ETABS 2013 Model Downgrade to ETABS 9

You and I both want to use ETABS 2013 and gain access to the faster analysis times and new features. But we haven’t made the plunge, at least in part because we might lose work if we get a week or three down the road and realize that we need to revert to ETABS 9.

Good news: now you can downgrade your file! That is, I can downgrade your file. The tool is not ready release to the world, but I have tested it and am confident that it works to take any ETABS 2013 model and push it back to ETABS 9, with no loss of model data (except for any new features unavailable in the older version, of course).

If you’re interested, send me an email at drp181 [a t] gmail dot com. I can do the first few models for free, but if there’s much interest I may ask for a pittance (to help prioritize ‘real’ requests, mainly).

What other roadblocks are there to you using ETABS 2013? Leave a comment and let me know.

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Posted in Uncategorized | 27 Comments

Importance Factor for Large Office Buildings

This question seems to come up quite often: what importance factor should be used for a large office building? The answer is apparently less than clear. Here’s my take:

The intent of the 2006 IBC is to require Occupancy Category III (with its attendant 1.15 wind importance factor) for buildings “whose primary occupancy is public assembly with an occupant load greater than 300” (emphasis added). The principal staff engineer for ICC gave the following explanation in the November 2006 issue of Structural Engineer magazine:

Table 1604.5 (S7- 04/05)—The 2006 IBC criterion for Occupancy Category III buildings has been changed to read “covered structures whose primary occupancy is public assembly with an occupant load greater than 300” to achieve consistency with the legacy model codes and the NEHRP Provisions. The 2003 IBC table stated, “buildings where more than 300 people congregate in one area,” which was confusing and led to inconsistent interpretation and enforcement of the provision. It appeared to apply to any occupancy classification, even large commercial buildings where an occupant load greater than 300 in a single room is not unusual. That interpretation was a departure from past practice under the legacy codes; therefore, the language was changed in the 2006 IBC to better reflect the intent.

Even though the language of ASCE 7-05 is less clear, the intent of the code writers is evident: large office buildings should be in Occupancy Category II and have an importance factor of 1.0, even if certain rooms or areas can have more than 300 occupants. Note that the 2009 and 2012 IBC retain this same language. Only buildings whose primary occupancy is public assembly AND with an occupant load greater than 300 must be Occupancy Category III.

Disagree? Leave a comment and let me know why.

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ETABS 2013 Disorder

CSI changed the default background color in ETABS 13 to white. If this really bothers you and you feel a compulsion to change it back, just use the following easy-to-remember keyboard sequence:

Alt+O,C,D

What do you think: coincidence or CSI humor?

Posted in Uncategorized | 18 Comments

2013 Steel Conference Recap

Last month I was privileged to attend the 2013 NASCC Steel Conference in St. Louis. This was my first steel conference and I can now agree with the sentiment of a friend: if you’re a structural engineer and can only attend one conference a year, make it this one. I look forward to being able to attend again next year.

AISC routinely shares more free information than you would expect, and the conference presentations are no exception – every session was recorded and is now available, absolutely free, on their website. Here are a few of the sessions I found most useful.

Practical Use of the Direct Analysis Method

You can’t spell his name or sport a better mustache, but you will want to use the Direct Analysis Method after listening to Lou Geschwindner. Key takeaways:

  • Chapter C of the 2010 AISC Specification contains the requirements; alternatives including the K-factor approach are now in an appendix
  • K = 1.0 for all members when using DA (Direct Analysis)
  • “Most of you are not calculating K correctly anyway” – Lou
  • A “rigorous” second-order analysis is required. Lou’s simple definition of rigorous: “superposition does not apply”

50 Tips for Designing Constructible and Economical Steel Buildings

Cliff Schwinger shared a wealth of knowledge that he has accumulated over his decades of experience. Highlights:

  • Use the deepest practical columns (e.g. W14s rather than W12s)
  • Size truss members to have adequate strength on the net section
  • Use R=3 “unless you have the seismic code memorized”
  • Talk with the fabricator and understand their preferences for connections

Load Paths! The Most Common Source of Engineering Errors

Carol Drucker has earned a reputation as one of the leading steel connection designers. She shared wisdom gained from observing the work of many engineers of record on countless projects. One key insight from her talk was: understand diaphragm forces from vertical trusses.

Structural Innovation: Combining Classic Theories with New Technologies

Bill Baker is smarter than you. And smarter than me, and smarter than everyone else who was in the room for the T.R. Higgins Award Lecture on the final day of the conference. Much of what he said went over my head, but the designer best known for the Burj Khalifa was enthusiastic and engaging as he applied Maxwell’s Theorem (which we all know, right?) to structural optimization problems, with fascinating results. It speaks to his charisma that I came away wanting to learn graphic statics, which probably hasn’t been taught at any university in 50 years.

Were you in St. Louis? Leave a comment and share your favorite session or design tip!

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ETABS 2013 Initial Reactions

The wait is over: ETABS 2013 was released last Tuesday, April 23rd. CSI was kind enough to upgrade my firm’s licenses that same day, so I’ve spent the better part of two weeks tinkering with the new version. Following more than half a decade after the release of ETABS 9, the new release has gobs of improvements and changes, meaning it will be months before I explore and understand all of the intricacies. What follows is a “first look” at the release along with general impressions.

Improvements

  • Better Graphics – Spinning a model in a 3D view is no longer painful.
  • Improved Solvers – ETABS 2013 implements the SAPfire solver used by SAP, leading to vastly improved run times. Using the advanced solver can reduce run times by 80% or more compared to ETABS 9, while the standard solver is at least twice as fast. One caveat: the “percent force and moment errors” given in the log file by ETABS 9 are apparently no longer available, even with the standard solver.
  • Rigorous P-Delta – The best ETABS 9 could do was an iterative P-Delta performed after the elastic solution. The results from this analysis could only match the AISC benchmarks if columns were split into at least two elements. In ETABS 2013, the P-Delta analysis is performed prior to running the elastic analysis, which more closely approximates a full “rigorous” approach. ETABS 2013 can match the AISC benchmark results without subdividing elements. Note that this is still not true “rigorous” analysis (where superposition does not apply), but is close enough for all but the most extreme theoretical cases.
  • Text File – The $et/e2k files that permit text-based model manipulation are in the same format as version 9. This is not an improvement per se, but the fact that they are still easily readable (unlike SAFE v12 text files) is a win.

Issues

  • Lateral Point Loads – After importing existing ETABS 9 models, I am consistently seeing numerical errors at points where lateral loads are applied to diaphragms. These nodes are not directly connected to model elements, but are used by the program to deliver, say, wind loads to the structure. ETABS 9 never batted an eye; ETABS 2013 returns an error message for every wind load at every floor. Even though this may be trivial by itself (no load is lost), it could obscure other ‘true’ errors in the long list.
  • Frame Subdivision – The very first model I ran in 2013 was the single column AISC benchmark (See figure C-C2.2 in the 2010 Specification, page 16.1-276). When I subdivided this column into two elements, it created an overlapping double element for the top half of the column. After I found this problem and deleted the redundant member, it recreated it when the analysis was run. This error is easy to miss and is not found by the “Check Model” command, and is thus very problematic.
  • 64-bit vs. 32-bit – I initially installed the 64-bit version, thinking that bigger is better. Not so. The 64-bit version does not play nicely with Microsoft Office 32-bit, meaning that exporting tables to Excel or Access is not possible. Because Microsoft discourages using 64-bit Office (due to limited compatibility with add-ins), I installed the 32-bit version of ETABS. CSI tells me and I have confirmed that analysis times do not suffer; the sole advantage of 64-bit is that the 2GB memory limitation is removed. Any model larger than this ought to be simplified anyway.

Summary

ETABS 2013 represents a major step forward for design software and will be my default software – in a few months. My initial recommendation is to not use it for production; let the chumps like me sort out the bugs. As they say with Windows: wait for the first service pack.

So, what do you like or dislike in the new version? Leave a comment and let me know!

Posted in ETABS | 35 Comments

Rigid Thinking

Quick tip: when applying temperature loads to a diaphragm (for example, in a parking structure), make sure the diaphragm is set to semirigid, not rigid. I definitely did not just make this mistake in my model and I don’t know why you’re asking. But if I did I fixed it immediately.

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ETABS Keyboard Shortcuts

My wife and I had a standing disagreement for the first year after moving into our house: she thought it was faster to turn at the stoplight; I thought it was quicker to get there the back way. It turns out we were both wrong – a middle route is nearly always the fastest way to reach our house from the west. Though I was proven wrong, I was still happy to have found a shortcut.

Why Shortcuts?

Shortcuts on the keyboard can dramatically improve your productivity when using software. Knowing the shortcut keys for common commands reduces the ‘friction’ of having to select a certain button among dozens or digging through menus. Saving a few seconds a hundred times a day adds up, both in time saved and frustration avoided.

Shortcuts in ETABS

Out of the box, ETABS does not have many shortcuts, but thankfully it does allow customization. To add your own, click the drop-down arrow at the right-end of any of the toolbars and select “Customize”:

CustomizeShortcutsStep1

Hit “Keyboard” at the bottom. You’re now in the “Customize Keyboard” dialog, where you can specify new shortcuts for any commands (including some that are not active and hint at features that never made it to prime time, e.g. Steel Beam Connection Design). Note that the ‘Shift’ key cannot be used alone nor can the ‘Escape’ key (which should be used for clearing the selection, but isn’t).

CustomizeShortcutsStep2

Shortcut to Shortcuts

You were hoping for this, I know: you don’t have to create your own. At the link below, you can download a .tb2 file containing all of my custom shortcuts. Here’s a partial list:

  • F2 – Rubber Band Zoom (Zoom Window)
  • F3 – Restore Full View (Zoom All)
  • F5 – Run Analysis (default shortcut)
  • Insert – Set Default 3D View
  • Home – Set Plan View
  • End – Set Elevation View
  • Page Up – Move Up in List (moves one story up in plan)
  • Page Down – Move Down in List (moves one story down in plan)
  • Ctrl+E – Set Building View Options
  • Ctrl+Shift+W – Show Selection Only
  • Ctrl+Shift+Q – Show All

All shortcut keys are displayed in the tooltip (displayed when hovering over a button) and on the menus. To install the shortcuts, close ETABS and then paste the .tb2 file here:

C:\Users\%username%\AppData\Local\VirtualStore\Windows

(If UAC is disabled or if you’re still inexplicably using XP, paste it here: C:\Windows)

Download Custom ETABS Shortcuts

Have any favorites I’ve left out? Let me know in the comments.

Posted in ETABS, software | 6 Comments