Holdfast Training Services Trial Blog

Hopefully this blog won’t be as boring as the title suggests?….

Even though it was a very long time ago…..  I remember being at university and learning about permissible stress design (it obviously wasn’t a Thursday morning).  At the end of the lecture a wise old professor informed us all that we’d never actually design anything this way, that it was pretty much obsolete because everything was moving well and truly towards the future…. and the limit state approach.  Indeed, when I worked in industry prior to joining the military I found this prediction to be true, and the idea was reinforced during Phase 1!

Imagine my surprise when I arrived in the US and found that, rather than being a bastion of enterprise and advancement, the US actually embraces and encourages the two design philosophies to be used in parallel!  Over here they are known as ‘Allowable Stress Design’ (permissible stress), and ‘Load Resistance Factor Design’ (limit state).  When I say ‘encourages’, this isn’t quite fair.  What I mean is that the American Institute for Steel Construction (AISC) publishes both methods side by side in its design manuals, allowing designers to pick which method they prefer.  Interestingly, concrete did make the jump years ago, and no longer practices ASD.

So I’m currently designing a glorified giant dog kennel and training facility for some fierce secret service working dogs.  I’m working to ASD for the steel and masonry, which is nice because it makes things simpler and I don’t have to worry about any complicated plastic analysis or behavior etc…. it’s also handy because all of the joists come from the manufacturer designed and specified using ASD.  Nice and simple.  Until it comes to designing the footings, which of course being concrete mean that I had to alter my loads using a crude conversion factor of 1.4 to ‘upgrade’ them from ASD service loads to LRFD design loads!  But even then the fun isn’t over; the soil bearing capacity (4000psf John, very stiff!) isn’t factored, so I need to multiply this by an additional ‘resistance factor’ so I can work in LRFD, making sure I don’t accidentally divide by the ASD global ‘safety factor’ instead, because that would be a disaster!  Who needs a sinking building and (expensive) squashed mutts!?

My Design:  Permissible Stress or Limit State?  Why choose, have both!

Apparently things are improving!  Until recently live load and dead load factor of safeties for steel and concrete construction using LRFD where different!  1.6 & 1.2 for steel and 1.7 & 1.4 for concrete; imagine having to swap between the two sets whilst working with the same philosophy on the same design!  You all thought Euro-codes were frustrating, you’ve got it easy!  Also, (and back to the point) the AISC are apparently considering removing ASD from their manuals which will be a step in the right direction!  However I’m not holding my breath; it takes a long time to buy a stamp over here, imagine how long it might take to alter something as ingrained and controversial as this.

This all seems like madness and an accident waiting to happen… and it nearly it!  During my time on the JOC project over at East Campus it was discovered that a series of giant steel trusses had been sub-contracted out to a ‘specialist’ by the designer.  When they were finally delivered to site it was discovered that they, together with all the connections had been designed using ASD rather than LRFD loads.  On checking the truss turned out to be fine (phew, almost a very expensive error for someone), but all the connections were under-designed by approximately 40%!  These trusses were very nearly installed, and it was only scrutiny of the drawings at the beginning of the week of installation that prevented fundamentally unsafe construction!  Whilst it is considered bad practice to mix design philosophies, in my experience it seems to happen an awful lot.  Particularly on simpler, small projects that need to be turned out quickly, and by older-generation engineers who grew up with ASD and see no reason to change.

I was wondering if anyone else had had any experience of permissible stress design during their attachments?  Surely if there’s a slow backwards nation that can stand with America on this one it’s Australia?!  Or is it really only America that lacks the will to embrace change?

BLUF: Can any of you geeks point me towards any discussions or case studies on the effect that fat, oil and grease (FOG) has on (NRVs?).  I’m guessing it’s not going to be good but I need to know how bad it could be, maintenance periods, dosing options.

Background:

I’m working on the adaptation of the drainage for an existing department store in London.  Over the years the store has developed a maze of drainage, both foul and rainwater, no-one really knows how it works and there is a history of FOG build ups causing blockages, and being London concern regarding rat ingress.

There are around 20 proposals to fix all the issues.  Two of these are flap type NRVs to prevent rat ingress, and grease traps with a wider grease management process.  However, there are areas where in the short term (2-10years) we will not be putting in grease traps for commercial reasons and so there is concern over whether fitting the NRVs to this section would make grease problems worse. The store doesn’t have a great record of maintaining the system so  “just check regularly to prevent blockage” probably won’t be successful.

Optimistically I’m hoping that the pressure required to operate the valve may be enough to clear any FOG build up but I guess this would depend on what volume and the frequency of flow, and what type of valve we specify.