Site response spectrum 1

[WARose]

I have testing service that is coming up with a site specific study for me. I was forced into this by ASCE 7 because of the poor site soils.

Today I swung by their office and looked at some numbers/preliminary results. One thing that jumped out at me is their (plotted) site response spectrum. (Based on background sources if I understood them correctly.) It appeared to me to have several peaks that could result in amplification corresponding to about 2 to 3 different frequencies.

For those of you who have had these types of studies done: is this normal? (I don't have anything to post because they are still working up the results.) Seems like I've seen something similar previously but I wanted to ask here to see what others think. Thanks.

 

[Wallache]

On my recent project that required a site specific survey, the survey data had peaks like yours. However, the engineer gave a design spectrum to use that was similar to what we are used to seeing. See attached screenshot.

 

[Agent666]

The peaks probably correspond to the natural frequencies of the soil strata at the site. I believe it is quite normal. Geotech engineer can probably offer some better explanation.

 

[Shu Jiang PEng SE]

As the drawing of wallache shows, there is a “general“ design response spectrum and a design response spectrum derived per site-specific ground motion procedure for spectrum analysis. Per section 11.4.7 of ASCE 7, a site-specific ground motion procedure can be used for any structure. It shall be performed for structure on site class F. “general” design spectrum can be used for structure analysis in most cases.

Site response spectrum is derived per site ground motion recorded before.

 

[WARose]

I really appreciate it Wallache. Do you know what they used as excitation forces at the source for your plot?

 

[Wallache]

The report says sledge hammer for close receiver spacings and bulldozer for further receiver spacings.

 

[WARose]

Thanks again Wallache.

 

[WARose]

Wallache, one last question: did they give you the forcing frequencies for either sources? For the dozer, I would think they couldn't filter it out because it is likely higher than the site's frequencies.....but for some of the hammers they use.....the impacted end sometimes has sensors.

 

[Wallache]

No, nothing like that was provided.

 

[WARose]

Ok.