Asce 7-05 Seismic Pdf
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Section 12.4.3.3 of ASCE 7-05 (or -10) deals with overstrength (Ωo) load combinations and allows a 1.2 increase in allowable stress when using these combinations. We received a question from a customer last week asking if the 20% increase applies to Simpson Strong-Tie connectors. The simple answer is yes. When demand loads are based on amplified seismic forces, connector allowable loads may be increased by 1.2 per Section 12.4.3.3.
Since the increase may be combined with the duration of load increases permitted in the NDS, you would apply the 1.2 increase to connector allowable loads at a load duration of 1.6, which makes the overstrength factor a little less terrible.
ASCE 7 and other model building codes acknowledge that structures will be loaded beyond their elastic range during seismic events. Damping and ductile yielding make it unnecessary to design for the full inelastic design force, so the code divides the seismic response by the R-factor to get a lower elastic design force or base shear. Higher R-factors represent more ductile systems and, therefore, yield a lower seismic design force. Deflections are multiplied by the Deflection Amplification Factor, Cd, to obtain the expected inelastic deflections. Similarly, the System Overstrength Factor, Ωo, is an amplification factor that is applied to the elastic design forces to estimate the maximum expected force that will develop.
ASCE 7 Section 12.3.3 addresses limitations and additional design requirements for structural systems with irregularities. Tables 12.3-1 and 12.3-2 define horizontal and vertical structural irregularities and reference the code requirements applicable to each type. In some cases, the irregularities are simply prohibited for high seismic areas.
Many of the irregularities are allowed, albeit with additional design requirements that make use of the load combinations with the overstrength factor. The purpose of applying the overstrength load combinations to irregularities is to prevent non-ductile failures in the structural system.
Designing for amplified forces can be a real challenge, but the alternative would be the building code not allowing structural irregularities at all, which would not be realistic. I have always thought of the overstrength factor, Ωo, as being a sensible compromise.
The code has become to complex. Most buildings in USA can have a simple design load, and the penalty would be 5% for the structure, and less than .005% for the cost of the building. What is wrong wiih haveing extra reserve strength in buildings. The final cost is not much. The way real building are designed is with simple loads simple concepts. Who knows how a computer designs a building. We ignor parts of the Code We do not know what the loads are. WE know how to design so it does not fall. That is great . Spend the extra 0.005%
There is a change to the provisions of ASCE 7-05 noted in the blog post on the overstrength factor that are now found in ASCE 7-10. In Sec. 12.4.3.3, the NDS load duration factor is expanded to all NDS adjustment factors. Also, there are also requirements In Sec. 12.2.5.2 and 12.10.2.1 of ASCE 7-05 and 7-10, requiring use of the overstrength factor.
I have also had trouble determining when to apply O.S. in my residential light framed wood structures, or any structure for that matter. Are you saying, as C Jones summarized, that they are only applicable when we have structural irregularities, in order to compensate for them? That seems too simplified. They are clearly required in Sec. 12.2.5.2 and 12.10.2.1.
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In this equation, δ is the maximum elastic displacement that occurs anywhere in a floor or roof from the application of the design base shear to the structure. The Deflection Amplification Factor, C, is obtained from ASCE 7-05 Table 12.2-1 and the Importance Factor for seismic loading, I, is obtained from ASCE 7-05 Table 11.5-1. The maximum elastic displacement, δ, includes the effects of translation as well as rotation due to torsion (inherent and accidental), but not the amplification of the accidental torsion. It must be noted that δ is different from the elastic displacement, δ, which is determined at the center of mass of floor x or the roof and is used in ASCE 7-05 Equation 12.8-15 to compute inelastic deflection.
Building occupancy
It also is worth noting that the maximum inelastic displacement is computed independently of the occupancy of the building, as the expression for the quantity includes the Importance Factor, I, which depends on the occupancy, in the denominator. The importance factor gets included in the maximum elastic displacement, δ, because it is computed under the design base shear, V, the expressions for which include the Importance Factor, I, in the numerator. Thus, when the Cδ is divided by I, the effect of building occupancy is eliminated.
S.K. Ghosh Associates Inc., is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. Presidents S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skg...@aol.com and susan...@gmail.com, respectively, or at www.skghoshassociates.com.
With the Medeek API, you can develop roofing, building, solar panel or other construction related applications that rely on accurate ground snow load, wind load or seismic data. Please feel free to download and test the following sample applications listed below:
- ASCE 7-10 Wind Lookup Tool (Microsoft Excel Spreadsheet with VBA)
This document is intended for website and mobile developers who want to use ASCE ground snow load data, ASCE wind load data, IRC 2012 seismic data or IRC wind load data within their own web based or mobile applications. It provides an introduction to using the API and reference material on the available parameters.
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- API Key required for all requests.
- 250 requests per 24 hour period.
- 1 location per request.
If you need a higher usage limit please contact us in order to make special arrangements.
Tip: You can also access the Medeek API using SSL, by substituting https for http in the request URL. HTTPS is recommended for applications that include sensitive user data, such as a user's location, in requests.
Outputs formats are specified using the optional output parameter in the request URL. The Medeek API currently supports the following output formats:
- output=json returns results in JavaScript Object Notation (JSON).
- output=xml returns results in XML, wrapped within a node.
- output=text returns results in plain text form. If output parameter is omitted this is the default output format.
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A status code, which may be one of the following:
- OK indicating the API request was successful.
- INVALID_REQUEST indicating the API request was malformed or the action parameter was not properly specified.
- OVER_QUERY_LIMIT indicating the requestor has exceeded quota for that 24 hour period.
- ACCESS_DENIED indicating the API did not complete the request, likely because the requestor failed to include a valid API key.
- UNKNOWN_ERROR indicating an unknown error.
An array of results containing the following applicable elements:
- A pg element indicating the ground snow load for that location.
- An sdc element indicating the seismic design category for that location.
- An sds element indicating the design spectral response acceleration for that location.
- A windasd element indicating the IRC 2012 basic wind speed for that location.
- A windi element indicating the ASCE 7-10 basic wind speed (Risk Category I) for that location.
- A windii element indicating the ASCE 7-10 basic wind speed (Risk Category II) for that location.
- A windiii element indicating the ASCE 7-10 basic wind speed (Risk Category III) for that location.
- A wind05 element indicating the ASCE 7-05 basic wind speed for that location.
- An elevation element indicating the elevation of the location in feet (as determined by Google Elevation API) if the elevation parameter is enabled, otherwise this element will report "N/A".
- A keyexp element indicating the expiration data (mm/yyyy) of your API key.
- A requests element indicating the number or requests that are still available for that 24 hour period.