Choosing The Correct Seismic Roof Curb – Understanding The Specifications

Matt Emberson At MicroMetlA previous MicroMetl Blog defined Seismic Curbs. Now that we understand the definition of seismic roof curbs, how are they applied in the real world?

It is the responsibility of the design team to ensure a building meets the safety, function, and economical goals set forth by local codes and building owners. Within your building’s plans, the appropriate code requirements for the job are specified. Curb requirements for the mechanical equipment may be specified within the mechanical schedule detail, and/or within a written specification.

Being awarded a project with improper equipment can be costly!

I discovered a tool that can ensure your curbs meet all job site requirements. However, as a prerequisite you must acquire the following information…

The Design Code;

  • 2013 ASCE 41
  • 2006/09, or 2012 IBC
  • 2010 ASCE7 (With July 2013 Erratta)
  • 2005 ASCE 7
  • 2003, or 2009 NEHRP
  • 2009 ASSHTO

International Building Code: 2012, 2009, and 2006
ASCE/SEI 7 Standard: 2010 and 2005
NEHRP Recommended Seismic Provisions: 2009
09 AASHTO Guide Specifications for LRFD Seismic Bridge Design

Site Soil Classification;

  • Site Class A – “Hard Rock”
  • Site Class B – “Rock”
  • Site Class C – “Very Dense Soil And Soft Rock”
  • Site Class D – “Still Soil” – The Default
  • Site Class E – “Soft Clay Soil”

Consult your building code reference document for this information. Each document provides guidance on how to use shear wave velocity, field standard penetration resistance, or shear strength data to classify the site soil. The U.S. Seismic Design Maps tool does not accommodate Site Class F!

The Risk Category – The Seismic Design Category;

  • Risk Category I – Low Hazard
  • Risk Category II – Other
  • Risk Category III – Substantial Hazard
  • Risk Category IV – Essential

Follow this link to further research “Seismic Design Categories”.

Site Location – (This classification has replaced the old seismic zone map);

  • Latitude & Longitude – Visual Map Available!

With the gathered information, go to the U.S. Geological Survey Website, (UGSS) input your data and compute the values. A visual aide map is available allowing you to pinpoint the site’s precise location so a numerical latitude and longitude is not necessary. The resulting report provides the seismic design for your specific criteria.

SS Tag

The goal is to match or exceed the seismic roof curb design value referred to as the “Ss” value. “Ss” is the mapped Maximum Credible Earthquake, five percent dampened, spectral response acceleration parameter at short periods as defined in ASCE 7-10 Section 11.4.1.

The conclusive value will be between 0 and 3.73 (highest value in California.) Compare this value located on your structurally stamped certified submittal. The “Ss” value should be greater than, or equal to the requirement for your area. If not, be certain to contact the curb manufacturer to ensure you’re submitting the correct product for the job.

Other necessary data is the “Ip,” value known as the importance factor. This information isn’t found in the UGSS  report, but is included in all MicroMetl seismic rated curb submittals. It represents the life-safety importance factor of the Equipment or Component as defined in ASCE 7 Section 13.1.3.

The final detail for consideration are the attachment requirements. The attachment detail Anchoring The Curb At MicroMetl

provided with your certified submittal should be followed when installing the product. If not, the structure may fail due to insufficient strength and stiffness between the structure, curb component, and between the curb component & mechanical equipment. When all factors are carefully considered, you can rest assured you are providing quality equipment and components that will withstand the prescribed seismic event.

Wind Calculations – The Added Bonus!

Perhaps you noticed the MicroMetl seismically certified submittal includes wind design criteria in addition to the seismic design criteria. This means you can use the same curb for inland hurricane prone areas where the wind specifications are met.

For example, downtown Houston, Texas requires a curb rated for 145MPH, 3-second gust wind speed, according to ASCE 7-10 wind chart. The structurally calculated MicroMetl curb is rated for up to 155MPH, 3-second gust wind speed, risk category IV, exposure C, for a building up to 60 feet tall.

A site offered by Applied Technology Council (ATC) allows you to retrieve wind speeds by location, thereby matching the wind speed requirement for your next job when building to an ASCE 7 code. As an example – At La Marque, Texas, your wind calculation for a building with a risk category III-IV would need a curb rated for 157 MPH. At this speed, you would require a curb specifically designed for hurricane winds.

As noted with the seismic stamped curbs, the hurricane rated curb calculation is based upon the assumption you will properly anchor the curb to the building’s roof, and the curb to the mechanical equipment in accordance to the submittal specifications. If the specified method is not followed, the likelihood of failure increases during a weather event.

If you have any questions regarding curb selections, feel free to contact your MicroMetl sales representative or customer service department.