Gravity Minimum Weight Design
This is the situation where usually an inexperienced or quite frankly rogue designer will consider the mathematics of design only in the final condition. The condition when the building is structurally complete and before it is to be dismantled. They will consider wind loading of course, but only as much as it effects the final condition. Temporary design considerations, connection design and any practical engineering considerations are excluded from their tender. Under the CDM regulations this is all forbidden, it’s even against the law, but time and time again they push their CDM responsibilities down the chain to other designers working for the Main Contractor and Tier 2 sub-contractor. “gravity minimum weight design” will yield the lowest weight of the primary members, but at what cost!
Practical Minimum Weight Design
This is a steel design where the designer has allowed for everything allowed for in the approach used for “gravity minimum weight” design but has also allowed for the secondary issues that should be thought about when designing steel, to name a few, but the list is not exhaustive:
- Consideration on how the steel structure is to be safely fabricated. Long slender beams need to be safely moved around a fabrication shop, the majority of welding is in the PA or PB position (downhand), therefore beams need to be turned. It isn’t normally a problem for experienced people, but consider the slenderness of a 22m long 406x140x46UB as compared to the slenderness of a 406x 178x 54UB. In the example I’m giving you, which is a situation was faced the steelwork contractor wanted to introduce a splice, but it was not allowed by the designer. It would have been much safer for the fabrication personal if the much stockier 406x178x54UB had been designed. These beams were fabricated to be portal rafters for a monopitch portal span, so additional stability considerations were required for the safe welding of the haunches at each end of the beam.
- For the same portal rafters in (1) additional concerns were raised by the fabricator on how these portal rafters were to be unloaded on site safely and how they were to be erected safely. To illustrate the point, the erector, who was highly competent and who has successfully worked on many of the largest portal frame contracts executed in the UK wanted to initially pick these beams up on their minor axis (web horizontal) with one fork lift vehicle using extended forks. After several discussions, a trial lift was done at the fabricators site, the beam was lifted off the floor more than 2m, the ends of the beam were still touching the floor. In the final reckoning, two fork lift vehicles had to be used on site, working in tandem to safely unload the portal rafters. Again special measures were required to ensure the stability of the portal frame during erection to ensure it’s stability at all times.
- Consideration of the requirement of temporary bracing, or additional checks for temporary loading.
- Truss hollow section minimum weight design fails to consider punching shear and all the other salient checks required to ensure that the boom members do not require considerable additional reinforcement. These checks commonly referred to as the “CIDECT” checks are a primary member design issue, not a connection design issue.
- If beams and columns are designed absolutely down to the bone, there is no scope in the design to take on any additional weight due to unforeseen circumstances. If the variation is necessary, then very expensive additional reinforcement will be required.
- Floor beams designed where there is not sufficient thought on the tolerances of steel beams during rolling, fabrication and erection. Often there is not enough landing for say PC units or PC stairs because a narrow flange minimum weight beam has been designed.
- Floor beams designed where there is no consideration of the temporary torsion induced in the beam during the erection of the PC units. In the final condition the beam is adequate, but in the temporary condition it isn’t.
- Large depth floor beams connecting to another smaller, sometimes much smaller depth beam. It’s not uncommon to see a 914 UB connecting into a 305 UC floor beam.
- Floor beams designed where their flanges are too narrow to shear stud, or their flanges are too thin to shear stud weld.
- Failure to make allowances for secondary steelwork necessary to give the following trades a fighting chance of fixing their products to the steel frame.
- Absolute minimum weight design may increase the number of pieces put on one single wagon, which could lead to (a) unsafe loads or (b) no change in the number of actual pieces going to site, hence no transport saving and (c) potentially of smaller, slender pieces becoming unstable in transit.
- Floor beams designed for the final condition, but not adequate to take the temporary torsional loads of edge protection systems.
- Minimum weight deign of columns supporting floor beams, typically a 152 UC could be designed but experienced designers will often use a 203UC to make connection design more sensible, reduce the number of flanges on floor beams that require notching of their flanges. Minimum weight, but maximum problems designing, detailing, fabricating and erecting the minimum weight design.
- Minimum weight design of floor beams could lead to beams requiring expensive and perhaps unnecessary temporary propping of the floor beams.
- Minimum weight design for essentially strength checks is prescriptive. There are mathematical rules to determine in most cases whether the strength check is adequate or not. What isn’t prescriptive is the treatment of serviceability checks, these are decisions based on engineering judgment. For minimum weight design and with the increasing use of section design with larger design strengths will reduce the weight of the primary section, which will make it more likely that the serviceability checks will be more of a governing factor than traditionally they have ever been.
- Poor section choice for hollow section beams and columns requiring fire protection. Sections with thin walls will need unpracticable thicknesses of intumescent paint to attain the fire rating, we have seen cases of 6mm of paint being required to suit the fire rating and section wall thickness designed.
- From a sustainability point of view, minimum weight design seems to tick all the boxes, but will contribute to poor selection of sub-contractors. Poor sub-contractors are more likely to estimate work at a global level, merely putting on global percentage fittings rates and global average fabrication rates. When the design is minimum weight, they are frequently wrong, so the poor quality sub-contractor that has a poor estimating function is more likely to win the contract. If a steelwork company has a poor estimating function, they are more likely to have a poor technical function, more likely to have a poorer fabrication function and will more than likely be poorer at considering temporary erection considerations.
It is of the opinion that sound “practical minimum weight design” has given sensible results in an often un-sensible market for years. There is a wide choice of structural steelwork contractors available for Tier 1 Main Contractors, so the market price of steelwork is very reasonable. Minimum weight design leads to lower embodied carbon, but I don’t think it gives the end user the minimum sustainability impact if all the additional problems and work is done to safely design, detailing, fabrication and erection of the structure.