visibility scale

[jim]

hi, just a general question when considering variable C values for visibility factor, ie. 3 for reflective signage and 8 for illuminated signage. 

is it considered best practice to re-scale results in smokeview dependent on the value used, for example:

For a light reflecting sign C= 3.0 and measured using a scale 0-10m. (default C term)

For a light illuminating sign C= 8.0 and measured using a scale 0-30m.

thanks 

[Sean McCready]

Jim,

I think I may understand your question - you're asking which is the more appropriate C value to use in most cases.  If so, that's a very interesting and underasked question!  It's been my experience you should attempt to find some literature on acceptable C values to apply depending on some circumstances of the space.  However this information doesn't seem so readily available and best practices are not well documented in this area.  Thus, the engineer must attempt to justify the selected C value and include the reasoning in the documentation and analysis.  

The engineer should take into consideration many different pieces of information.  One should consider what is the hazard at play - are the occupants unfamiliar with the space and at risk of a falling hazard, say over a railing, or down unfamiliar and perhaps existing non-compliant stairs?  Is the building only occupied in the daytime and plenty of natural light is available surrounding the exits?  In cases where I feel that improper nagivation can result in an injury, I'm much more likely to use a C value of 3 or lower, even if lit exit signs are in use.  If the space is a higher education facility,  where the occupants are familiar, you can with confidence trust that they will know how to exit, and understand the options to exit.  In that case, the engineer may consider that visualizing exit signage at C=8 will be an appropriate metric, and I would normally take that data at the level of exit signage, rather than the typical 6 ft AFF.  If the exits are surrounded by glazing, and the structure is only open to the public during the daylight hours, you might take a similar approach and evaluate at C=8.

I'm very interested in this particular question so I hope that someone disagrees with me!

If I've misunderstood your question, please let me know.

Best,

Sean

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[jim]

hi, thanks, but my main point was more related to the scales. so is it appropriate to use the same scale, say 0-30m scale to present data from any C value?

[Sean McCready]

I don't know your specific application so I can't comment.  I'm not sure what for what reason you are evaluating.  If your goal is tenability, then you will likely want a scale that can accurately show your danger and critical points of tenability.

[dr_jfloyd]

I agree with Sean. What poor visibility is has a dependence on the occupants and the space. I think a requirement should be established based upon that and then pick C and scale the Smokeview presentation so that a reviewer can clearly identify if success is met.

For example if we happened to be concerned with can a person find there way out of a 5 m x 5 m x 3 m hotel room into the corridor, then I'd argue visibility could very low.  You don't need to see 10 if the mean beam length is ~2 m.  You just need to be able to orient yourself to the general direction of the door.

If instead we are concerned with a large department store where the occupant may not be familiar with the space and it is also cluttered with displays, then I'd argue you would need decent visibility to be able to see distant  lit exit signs. 

On Wednesday, October 11, 2017 at 12:30:20 PM UTC-4, jim wrote:

[fde]

Not an answer to question but it made me think about VISIBILITY output. What does SMV quantifies when we check VISIBILITY slicefile? Is it for C=3 or C=8? Also would the results change when we change MAXIMIM_VISIBILITY? Where does 30m default value come from? 

Thank you. 

[dr_jfloyd]

User's Guide 17.10.2 Visibility and Obscuration

The third parameter, VISIBILITY_FACTOR on the MISC line, is the constant C in Eq. (17.6). It is 3 by default.

MAXIMUM_VISIBILITY is converted to a soot mass density using Eq. 17.5 and 17.6,  Then when evaluating 17.6 for the VISIBILITY output, FDS uses the maximum of the MAXIMUM_VISIBILITY equivalent density or the predicted soot density. We can't divide by zero so we need to ensure that the denominator of 17.6 is greater than zero. 30 m was picked to be large enough for a user with VISIBILITY slice files to see smoke moving about a building early in a fire but not so large that the plot limits picked by Smokeview would mean not being able to discern values typically used as performance criteria.

[Cian Davis]

Wojciech Węgrzyński from the Building Research Institute in Poland has done a huge amount of work on how visibility is calculated. He gave an excellent presentation at the Fire and Smoke Modelling Forum in London in November. The presentation and video is available at http://www.fsmf.uk/archive.php

Regards,

Cian

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