Washington State Snow Loads
Washington State Snow Loads
(OP)
thread1066-166368: WA Map Showing Snow Load Zones
If you do a little digging about you will find that every county and municipality has their own little formula for determining snow loads in the State of Washington. Some states are much more organized in this respect and have a statewide system (ie. Oregon) or a simple equation with a limited number of exceptions (ie. Utah).
Some counties in the state defer to the 1995 snow load study or require separate case studies for each building site. A few jurisdictions now have online mapping tools that automate this process.
Others utilized an across the board snow load or ask that you call if elevations are "high".
The ASCE 7-10 map for Washington is virtually useless for much of the state.
In an effort to consolidate all of this data I've finally taken a couple of days and put together a map here:
http://design.medeek.com/resources/snow/washington...
I am still vetting the results of the interpolation algorithm against the previous snow load study and looking for anomalies or locations where the interpolation algorithm might break down.
It is interesting to three-way compare the results of the map with the SEAW snow load study and the AHJ (county or city). In some cases there is very good agreement and in others its a bit more chaotic.
If you do a little digging about you will find that every county and municipality has their own little formula for determining snow loads in the State of Washington. Some states are much more organized in this respect and have a statewide system (ie. Oregon) or a simple equation with a limited number of exceptions (ie. Utah).
Some counties in the state defer to the 1995 snow load study or require separate case studies for each building site. A few jurisdictions now have online mapping tools that automate this process.
Others utilized an across the board snow load or ask that you call if elevations are "high".
The ASCE 7-10 map for Washington is virtually useless for much of the state.
In an effort to consolidate all of this data I've finally taken a couple of days and put together a map here:
http://design.medeek.com/resources/snow/washington...
I am still vetting the results of the interpolation algorithm against the previous snow load study and looking for anomalies or locations where the interpolation algorithm might break down.
It is interesting to three-way compare the results of the map with the SEAW snow load study and the AHJ (county or city). In some cases there is very good agreement and in others its a bit more chaotic.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com





RE: Washington State Snow Loads
Mike McCann, PE, SE (WA)
RE: Washington State Snow Loads
RE: Washington State Snow Loads
If you mind me asking what snow load did you use at that location and how did you arrive at it?
I've actually only used the 1995 Snow Load Study once with its associated isolines for a project, all other instances have been simply using building dept. assigned values. I was actually wanting to incorporate the isolines into my online map but after speaking with the original authors of the snow load study (SEAW) they were concerned with the accuracy of a map that relied upon auto-interpolation of the isoline values.
The method I chose to use is my own algorithm based loosely on the method used by Montana State University in their "Montana Snow Load Finder". My original idea was to simply use the closest data points to the point of interest and then to inverse distance weight them, however the DeLauney triangle method used by MSW and also used in my Arizona map made sense so I employed it. The idea is to normalize the data with respect to elevation and then generate a smooth 3D surface using interpolation between data points. The NGSL map overlay in Google Earth is shown below:
This also shows the location of all the NRCS and NWS stations. Note that this is not the ground snow load but the "normalized" ground snow load. As you can see there is some scatter with regards to some stations so the expected nice curves (isolines) don't just jump out as one would hope. Also note that I followed the method of the 1995 SEAW Snow Load Study where the NGSL values are generated using a min. 300 ft. elev. for points west of the Cascades and a 1000 ft. elev. for points east of the Cascades, otherwise some of the NGSL values at low elevations are too "high". This image was generated with sample points on a 0.1 x 0.1 degree grid, approximately every 5 miles.
The problem with the data is that for certain sections of the state there is a lack of data points and then for others they are tightly grouped (ie. Snoqualmie Pass near I90). When they are that tightly grouped and some of the NGSL values differ significantly it makes one wonder if some of these point are either in error and should be discarded or there is really something going on at this location (micro-climate) with respect to the abrupt changes in terrain or lake effects.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Washington State Snow Loads
http://design.medeek.com/resources/snow/images/WAS...
If you look at this image closely you will notice a few instances where two stations are very close together and yet their NGSL values are different, this is probably due to the fact that one is a NRCS station (in. of water) and the other is a NWS station (in. of snow). My conversion method for the NWS stations uses the Rocky Mountain Conversion Density (RMCD) developed by Ronald Sack in his 1986 Idaho Snow Load Study. This is not an exact science so different data from different types of stations with a conversion factor involved all play into this, the question is how to deal with it?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Washington State Snow Loads
http://design.medeek.com/resources/snow/washington...
Perhaps the NGSL should be generated by some power of the elevation which might smooth things out better (ie. NGSL = Pg / sqrt(elevation)).
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Washington State Snow Loads
None of my projects have been that high yet. What can I say?
Mike McCann, PE, SE (WA)
RE: Washington State Snow Loads
A min. elevation of 300 ft. and 1,000 ft. is used in the determination of normalized ground snow loads west and east of the Cascades respectively. Additionally, the NGSL is restricted to a min. of 0.050 psf/ft and 0.0150 psf/ft west and east of the Cascades respectively and a max. of 0.150 psf/ft statewide, regardless of elevation.
This effectively limits the min. calculated snow load to 15 psf for the entire state. Some locations at or near sea level and in the interior were giving values for the ground snow load at about 10 psf.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Washington State Snow Loads
I exaggerated a little giving Mike a hard time about the 400 psf. The ground snow load was actually 250 psf and Kittitas County provided it on a design spec sheet they gave to the client in a pre-permit meeting. They have been very good about giving out the snow load if you give them the address or parcel number.
RE: Washington State Snow Loads
I've actually never done a Log Pearson III analysis before so I'm wondering if I've got it right.
If someone has a chance please take a look at Station 832, I placed the Excel file as well as a PDF printout of my 50 yr. MRI calculations:
http://design.medeek.com/resources/snow/MEDEEK_SNO...
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Washington State Snow Loads
http://design.medeek.com/resources/snow/washington...
In this particular case Snotel Station 974 has data for 16 years, clicking on the Station ID will give all the pertinent information.
Unfortunately, I don't seem to be able to extract any new data for any of the older snow course stations from the NRCS website.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Washington State Snow Loads
This area has always fascinated me with its crazy snow loads. I worked out the algorithm for Nevada County and Truckee about 6 months ago and added it to my API but I finally got around to putting together a map for it this afternoon and evening:
http://design.medeek.com/resources/snow/nevadacoun...
The transition zone between the west and east sides of the town and county is probably what makes this one the most interesting out of any snow load zone I've looked at thus far.
How does one even design residential roof and walls for snow loads that come in at 300 psf? Even the footings at your stem walls would have to be massive given a bearing pressure of 1,500 psf.
I would love to look at the plan set and structural report for a typical one story residence or garage in this neck of the woods.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Washington State Snow Loads
RE: Washington State Snow Loads
Where most of the high snow loads occur, any structure would likely be founded on rock. As alluded to, adding up to 100 psf for a 400 psf snow load to the seismic calculations in a high seismic zone would be interesting to say the least.
Mike McCann, PE, SE (WA)