you already got rid of this, don’t you?

Tension/Compression

When the temperature rises, a body tends to expand. But for the rigid bar, the expansion is prevented by the fixed support and thus, the internal force that resulted due to the rise in temperature tends to “compress” our rigid bar. Are you able to follow?

And when the temperature drops, a body tends to shrink. And so for our rigid bar, this shrinking or shortening is prevented by the fixed support and thus the resulting internal force is tension.

Cracking

When a beam, slab, column, or wall or any part of the structure bends and exceeds the modulus of rupture, it cracks. Depending on the magnitude of the bending moment, it can generate cracks ranging from microscopic to visible cracks. Although there are no researches yet that link cracks to reinforcement corrosion, cracks can be very unsightly. And though there are no significant reduction in the strength of structural members due to “normal” cracks, non-technical people can get overly concerned and the said cracks might cause undue panic in extreme instances.

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Cracks due to bending. Photo from http://www.answers.com

Now, pulling a cable for example, beyond its allowable pulling force will produce tears or cracks in its strands before rupturing.

For our rigid bar bending under its own weight and other imposed loads while being simultaneously subjected to tensile stresses can produce significant cracks due to the combination of pulling and bending. And because concrete has a very low tensile strength, additional bars must be provided to limit the formation of the said cracks.

Temperature gains that induce compressive stresses tend to “seal” the cracks and thus the compression – bending action is not as critical as the tension-bending action.

Confused yet? It can get confusing alright. The rigid bar is simple. However, for a building with varying stiffness and complex interaction between structural members, calculation of combined tensile and bending stresses is next to impossible, except with the use of computers.

Ok, hold on tight. We’re about to include friction in the chaos.

The Fun Friction

rigid-friction
Like this one!

For our rigid bar model, let’s replace the fixed supports by a friction mechanism. If we apply a drop in temperature, the bar tends to shorten and shrink. But depending on the end reaction and coefficient of friction, the support may not yield, and so there is tension in the bar. But if the end support moves, the bar is relieved from tension.

For structures founded on soil which serves as the point of contact between the structure and friction, the interaction is much more complex. Normally, one would expect that there will be no movement under the presence of heavy loads such as columns and walls supporting multi-storeys compared to columns supporting just one storey. Again this depends on the force normal (perpendicular) to friction forces and the coefficient of friction. And again, throw in all the geometrical complexities, variety in loading and differences in temperature exposure, and it will render the structure very complicated. Where there are large restraints such as walls, there tends to exist large amounts of tensile stresses due to the change in temperature.

And once you were able to model all of these, all you need to do is look for hotspots: large tensile stresses including the bending moments (recall that cracks are produced by bending and tensile stresses), key in the forces on spreadsheets that check crack widths and you will be able to find the adequate reinforcement.

Modelling Friction in ETABS

Springs are not the correct means of modelling friction. It is only applicable for piles, since we can derive the horizontal springs with ease. But not friction, because once the area of concern moved, the restraint is now released and the horizontal spring already has a constant value.

Credit goes to Dale, my design manager who dug deep into ETABS to explore the friction isolator link.

First, we need to model a link. That is something like a “column” support between the raft and the soil. Height of which does not matter so long as we set the parameters correctly. But first, the point support below the link should be fixed.

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Modelling of links under a raft.

Now to define these links, we go to the Link Property Data. For Link type, we chose “Friction Isolator”, tick Directions U1, U2, and U3 and tick all boxes under Non Linear.

Link Corner Front.JPG
The Friction Isolator Link Type

Under Properties, click “Modify/Show for U1…” U1 is the direction of Z axis – translation (up/down). Stiffness under the Linear and Nonlinear Properties is the tributary soil subgrade modulus, hence we have Edge, Corner, and Interior. Interior has a full value of the subgrade modulus, Edge has only half, and Corner is a quarter of the total value of subgrade modulus.

Link Corner U1.JPG
Under U1

For both U2 and U3 that denote the X and Y translation (horizontal), set everything else to zero except for friction which should be as per ground investigation report. And the Effective stiffness should be as big as possible (will discuss that in the next paragraphs). According to Dale, he hasn’t yet discovered why there should be an effective lateral spring for friction but based on preliminary analysis, the larger the value, the more accurate the results will be.

Link Corner U2 U3.JPG
Under U2

Having set all of those, we must now define nonlinear cases for the load cases. The aim of which is, to load the gravity loads first (that is, normal forces should engage first before friction can take place). And so for this example, we started our nonlinear case with both self-weight and superimposed dead loads. And we continue from this nonlinear state in applying the temperature change.

dl-nl
Dead load nonlinear load case.
temp-nl
Temperature nonlinear load case. Continuation from nonlinear dead load.

Now we have to verify the results. Friction force is a function of the coefficient of friction and the force normal to the surface. The shear forces of the links in the X and Y direction must be combined in vector form (that is SRSS or square root of sum of squares) and the result should be equal to the product of friction coefficient and the axial load on the link. And the greater the horizontal effective stiffness is, the greater is the accuracy of the results. Why? We cannot answer it just yet. We still have a lot to read to answer that.

We haven’t used this method yet extensively as this is still relatively new to all of us. This may be crude but in my judgment, this is the best way to model friction compared to how we did it previously. It’s just a lot more tedious especially in the modelling process but this is the nearest exact mathematical solution that we can find. And comparing it to the crude methods we did previously, this is the next best invaluable tool for thermal analysis.

The Handsome Micromanager

They need someone to serve as an inspiration to remind them that “this man has been through a lot and yet he successfully overcame all those resistance. He’s one tough motherfucker. I want to be like him.”

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Photo from http://www.business2community.com

I am a good friend and officemate but when it comes to work, I want nothing but the best. And in that case, I tend to micromanage.

Not that I didn’t produce crappy results since the beginning of my professional career but I want the work I delegate to be taken seriously and given attention and passion the way I do.

If you’ve been through some of my posts, you would know that my work had been my social life. It still is. It’s not a drag, nor something that I wish would be over soon the moment I wake up. I love my job, I’m happy and fulfilled. In other words, it’s the extension of myself. It’s a part of me. IT IS ME.

My work is the reward in itself.

I’ve had my shares of downs in my career when I didn’t know a thing of what I was doing. But since I am so eager to learn and be better, I used those depressing times to dig deep into certain aspects of structural design that I don’t fully understand. I’ve read. I asked. I abused my calculator and used tons of paper to work with my calculations. And sure enough I found answers.

And lately, I’ve been handing down those answers myself to junior engineers who are still taking baby steps towards their continuing improvement.

I don’t claim to be a master now. I’m still learning and will always love learning especially in my chosen craft. It’s just that I am no longer qualified to be called a greenhorn. I’ve already grown a considerable size of tough horns out of my obstinate head.

So why all this ruckus?

It’s very frustrating for me wasting my time and effort explaining something over deaf ears. I’ve experienced talking to juniors who are not listening – who do not want to listen, rather. I can tell if a person I’m talking to is listening or is just pretending that he does.

And the problem is they don’t even ask. They pretend they know their way out. And when the moment of truth comes, I would know, albeit too late that the methodology used is all wrong. It’s very easy to ask. Doesn’t take the whole day to ask, does it?

The next thing that I’m so frustrated about is a structural model that’s not thought of properly.

While I know that it takes a certain degree of experience to create a quality model, I can tell if it was done in a delinquent or “bahala na” manner which is a very irresponsible way to do it. Structural modelling should be done with utmost care, like procreating. You should do it with passion, it should be put together detail by intricate detail.

I’m relatively new to supervising junior engineers because I am used to doing the job myself. There is fulfilment that comes from piecing it all together and making sense out of the mathematical model in front of me and so I’m hesitant to let that part go and let somebody else do it.

And sometimes, I have this temporary amnesia that I tend to forget that we have different levels of learning capabilities.

The style that works for me in learning my craft is to be left alone to discover things for myself. I ask questions when I get stuck, but I only ask after I’ve already exhausted all the means to answer my own inquiry. Thankfully I have managers whose management style complements my style of learning. I am also thankful for their insane patience with my short comings as I learn further still.

As I’ve said, I am no longer qualified to be called a greenhorn. And as much as I don’t like it, I need to include mentoring to the tasks assigned to me. I’ve been at the front of the battlefield, but now I need to leave my ranks and be a part of trainers of new privates to fill my place. I guess it’s a part of the corporate ladder. And of life in general.

There is a stage where you start from nothing but your guts and your will to succeed. You will not be able to get all the wisdom you need in one seating, nor will you be able to read everything you need in one book. A big chunk of learning is dependent on the TAE method (trial and error). And effective learning is experiential learning. Just like in structural engineering where one may know all the contents of a textbook. But without experiencing the issues firsthand, the engineer’s nothing but a mere storage drive where files are kept instead of a walking and thinking human being.

And there is this stage where we need to start weaning the young ourselves. Whether we like it or not, we’re no longer the young anymore because we’ve already learned a lot now. And if there’s a reason why we’ve learned our lessons and we’re still alive is that we are meant to share what we’ve learned.

It’s now time for us to hand those wisdom down to the juniors who need our guidance because as of now, they don’t know a lot yet about what they’re doing. They need someone to show them another way out because they will run out of ideas on how to get their asses off the rut they’ve been stuck into.

The juniors need our patience as they step forward, shakily at first. They will fall as a result of piecemeal experiential learning. They need our patience to be there when they fall occasionally to tell them to stand once more, and for us to tell them to keep going.

The juniors need our presence so that they have someone to look up to during the trying times. They need someone to serve as an inspiration to remind them that “this man has been through a lot and yet he successfully overcame all those resistance. He’s one tough motherfucker. I want to be like him.”

Things to Remember When Designing a Foundation

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Massive reinforcement for a raft foundation. Photo from http://www.aci-net.com

Designing the foundation is by no means the most boring task a design engineer will undergo. Nor does it mean that it is the least prestigious. You see dude, the mightiest skyscraper needs to rest on a mightier foundation. Otherwise, its glory will be mythical, one that would remain on the design drawings and will never be constructed to reality.

Foundation design requires a lot of common sense. A lot of it. In fact, foundation design is all about making sense of the super substructure that will support the superstructure.

For a few guidelines, I’ve listed the essential ones below. If I missed anything or if you might find anything missing or you want to augment my discussions, please hit reply and key in your thoughts. I, including the technical community of structural engineers will greatly appreciate your thoughts about it.

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Action not equal to reaction = Broken Jaw. Photo from http://www.fxguide.com
  • Make sure that all loads above the foundation are transferred correctly. Check base reactions. Check applied vertical point loads for gravity loads such as dead and live loads and see that they match the overall base reaction from the structural 3D model. Check for horizontal base reactions for wind and seismic loadings.
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Burj Khalifa foundation. Photo from http://www.civilenggseminar.blogspot.com
  • Know thy geometry. This is an exciting part for me. If it’s just a pad foundation or a typical 3-pile or 4-pile-group pile cap, it can really get a little boring. But if the foundation system is supported directly on soil or a piled raft with a playful rise and fall of elevations and mind-bending underside crankings then it’s game on. This also implies the designer to be mindful of possible MEP trenches or pits. It is important to model the geometry as near to the actual situation as possible.
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Modelling the pile and the pile spring to account for the effects of strut and tie. My snippet from SAFE.
  • Model the correct soil support. Consult the geotechnical reports for pile spring values and soil springs for rafts usually defined as the subgrade modulus. Take note that the subgrade modulus is defined as the allowable load (kN, kips) per square meter (or square feet) per millimeter (or inch) settlement. Usually, the unit of the subgrade modulus in SI is in kN/cu.m There were also instances when we modelled the pile, 75 mm into the cap soffit to account for the strut and tie effects.
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You don’t want your house floating like a butterfly, do you? Photo from http://www.daad.wb.tu-harburg.de
  • Apply hydrostatic uplifts where required (emanating from ground water table). Aside from gravity floor loads, the hydrostatic uplift can be critical for piles with high tensile loads and bearing pressure for rafts. More of that on the proceeding item. The knowledge on the underside of raft topography is critically at play here. Remember that the deeper the pits and depressions are, the greater the uplift loads are.
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Cant find any related photo so here goes something else… Photo from http://www.zyg.com.sg
  • There should be no soil tensile stresses because obviously, only compressive stresses (bearing pressure) are resisted by the soil. Only piles can carry tension loads due to lateral loads and hydrostatic uplift. If there are positive tensile stresses, it should be investigated. And if there really is tension to be resisted, it might be required to run a non-linear analysis to capture the effects of uplift or to alter the existing geometry of the foundation in order to eliminate the uplift.
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Nothing beats hand calculations, hey honey, did you hear me? Photo from http://www.mibb-design.com
  • Never underestimate the power of manual load take downs. This is the most powerful tool for foundation sensibility checks especially for piled foundations.
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Same same. Photo from http://www.betanews.com
  • Same layout and loading, same answer. Be it column loads or pile reactions, it should be almost the same. The “fishy” one will stand out with this check.
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An extreme version of “settlement”. Photo form http://www.sott.net
  • Check for settlements. Differential settlements should be kept below the allowable levels because it will cause additional stresses to the whole building. How? Imagine a portal frame and one column settles by a certain amount of vertical displacement. The beam connecting the columns will incur an additional bending moment which might be very significant. So the whole system is not really “at rest” because of the initial stresses brought about by excessive differential settlement. Another visual effect of which is cracking of brittle partitions such as glass façade, block works, and cracks on corners of doors and windows.
floating-house
At least Noah’s ark floated! Photo from http://www.article.wn.com
  • Check the flotation. Your structure is not Noah’s ark! Large hydrostatic uplift can cause a building or structure to float. Don’t ignore uplift forces, it’s a must to consider this in the design process because it’s a basic serviceability requirement.
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Agree bro! Photo from http://www.jitbit.com
  • Extra serviceability requirements. Aside from flotation, one should check that the structure is stable by checking the sliding and overturning.
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That should hurt! Photo from http://www.imgarcade.com
  • Avoid punching the raft. Other than one way shear and bending, one must ensure that columns or pile caps especially single-pile pile caps do not punch through the supporting raft. What we do is we do a handcalc of the actual stress from the column load to the critical perimeter and compare it to the allowable stress.
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I don’t think you can minimize that do you? Photo from http://www.ahmadalikarim.wordpress.com
  • Minimize the crack. In some parts of the world just like in the Middle East, it is but common to check crack widths emanating from service bending moments to mitigate the resulting crack and its probable effect on the corrosion of reinforcement. This aims to provide a compact and robust section with optimum reinforcement. We usually delimit our cracks to 0.20mm for raft/foundation undersides and 0.3mm on top (except where there is a water tank on top of the raft, and that we take 0.20mm as a limit, or it also depends on the nature of the fluid retained where a more stringent requirement of 0.10mm crack or lower is required).

How to evaluate the mass moment of inertia in ETABS

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Mass moment of inertia. Photo taken from http://www.physics.louisville.edu

 

Good times, good times.

The project I am currently involved in will undergo a wind tunnel test in order to derive a more realistic wind loading for the building.

But before they can generate results, they asked for the following data to be furnished to them :

  1. Modal displacements per floor. We need to provide the displacements for the first 3 modes of vibration.
  2. Seismic mass, center of mass, and center of rigidity per floor.
  3. Mass moments of inertia per floor.

The last one was relatively new to me because honestly, I haven’t given much thought about it. I cannot find any sense of urgency to learn the said concept, and so I’m glad to be assisted by a design manager when the time came. Luckily, he is a part-time nerd like me who has a knack for deep technical discussions such as this topic on the mass moment of inertia and its role in dynamic analysis and in the overall structural response.

By definition from http://www.engineersedge.com, the “mass moment of inertia, usually denoted I, measures the extent to which an object resists rotational acceleration about an axis, and is the rotational analogue to mass.” This should not be confused with the stiffness used in defining the global and local stiffness matrices of members used for structural analysis that is:

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Hooke’s law. Taken from http://www.hendrix2.uoregon.edu

and that it is not the same with the K matrix of the equation of motion:

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Engineering Dynamics Equations. Taken from http://www.formulasheet.com

The stiffness matrices used for modal analysis are the translational and rotational stiffness. Note again that the rotational stiffness stated is a function of mass and it’s distance to the center of rigidity while the rotational stiffness due to its rigidity is based on its geometric properties.

mass moment.JPG
ETABS generated mass moment of inertia…

ETABS produces a mass moment of inertia about its center of mass (by the way, you need to set all diaphragms to “Rigid” for it to generate this calculation). This is what I’ll call the local rotational stiffness. But since we’re dealing with a system, we need to get the global rotational inertia which is the building twist about a point called the center of rigidity. And so just like moment of inertia where we need to transfer it to a certain location other than its centroidal axes, the mass needs to be transferred to the center of rigidity as well.

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Center of Rigidity, CT and center of mass, CM. Photo from http://www.debug.pi.gr

So how did we figure out that one? By manual calcs. How? By summing the mass of individual elements (floor elements, walls, beams and columns including the applied line loads and floor loads that constitute the seismic mass or the “mass source”) multiplied by the square of the distance of its centroid to the center of rigidity. It’s an onerous process but there’s really no other way to verify the result from ETABS. Luckily, based on our calculations, when we added the centroidal mass moment of inertia to the product of mass and the square of its distance to the center of rigidity, the results can be safely considered acceptable (discrepancy was kept to a minimum).

capture
Table above are values from ETABS while the mass moments below are based on manual calculations. The results yielded a relative error of +/- 2.19%

Were you able to follow?

Another eureka moment indeed, thanks to the guidance of a very patient design manager. I cannot hide my satisfaction and elation in being a part of decoding that one, it’s like finding gold in my backyard. But as usual, not a lot would share my excitement. Mostly, I’m given the blank are-you-going-nuts look. And most of the time, I just try to understand but sometimes it is kinda frustrating when no one seems to share that joy with you.

Hell I’m still happy despite all that. That’s why I’ve written it here…

Static vs Dynamic Earthquake Load Case: When Do We Use Either or Both?

So will it be dynamic only, or both?

Do we always have to use both the static and dynamic earthquake load cases for design? When can we use the dynamic case only?

There is no exact rule as to when can we exclude the static and consider only the dynamic case only. And as far as I know there are no codes to regulate this, because the prompting situations cannot be generalized and because it really depends on the situation.

I was fortunate enough to encounter this situation in one of my projects. Fortunate because even if it gave me a hell of a headache in trying to solve the issue, I learned a ton in exchange.

The project is located in the middle east. Seismic code is UBC 97, zone 2B with soil type Sc. It is a 24-storey building with a bearing wall lateral load resisting system. The placement of the walls in my judgement is “odd” – a very eccentric layout of walls favoring the right side and a lot of horizontal elements resisting the shear along X direction and fewer walls on the north-south direction.

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Odd ain’t it? All columns except the 6 columns in the middle are engaged or participates with the wall in resisting lateral loads. Told you…

True enough the walls were a pain in the ass. Nothing’s working!

With my project lead and design manager, we desperately tried to find ways to make the walls work, since the current arrangement of walls and columns was the only arrangement that was allowed by the current architectural layout. The possible solutions that we explored were to add link beams to tie the lateral system. And the other is we cracked the walls that exceeded the modulus of rupture due to tensile stresses brought about by tensile forces and in-plane bending due to lateral loads. The last solution worked, albeit the resulting wall thicknesses were insanely huge for a 24 storey building and reinforcement ratios were still very high – ranging from 2% minimum to the highest requirement of 3.95% (limit is 4% based on the ACI 318M-11). Aside from that, it’s very odd cracking almost all the walls because almost all walls are in tension, I am not exaggerating on this one! It’s as if I’ve cracked all the walls such that redistribution is simply impossible.

 

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The gray walls are “cracked walls” i.e. property modifiers for f11, f22, m11, m22, and m12 are reduced from 0.70 to 0.35. Can you believe that a large portion of the wall base is in tension?!

When I looked at the governing load combinations, I found out that governing load combination for most of the walls are the static earthquake load combinations. In an effort to understand why, I decided to plot the storey shear. True enough, there is a great disparity between static and dynamic, no wonder everything is struggling!

 

Shear X.jpg
See the gap between static (EQX – orange line) and dynamic (RSX – gray) for the X – direction …
shear-y
See the gap between static (EQY – orange line) and dynamic (RSY – gray) for the Y – direction …

I’ve tried to exclude the load combinations containing the static earthquake load and lo and behold, the largest reinforcement percentage is 2.5% which occurred at the bottom. So that’s the culprit!

I presented this to my design manager and he was convinced also that we can use the design forces using dynamic earthquake loads only or else nothing will work (wind loads and gravity loads were included too although the resulting storey shears for wind loads are way below the storey shears generated by earthquake hence it was not critical.)

Aside from design, this static-vs-dynamic dilemma have profound impacts when calculating the storey drifts and required building separations. A friend told me that they don’t use static earthquake load cases because it’s not realistic. I can see his point. Deriving the earthquake loads via the static force procedure gives only an estimate of the earthquake forces ideal for buildings with very regular layouts. But as the building layout gets complicated, a more complicated and rigorous structural analysis is needed to capture the real behavior of the building. This is done by employing the dynamic analysis which accommodates all combinations of translational and torsional motion due to ground acceleration that is produced during a seismic event.

So will it be dynamic only, or both? Without any code regulating this, it really depends on the results. If the resulting design and wall sections are over the top from expectations, removing the static earthquake load combinations can be an option. Otherwise if both will yield acceptable results based on the calculations and wise engineering judgment, it never hurts to use both dynamic and static earthquake load cases.

To the Technical People Reading My Technical Blog

I don’t have all the answers and neither do I know all the possible questions. What I can assure is that I’ve been there for all the situations I was in as a structural designer – a bloodied soldier in the battle field and a boxer receiving punches and knocking down opponents inside the ring.

Just to set things straight, this portion of my blog dedicated to structural engineering is not a one-stop-shop for the vast turf that our profession covers. There is always something left to learn and the learning never stops because as the saying goes, “the more you know, the more you don’t know” is very much applicable to us. Even the old and renowned structural engineers in the past and present have not mastered every detail nor unturned all the stones in every corner. Much so for me. Besides, there are a lot of forums, technical papers and printed journals and books out in the world today which are great sources of information for specific structural engineering issues.

I don’t have all the answers and neither do I know all the possible questions. What I can assure is that I’ve been there for all the situations I was in as a structural designer – a bloodied soldier in the battle field and a boxer receiving punches and knocking down opponents inside the ring.

What I share are by-products of my TAEing (TRIAL AND ERRORing).

What I do is I apply what I learned from forums, from a friend, officemates and technical literatures no matter how obvious or trivial as long as it captures the interest of the technical beast in me. How do I do this? I model in ETABS and SAFE or any structural engineering software a simple prototype and study the behavior until everything makes sense. It doesn’t matter if it seems stupid or a waste of time to others. The important thing is it captures my imagination. And in the end, the deduction will either reinforce what I already know or disprove what I previously believed to be true.

Just as I mentioned before, this serves as my technical diary. What I write about are the things that I discovered through my initiative, discussions with my superiors and colleagues, and the projects I was assigned to. The procedures I lay in this blog are the same procedures I execute.

I don’t have the monopoly of knowledge and wisdom in my profession nor do I claim to be a grand master of the subject. I acknowledge that there will always be those who are smarter than me and more experienced than me.

But what fires me up is the learning process. Learning in itself, the flow that comes with analyzing and drawing conclusions from the mathematical calculations are the rewards themselves. Of course money is necessary. I have myself and a family to feed and tend to. But I’ve been happier not by putting money as my ultimate prize but learning.

I don’t want to be stuck as a teacher when the time comes that I will teach younger generations of structural engineers. Life’s a journey not a destination, Aerosmith said in one of their songs. We all get our turns in teaching the young minds in whatever profession that we choose. I always want to be a student. And I am loving the journey of learning structural engineering. What drives my passion for structural engineering is finding answers, understanding the behavior of structures and making sense out the outputs – it’s what separates the handsome intelligence from the artificial one.

This inherent desire to be better and to strive for excellence (not perfection) are the ultimate qualities that a professional must have because these are the things that will give him satisfaction in his chosen career and it’s that something that no machines will ever be able to emulate.

And I wish that as a structural engineer like myself, we’ll both take the same stand to always be better than ourselves today than yesterday, and that neither do we allow AI nor programmers to replace the work that only the structural engineer does and understands. And most importantly, to always tell ourselves that the structural engineer’s judgment will ALWAYS be superior because he has this gut feel that no machine will ever have.

I’ll try to write more about technical topics since based on my stats, there are more views on topics regarding structural engineering rather than my creative compositions. I know I have a hard time blogging about my work especially when I’m preoccupied with deadlines and when I am in the flow. But I’ll try to post frequently about technical stuff. And by the way folks, thank you for your visits. I hope my writings helped enlighten you. Please feel free to communicate on how can I further improve so that I will be able to reach more graduate and assistant structural engineers who need assistance to master the basics.

So if you have comments/suggestions/doubts/thoughts or anything about our profession that you are hesitant to bring out in technical forums, let’s discuss it here. Feel free to leave a comment. I will try to answer what I can answer.

Cheers mates ml/!!!

The handsome man behind the blog,

Angmamangenhinyero

 

PS

I will also take this opportunity to pay my last respects to our iron lady, the late Senator Miriam Defensor-Santiago who served her country until the last ounce of her strength. All I have left in memory of you is your book. Thank you for being a Filipino. May you rest in peace – and your wisdom given to those who need it the most…

ETABS Life Saving Hacks: Copy/Paste to $et File

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Load Combinations definition in $et file of ETABS

 

Unlike in SAFE where you can import/export Excel files or copy from Excel and paste to SAFE, ETABS doesn’t have that feature – yet (maybe they will eventually incorporate this on their succeeding upgrades or maybe they won’t.) This would have been very handy especially if you start from scratch to define a hundred or so load combinations. But until then, you need to do it manually by defining it one by one in the GUI.

It’s not a pleasant task but it is necessary. Just imagine creating service wind load combinations to check the wind drift. In ASCE, there are 4 cases to consider including the possible combinations of the eccentricity (ETABS generates 12 wind load cases). Add to that the SLS (service limit state) gravity and seismic load combinations to complete the piling loads or to check and ensure that the allowable soil bearing pressure is not exceeded. And of course the ultimate limit state or ULS load combinations such as ultimate wind load combinations to design the columns and wall reinforcement, and the ultimate seismic load combinations to assess the seismic drift and to complete the necessary strength level combos.

But the list doesn’t end there.

If you have hydrostatic uplift, retained soil, Mononobe-Okabe loads and envelopes, that’s quite a lot to accomplish. And as I said, if you’re starting from scratch, it’s quite a handful. But if you don’t have the luxury of time to build the new model on an old ETABS template where your load combinations are already defined, don’t fret my friend because this is your lucky day. Like Staad’s text editor, you can use the text file $et of ETABS.

First, you have to rename the load patterns and load cases (including the necessary load factors and other parameters) with the same load patterns and load cases you are copying from. Study the syntax (how ETABS defines load patterns, cases, combos, and combos within a combo, see sample above), generate the syntax considering all load combinations in Excel, paste in the $et file, save, and import the said text file and that’s it! A half day’s work done in less than half an hour.

The same can also be done for frame column definitions. In one of our projects, the client required that we input the rebars reflected in the column schedule to the model so that when a third party reviewer will run the design, he can see in an instant if the columns pass by just looking at the PMM ratio or the demand/capacity ratio. In order to shorten the time required for this, our design manager defined the column tags, sections, and reinforcements. After which it’s just copy, paste, and import and it’s done.

Technical Pulls of Angmamangenhinyero (Mga Teknikal na Hugot ng Mamang Enhinyero): Welded Connection

Welded connection tayo. Di tayo basta-basta bibigay dahil pinag-isa tayo ng apoy.

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Welded connection tayo. Photo from http://www.tboake.com

May natatanging koneksyon tayo at ang koneksyong ito ay parang isang welded connection.

Hininang tayo at pinag-isa gamit ang apoy. Masakit at hindi madali ang prosesong ito dahil lulusawin ng apoy (welding) ang ibang bahagi natin upang tayo ay pagdikitin at pag-isahin. Kelangan nating isakripisyo ang ilang mga pansarili nating mga kagustuhan upang bigyang daan ng pag-iisa nating dalawa.

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Hindi tayo bolted connection. Photo from http://www.buildingsafeinc.yolasite.com

Hindi tayo gaya ng bolted connection dahil ‘di natin kailangan na may iba pang mamagitan sa atin upang tayo ay matibay na magsamang dalawa.

Kaya nating tumayo sa ating mga sarili, gampanan ang ating misyon na maging bahagi ng “bigger picture” at manatiling matatag na magkasama sa kahit ano mang dumating – mapa unos (wind loading), lindol (seismic loads) at mga pang-araw araw na pagsubok (service loads).

Welded connection tayo. Di tayo basta-basta bibigay dahil pinag-isa tayo ng apoy.