Wednesday, 23 November 2016

Top 3 Learnings for Solar Mounting Structures Design

AGAMI has been fortunate to have worked in a broad range of areas of module mounting solutions for Solar Utility Projects. We have done designs of mounting systems as well as peer reviews. AGAMI might be one of the few , if not the only consultant in India currently, to have done a full-fledged structural design for a Tracker Structure. This provided very deep insights into wind tunnel studies, dynamic failure modes of structures in wind and quantification of complex wind effects.

As this is a nascent field, and the engineering consulting industry is taking its first steps, there are many critical errors in the design of these seemingly simple structures. The below is a round-up of common errors that consultants/EPC design teams need to watch out for, which can save your project in extreme wind events.

Learning 1: Forgetting the “NOTE 1” in Wind Load Application from IS 875:3

IS 875:3 Table 11: Monoslope roof coefficients, which is the wind table referred for all solar MMS design, has hidden the most important criteria which has the ability to govern design of your members as a footnote, which reads thus -
..the center of pressure should be taken to act at 0.3*w from the windward edge.  
In Layman terms, what this means is, the wind pressure is not symmetric but is BIASED towards the windward end. This is to be expected , for why would something as complex as wind, be completely uniform across your panel?

Applying this criteria results in lot of bending moments or torsions in most systems, and has a significant impact. This has been caught time and again in our peer reviews with large EPCs, as also with reviewing designs of other consultants, and is indeed a “rookie” mistake many commit.

Learning 2: Providing Specs to the Geotechnical Agencies

The geotechnical agencies investigating the parcel are often given no idea of what kind of structures would be coming , and what their output needs to be. The report often comes up with recommendations that are more aligned towards normal Building structures.

For e.g. the recommended “type” of foundation and example calculation in the report would be of a PAD FOOTING, at 3m depth below ground. Whereas a pad footing is never used in Solar MMS.
This creates various problems, say if the first 2 metres of soil were problematic, the geotechnical consultant solved the problem by saying give pad footings at 3m, visualizing a building coming up in the parcels. Other times, the report would have no testing data about first 1-1.5 m of soil.This results in bad foundation design, based on interpolations and speculations where none is needed.

Hence, detailed specifications are necessary to prime a Geotechnical agency into what needs to be done w.r.t. Solar Projects. The geotechnical specs should provide info about the type of structures, and request for calculation of capacities of a bored pile 1.5 or 2m deep.

Learning No 3: Not Neglecting Dynamic Effects for low tonnage designs

It is seen that many large EPCs are doing some interesting experiments in module mounting to reduce tonnage per MW. Already most EPCs are in the 30 tons range, and still aggressively pushing further down. The approaches are based mostly on using very high strength steel or galvalume, which has twice the strength of usual steel. This creates very thin structures that pass all the static tests of design.

What is often neglected is that, as the structures become more slender, dynamic effects become more powerful. Wind creates vortices at a particular periodicity, and if you are unlucky and the period of oscillation of your structure matches the vortices, resonance happens. The structure will then most probably fly off or damage a lot of mounted equipment. A more comprehensive analysis of the adequacy of the structure is required at such competitive levels of tonnage.
Nimish Prabhukhanolkar, B.Tech/M.Tech IIT Bombay is Partner/Founder at AGAMI ENGINEERING CONSULTANTS where he heads the overall technical solutions by AGAMI.

Tuesday, 22 November 2016

The Role of a Structural Engineer: An Indian Perspective

About this Article.

This article is aimed at layman readers as well as people in the construction domain, but not people who are structural engineers. The motivation behind writing this is that the role of a structural engineer in a construction project is not really understood at all by a layman. Neither is it understood clearly by other construction stakeholders, as to what it is that a SE does with his time, apart from creating Structural Drawings. Or why he should be given ample time to do his work!

What are his credentials?

Typically SE's are B.E.s or B.Techs. although unfortunately i haven't heard of any government legislation that keeps a minimum qualification to render Structural Services. The higher limit would be an M.E, M.Tech. or a Phd. Typically Phd's in industry are hard to come by, and spend their time in really niche areas, like retrofitting, or government research agencies etc.

So What do they do?

Imagine a residential building being planned in an area.
The architect answers the question of how exactly does this building look like? How are the rooms planned. Are we sticking to govt. FSI norms? Firefighting norms? Drainage norms? The output of this step is a building shape which satisfies the previous constraints. So what is missing?
The building needs STRENGTH. Strength to withstand it's own humongous weight(in structural parlance, it is called DEAD LOAD). Strength to withstand loads from tenants cargo once they step in. The tenant should not be worried if he wants to have lot of furniture in his living room, or transform a spare room to a "store-room" as to whether the slabs would take it or collapse. It is a given that it would. Except it is not. The structural engineer ensures this in his design!
There's more though.
Apart from these daily loads, a structure needs to withstand any natural or unnatural loads expected atleast ONCE in it's lifetime. What can you think of?
Think an Earthquake, a Wind-Storm, and Fire. A building is engineered such that it can withstand it's own region's designated earthquake with minimal damage, and an exceptional earthquake WITH DAMAGE but without collapse! A building is engineered to withstand a 50 year wind event without damage. A building is engineered to provide atleast 1 hour fire protection in case of a fire, which ensures atleast the concrete-part would not collapse under fire.
Any typical building around Mumbai is designed to take a moderate earthquake of about 6.5 Richter scale without damage. And a hurricane consistent speed of 160kmph without breaking a sweat. This if it is properly designed.
Now thats a handful isn't it!
Interestingly, due to increasing percieved terrorist threats, some very important buildings are also being designed to take load from a bomb-detonation. They are designed to keep standing if a bomb blows of a couple of columns at the ground.
On another note, buildings on the sea-front, in Mumbai, are not designed to take the force of a tsunami wave-should it come. We might see these things entering our design codes in some more years.
All these things lie within the purview of the Structural Engineer.

How much is the compliance?

Like all other "desi" things you might be tempted to ask this question - how much are all these rules of design followed? Are our buildings really this safe? Don't contractors use cheap-grade material and undermine building safety?
I would say the situation out there is not extremely good, but not so bleak too! It is simply because everyone knows we are dealing with a sensitive thing here. What i mean is, if the contractor steals a lot, and uses lot of substandard materials, the engineer on his inspections can easily detect some of these transgressions and report this to the developer/client. The client could take strict action and send a stop work notice. This affects the contractors further prospects in the market. In my experience, I have seen some shoddy work being undertaken on site, but have yet to come across anything that makes a complete mockery of Structural Design. I have worked with some real good contractors too, who keep confirming with the engineer on what they are doing is right or wrong. This is more or less a good thing.

The Economics of Time & Money

Considering the year 2014, average civil cost of a project(cost of materials, labour, formwork) should be in the range of 600-1000/sq foot built-up. The fees of a structural engineer who controls the above material is from 1Re/per sq feet to 15 Re/ per sq feet. (This is leaving out ultra-premium luxury residences sector, where they go for designer labels for everything and bring in some MNC for its brand-value) Is this enough?
How can one answer the question of - how much is the structural engineer's work really worth?
Considering I am myself a SE, that makes me a biased party to this argument. I would try to approach this as rationally as possible. Let's consider one possible small-scale structural engineering business. This would consist of one engineer and one draftsperson. This represents a most "agile" setup, with very less overheads. They may or may not have an office, depending on their revenue.
This is what i feel is the rough time-line to deliver design of a typical 5-9 storied building which is the norm in metroes.
Initial Briefing and information gathering2
Finish structural calculations(modelling etc)3
Decide on Foundations after coordinating with site1
Design Foundations and release drawing3
Release column schedule till top3
Time to release each distinct slab3
Site queries, site visits5
Rework due to any changes to plan3
It seems from above the work takes almost a month to finish. Remaining days can be taken as a down-time, since it is not necessary to have business every single day of the year. Lets assume, ideally, that the designer-draftsperson duo get one such building every month for all months. Assuming per floor built-up area of 1500 sq feet, thats 1500X8 = 12000 sq feet per building. For 12 months, theres 144000 sq feet of work to be done. Now this is only an average. A more realistic spread might be, 1 bungalow, 2-3 short buildings of 4 floors, one 20 storied building, and so on. However the average might hover quite near the above.
Now lets see what revenue would be generated as a function of various per sq feet rates and it's implicatons.
Design Rate   per sq feet Revenue   Yearly       Comment
50 paise72000 INRNot viable at all. Engineer would curse the person who made him go in this field, citing "civil mein bahut scope hain!"
2 Re288000 INRStill not viable even for 1 person.
5 Re720000 INRBarely viable. With a 40:60 split, engineer gets 4.32lpa. Could get more as a bank clerk possibly with much better perks.
7.5 Re1080000 INRViable for a home-based business without office overheads.
10 Re144000 INRJust Viable for a office in suburban area with running cost of 5lpa.
15 Re2160000 INRViable in a proper way.
The above table makes it clear what represents the limits of viability. Note that the time considered for design is what represents a sound engineering time. Time in which an engineer can weight in options, optimize, and give a good design. If shortcharged, what an engineer would do is, reduce the time, reducing quality of design and increase yearly square footage of business, so that he can reach his targets. This represents a lose-lose situation for everyone involved. For saving 5 rs by paying the SE 5/sq feet instead of 10/sq feet, the client may have inadvertently increased his civil costs from say 700/sq feet to 750/sq feet, due to the engineer not being able to keep reserve adequate time for the work. Makes sense?

Author: Nimish Prabhukhanolkar 
Partner/Founder - Agami Engineering