The switch to a 700-year ultimate wind speed rather than a 50-year service level wind speed as the basis of design has left many jurisdictions and engineers perplexed. There is no significant change in design wind pressures when using the ASCE 7-10 load factor of 1.0 on a 700-year wind versus using the ASCE 7-05 load factor of 1.6 on a 50-year wind, but many jurisdictions have inadvertently reduced their design wind pressures by almost 40% by listing 50-year wind speeds as ultimate wind speeds in their code amendments to the 2012 and 2015 IBC. Please help eliminate this confusion by directing these jurisdictions to the 2013 Colorado Front Range Gust Map, endorsed by SEAC in 2014 and available for free download from the SEAC website.
SEAC also recently endorsed the 2016 Colorado Design Snow Loads Report produced by the SEAC Snow Load Committee. This report recommends the use of risk-based ground snow loads rather than hazard-based ground snow loads for design in order to achieve targeted reliability levels. Consequently, recommended ground snow loads have slightly decreased in the mountains and have increased in the plains. The SEAC Snow Load Committee has been working with the Colorado Chapter of the ICC to get the word out, but could use the help of individual SEAC members as well. This report is also available for free download from the SEAC website.
Both reports have accompanying .kmz files for use with Google Earth.
Many, if not all of us, have designed buildings or other structures in Colorado for the earthquake requirements of the building code, now the IBC. We have some moderate to high areas of seismicity in Colorado per the USGS hazard maps. Much of the eastern part of the state is considered by the USGS to be a low seismic region and the building code allows Seismic Design Category A (SDC A). But along the Front Range, where 80% of the population of the state reside, there is a transition to higher Seismic Design Categories. Along the Front Range for Site Class C soil conditions there is often a dramatic change from SDC A (1% g) to SDC B thereby requiring the actual mapped spectral accelerations to be used along with R and I factors.
So let’s hear your thoughts on the following questions:
What do you do if you are designing a police station, fire station, hospital, emergency operations center or a school along the Front Range and the spectral acceleration values, Sds and Sd1, have been determined based on a Site Class C site such that the code would allow you to design for SDC A?
What if the differences in the values of Sds or Sd1 from those that would require SDC B were at the 2nd or 3rd decimal point? Do you think that the certainty in the hazard maps is high enough to justify such precision in the application of this 1%g code provision?
Would it make any difference to you if the SDC A 1% lateral loads exceeded wind and governed the lateral design? Since earthquake is governing even at 1%g would you then feel any obligation to design the building for the actual Sds and Sd1 from the maps rather than using 1%?
Do you have any concerns over the occurrence of induced earthquakes in areas where naturally occurring seismicity has been low?
If you are very interested in seismic design, resiliency, building codes and improvements to all these aspects of structural engineering practice you might want to join the Seismic Committee of SEAC. Contact Rob Jackson at firstname.lastname@example.org.
SEAC is a Colorado organization. Here are some interesting links pertaining to the Colorado earthquake hazard:
Welcome to July. I hope everyone had a safe and fun holiday weekend. I am happy to announce that the launch of the new SEAC website is imminent. One key function of the new website will be an area for SEAC members to submit forum posts, as well as to submit responses. The idea is to create an opportunity for members to openly share their thoughts and experiences. Your board members are currently drafting the first posts so that they will be ready to submit as soon as the website is available. Please take time to read them, and to submit your responses.
Many of you may have noticed that David Poe has graciously volunteered to manage the audiovisual and WebEx setups for SEAC meetings for quite some time now. David has done an excellent job, but SEAC would like to give him a well-deserved break. So, please take note of the solicitation in this month’s newsletter. We are seeking technical support with the setup and take down of the audio-visual and WebEx systems during our general meetings and Fall seminars. The position does not need to be filled by a SEAC member, and it is a for-pay position. Please spread the word if you know someone who might be a good fit and want to earn a little pocket money.
In other news, many of our committees are hard at work. The precast concrete committee has prepared a white paper and this month’s presentation, the steel committee is developing a white paper and presentation for a future SEAC general meeting, the young members group is becoming more active every month (make sure to participate in the gingerbread bridge competition if you think your team can win, or if you just want to have some fun), the education committee is very active planning this year’s Fall seminar as well as other frequent activities, Peter Marxhausen has breathed new life into the ethics committee, The northern Colorado committee continues to grow thanks to our many colleagues from that part of our awesome state, and the business management committee is conducting the annual SEAC Business Practice Survey. Please consider volunteering for a committee if you do not already do so. I believe our members and our committees are SEAC’s lifeblood.
Finally, if you’re a cycling fan, don’t forget to catch a stage or two of the 2016 Tour de France this month.