One of my younger engineer friend told me that there was a design of sport stadium with 45 m. roof span to check. The former engineer's design was not economic so he gave this drawing to another engineer to do. The latter engineer said the cost cannot be reduced any more. He came to ask me whether I can do anything save.
The drawing is a stadium with 45 m. roof span.at bay 6 m. each . The assumption of the main roof truss is simple beam. The longitudinal members are 190.7 mm. pipe(7.5") .The depth is about 2 m. , diagonal members are 139.8 mm. pipe(5"). At each side of main trusses are proped with 5 trusses. The cross 15 mm.round bar tierods are between each another main truss spacing. At center of purlins are 12 mm. sagrod placed across the width of the roof.
Try to imagine . The former design was the bow without string so the bow must be large to withstand the moment(bending force). When the bow deflects due to the weight , there will be horizontal force act on column head. The columns were not reinforced to withstand this forces. It was observed by reinforcement in the columns that were at side instead of fore and back . The problem was solved by making the support plate slot holes to let the ends slide.
The drawing was revised. Structure of an arch was used.It moment diagram is like parabola (half moon curve) . and like the arch roof structure. The roof is the bow and the string is tierod which resist this horizontal force, about 15 tons, that act on column head. The moment arm lenght is the length from the center of the bow to the string. The moment arm length of new design is much longer than the former which is only 2 m. depth. ( a longer wrench arm needs less forces to turn the nut).
The side probe trusses were taken off . They are useless structures because when the main truss sway it will sway together. The cross tie bars were used instead. The new main truss was rectangular box section, 0.40 width 1.00 depth with 50x3.2 mm.(2") pipe at each corner. The box section was designed in order to resist side force, either from wind or handling. The mono plane section may cause buckling because the ratio of heigth to width was too large. The sagrods ( steel bar at center of purlin to keep the purlin straigth) across the roof were taken off because the steel roof was sagrod by itself.
When the cost of new design structure was estimated ,it was found that the cost was only 33% of the former design. About 3 m. baht was saved. This was only the roof exclude the step seats . The benefit of the step seats was the depth between each step,the more depth the more moment of inertia(material property to resist bending), the less reinforcement. The former design abandoned the depth of seat by using slab instead , and the depth of seat become load.This was the turning back of a good design.
It is obvious that saving construction cost not depend on bidding, but on the proper design.The difference in bidding is atmost 10% while the design is more than50%. The project owners always inspect only at bidding cost and overlook the skill of engineering designer. They do not realize what the principle of saving cost is. Some owners try to reduce designing fee. They do not know that this leads to less experience engineer hiring too.The unexperienced engineeer can harm a lot with their ignorance as well if the project owner lacks of cross check or review of the design.
From many cases of our bidding proposals, more than a half of the cases are very extravacant and weak. For example:
Warehouse: using piles on the stiff ground (sandy clay),where the capacity of piles are less than bearing capacity and no problem of consoldation.
Showroom: avoid piles on ground floor when this will cause diffential settlement in the future.
Factory: Using single pile at side column , the wall cannot stand moment due to wind force .Most of the design from subprovince administrations authority, the wind forces are always negleted.
This is only a part of misleading to the project owner of unreasonably employment.
วิศวกรที่ประหยัด จะต้องตั้งคำถามในใจตลอดเวลาว่า แบบอย่างนี้ จะปรับปรุงให้ดีขึ้นได้ไหม ถ้าในวิชาวิศวกรรมอุตสาหการ หลักการของญี่ปุ่น เรียก po ka yo ke ให้ทุกคนมีส่วนร่วมในการปรับปรุงพัฒนาผลิตภัณฑ์ ไม่ต้องแก้มากมาย แค่จุดละเล็กน้อย ปรับปรุงตลอดทุกจุด