05/07/2021
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COMMON MISTAKES IN THE CONSTRUCTION INDUSTRY:
1 (STORIED RESIDENTIAL BUILDING)
Often a time developers get held by the costs involved during construction of any of their developments right from the substructure (foundation) and super structure (upper elements that are above the ground i.e walls, columns, beams, roofing and many other). I would like to only focus on the foundations or substructures design for a single storied building in this afterthought1 which I know most of us do. Conventionally its design would start with soil investigations that is intended to establish the BEARING CAPACITY of the soils and other soil parameters that aid in the design of a safe foundation system of the structure. Fortunately, Uganda is blessed with its good soils at least quite a good portion of land has good soils unless otherwise for the cases of swamps and you would find that the magnitude or weight of such structures (single storied developments) don’t need much of the investigations and as designers / structural Engineers they adapt for the minimum possible value of the allowable soil pressure of 150 to 200kpa unless otherwise where the recommend for improvement of the foundation either by compacting the weak pockets at site and improve it with low grade concrete or use of hardcore which is normally in the swampy places where they have doubts of achieving the allowable bearing pressure 150 to 200 Kpa for the design of the foundation.
I have had a chance of reviewing a lot of designs and drawings from other structural designers whenever either clients or Architects presents to me and for a number of times I have noticed what seem to be a fallacy that most of designers do ** am sorry for this my fellow designers but this issue needed to be addressed for all of us to understand*** lets have value for the money mostly to clients who are developing and those wishing to develop. Some of Engineers present a very ambiguous design (They over design) and for this afterthought we looking at the design of the foundation pads (footings), yes it’s always safety first which I often do in all my designs and followed by the feasibility I mean the design should be safe (with sufficient safety margin or factor of safety) and it should also be economically feasible. I have interacted with one of the structural designer whom I was referred by the client because his design was purely ambiguous with all the pads over-sized (1800x1800mm) and given depth of around 600mm for just a storied residential development. Well we are also like doctors who prescribe medicine depending on some of the usual or familiar sign and symptoms that are seen on the patient’s body so do a good Engineers and should be able to scrutinize through the design to check for those which are critical. During our interaction I asked him why did you design for such, what was the motive because even if you had designed for such( I rendered in my opinion about that design), the structural integrity would still not be compromised and I was shocked by his reply; he said “ Boss you see I have designed for a number of clients of which some his building collapsed of late because he changed the design at site with the fundi , whatever I specified in the layouts was compromised and reduced , for example the columns I had put bars of diameter 16mm and the client changed to those of diameter 12mm so I decided to give such for allowances such that in case they change at site it won’t be affected “ those were his exact words , he seemed to have a point but I asked him how about those clients who comply with your design and they precede to implement the design ( your ambiguous design) should they suffer losses at costs of the non-complaints? He said “No” and I asked him do you know how much it costs them or how much would the client spend on each footing pad incase the client implements this design. Well back on the point my entire writing is not to allow clients to change the design specs as instructed or designed by the Engineer I repeat clients are not entitled to change the design at site but again to Engineers lets design what is safe and economically feasible not only rising the safety margins to ambiguous values.
I later got to know that the client had used him because he was cheap because I kept asking myself there is no professional who would give such an excuse. I often use simple examples to explain to clients on the formation of the structural system in the entire structure; for this case it’s about the foundation design, we all know that it has footing bases or pads and there are some which we refer as critical one in Engineering and each foot base disperses off the loads of the building right from the superstructure (upper part of the building) to the ground soils and from the layman’s view I would love to relate it to a mother who has many boys , let’s take a simple analysis i.e 35 boys this time refer them as pads in the foundations amongst these boys there those who eat a lot (whom I refer as critical columns) i.e 12 of them and the other 23 boys eat very averagely to an extent that others eat even below what their mother expects (these are the remaining footings and columns). It would be totally a waste of food for the mother to assume that all his boys eat equally or their eating capacity is the same because she will always struggle to make the food for all the 35 boys since her 12 boys eat a lot which is quite similar to structural elements there those we refer as critical one whom we often design for and others non critical but still plays a role in transmitting the load to the ground soils. Let me do a quick estimate for the example of boys as the pads using his design of 1800x1800x600mm depth… his design he had allocated all the footing bases to be of that dimensions. For residential we always use concrete class C25 which has a mix ratio of 1:1:2 ditto 0ne batch of cement : one batch of sand : 2 batch of aggregates I will only compute bags of cements for a single footing and you will refer the costs to the sand and the aggregates..
Volume of each pad (1800x1800x600)/1000000 =1.944 m^3 ** divided by 1000000 to convert mm^3 to m^3 , the density of cement is 1504 kg/m^3
Using the ration of 1:1:2 , computing for the bags of cement = (batch of cement/sum of ratio)*volume of the pad *density of cement * factor of 1.33 ) divided by 50kg –cement is packed in 50 kg
=((1/4)*1.944*1504*1.33)/50
=19.4 roughly 19 bags per footing pad. And now the if the engineer gave the same design through for the footing this makes it (19 * 35 number of pads )=665 bags of cement . if we look at a cost of I bag of cement with transport i.e at a rate of 32000, 32000x665= 21,280,000 ugx.
Let us do the case where design for every individual pad i.e by allocating those that are critical and those that are non-critical. Let’s take that for non- critical to take dimensions of 1200x 1200x350mm , going through the same calculations to quantify the materials and the price;
Volume of each pad for non-critical (1200x1200x350)/1000000 =0.504 m^3 ** divided by 1000000 to convert mm^3 to m^3 , the density of cement is 1504 kg/m^3 using the same ratio 1:1:2
=((1/4)*0.504*1504*1.33)/50= 5 bags of cement in each of the non- critical columns..
So using of example of boys, only 12 to be critical and 23 to non-critical
Total bags of cement for the revised design = (12*19 )+ (23*5) =343 bags. Compare the two values of 665 bags of cement if the design was ambiguous and 343 bags if the design was done for every individual structural component roughly saving 10.3 million according to my analysis and these are losses suffered by clients who are complaint to some ambiguous designs.
NOTE – I don’t really encourage developers or clients to change the designs at site but rather a void cheap thing, its true cheap things are very expensive imagine of how you would waste. And for the case of professionals, lets always effect safety in our design but at the same time the design should be economically feasible. Let’s always take clients projects to be ours
Clients always be careful, if a professional asks for a slight amount compared to any other designer on block, respect it ** I mean it should not be too much* ..
Thank you, let’s look forward for Afterthought 2 (super structure).
Compiled by HOSEA DAMIAN AHEEBWA
