bwt 210
Steel Construction
Discuss the elements of steel construction.
Advance planning :Large/complex projects require advanced planning & include divisioning the steel and planning shipping and erection procedures – thus dividing the structure into units (divisions) which are to schedule the fabrication & delivery of the structural members to the job siteFabrication & delivery to site When planning shop fabrication, the size & weight of members must be checked against plant capacity, transport requirements & the capacity of the erection equipment, long lead items should be ordered in advance and a deposit needs to be paid to ensure that the steel doesn’t keep the work up and forcing the team to get behind schedule if it doesn’t arrive on time.Field operations and equipment neededWhen planning erection procedures, the type of equipment to be utilized (is there space for cranes and etc.) and the procedures to be followed are determined by the type of structure erected & the anticipated site conditions.Steel elements are not shelve items and needs to be pre fabricated.
Explain the field operations when working with steel on a project
Include receiving, off-loading, sorting, inspecting, storing (yarding) & erection of the steel members
Off-loading from trucks are very important as improper handling can lead to the distortion of the members and damage to paint (if paint is damaged, the steel van start to rust)
Important:
When off-loading long flexible members and trusses, it is important to use double-sling lifting chains to avoid bending of the member
Inspect all members when off-loading, even if the inspection has been carried out in the manufacturing shop
Store all steel of the ground on platforms protecting it from dirt & corrosion
Explain the erection of steel structures by naming the different crews involved
Raising crew: lifts the steel members into position and makes temporary bolted connection
Fitting crew: takes over from the raising crew and brings members into proper alignment, temporarily bolting until final connections can be made
Fastening crew: makes the final connections *bolted * welded to meet the required specifications
Explain the advantages/disadvantages of using steel structures
Advantages:
Can be totally pre-fabricatedoff-site
Complex structures can be trial erected at the point of manufacture
Not subject to significant damage in transit & handling – why?
Capable of pre-erection into sub-assemblies on the ground to save erection time in the air
Erection generally straight-forward. With the use of profiled steel decking, welded to the beams, a safe place of work is created for the fixing of reinforcement and the concreting of the floors – all that is needed is perimeter protection.
Structural steel is far and away the best solution if the facility for future alteration of use is a built-in requirement
Erection time for the structure arguably faster than the alternatives
Long term retention of strength properties
Disadvantages:
Inaccuracies in manufacture not uncommon
If fire proofing is called for, steel not necessarily be encased into concrete – sprayed systems and box encasements are alternatives to concrete encasing which entails formwork, scaffolding etc.
Great care in detailing and erection are necessary when associated with pre-cast concrete cladding
Safe working conditions during the steel erection may be difficult to achieve – windy conditions/large sections
Discuss the application of sundry steel elements in a building
Deur handvatsels
Flooring
Industrial stair system
Guard Rails
Bollard
Cat ladders
Handrails
Water is required for several purposes:(11)
1. To provide moisture required for hydration of the cement to take place – hydration is the chemical reaction between cement and water produces hardened cement
2. If aggregates are not in a saturated surfacedry condition they may add or subtract water from the mix
3. The amount of water in a mix affects the plasticity or workability of plastic concrete
4. Strength, water tightness, durability and wear resistance of concrete is related to the water/cement ratio
5. The lower the water/cement ratio, the higher concrete strength and durability achieved
6. In terms of water quality – any water suitable for human consumption, will do
7. Where water quality is in doubt – make trial mixes
8. Organic material in mix water tends to prevent the cement paste from bonding properly to aggregate surfaces (clay content)
9. Alkalies or acids in mix water may react with cement and interfere with hydration
10. Seawater may be used – will result in concrete strengths 10 – 20% weaker than normal. The use of a lower water/cement ratio can compensate for this (avoid use of sea water unless approved by engineer)
11. Seawater should not be used for prestressed concrete where steel will come into contact with the concrete (due to the salt content = ?)
Additives:
Principal types:
• Air-entraining agents – increase resistance to freezing
• Water-reducing agents – increase the slump/workability of plastic concrete
• Retarders – slow the rate of hardening of concrete – use in high temp and when pumping over long distances
• Accelerators – they decrease setting time and increase early strength (act in opposite manner as retarders)
• Pozzolans – finely divided material – used to replace some of the cement in concrete mix. Reduce segregation.
• Workability agents – increase workability
Explain the term pre-cast concrete in full.
Concrete that has been cast into the desired shapes prior to placement in a structure or application
Done in a central plant where industrial production techniques may be used under controlled environment/procedures
Obtaining better surface quality & quality control than in-situ concrete
Distinguish by explaining in full the difference between hot and cold weather concrete practices
Hot Weather:
Setting time of concrete placed, is reduced
Higher temperatures also reduce the amount of slump for a given mix
Adding water to increase the slump = adding cement to the mix to correct the water/cement ratio
High temp. accompanied by wind/low humidity = increase the shrinkage of concrete leading to cracking.
Reduce the effects by:
1. Lower the temp. of the plastic concrete by cooling the mixing water and/or aggregates before mixing
2. Use low-heat cement or add retarder
3. Air-entraining agents, water-reducing agents or work-ability agents may be added to increase workability without changing the water/cement ratio
4. Discharge ready-mix concrete within 1 hour or less
4. Use shades/covers in controlling temp.
5. Start with moist curing immediately after finishing continue for at least 24 hours
Cold weather conditions:Problems are essentially opposite to that of hot weather concreting
1. Concrete should not be placed on a frozen surface
2. Must not be allowed to freeze within 24 hours after placing = permanent damage + loss of strength
3. Forms & reinforcing should be free from frost, ice, snow & be kept at a temp. above freezing
4. Do not concrete in temp. of 5°C, minimum 10°C for 3 days after placing
5. Water used may be heated to heat aggregate only prior to mixing with cement – no hot water to cement!
6. Use un-vented heaters inside an enclosure
during the 1st 36 hours after placing – important to let the concrete cool down
gradually to prevent cracking
1. Pre-tensioning:
* Places the pre-stressing material under tension in the concrete form before it is cast/poured
* After the concrete has hardened, the external tensioning devices are removed
* Bonding between the concrete and the prestressing steel holds the pre-stressing in place and places the concrete under pressure – See figure 12-10 page 210
2. Post-tensioning: NB!
* Places the pre-stressing steel under tension after the concrete member has been erected (poured)
* The pre-stressing is then tensioned by jacks placed at each end of the member
* After the pre-stressing load has been applied, the pre-stressing steel is anchored to the concrete member by a mechanical device at each end or by filling the pre-stressing tubes with a cementing agent
* After the steel has been anchored, jacks are removed and the pre-stressing steel is cut off flush at the ends of the member
Explain the importance of site inspection forms
The reality of a site is often quite different from the impression given by the drawings supplied (important for planning and pricing action). Site inspection meetings invitation… WHEN?
Why do we need site inspection forms
Follow structured form to make sure that nothing important has
Add to the site inspection form photo’s to give a good indication
of the main site features
Compile a site inspection form for pre-tender purposes
– Access (off site)
• Width/weight restrictions of existing roads which might influence the choice of plant to be used
• Bridges – height restrictions
– Access (on site)
• Obstructions – movement of plant and equipment
1. Overhead – power lines, trees
2. Underground – drainage, sewers, water tables etc.
3. A lot more – see book SITE INSPECTION FORM continue
– Boundary Conditions
• Adjacent buildings, trees – position of tower cranes? Ensure removal at completion?
• How much room between new works and site boundary – room for plant and storage
• Safety of public – right of way (deliveries)
• Adjacent watercourse – water level
• Airfields – may affect plant location – inform aviation of GPS co-ordinates of tower cranes
• Height of cranes’ influence
– Noise
• Noise is pollutant – see local authority for maximum permitted noise level – can affect the choice of plant
– Public utility supplies for construction
• Each public utility company will need to be contacted concerning availability of supply for construction purposes example water/electricity
• What may be adequate for the existing situation may not be adequate when construction is added to it• Developments stopped because of inadequate planning of local authorities
– Surface and below ground conditions
• Do an inspection of the site in dry and wet conditions – why?
• Where does the water flow to – natural drainage – may affect new construction
• Look at type of grass and wild flowers – can give you an indication of the type of soil
• Consult with the engineer – any difficulties with below ground conditions when excavating for foundations
• Insist on the Geo-Technical report from EngineerSITE INSPECTION FORM continue
– Local working week & local holidays
• Necessary to planner in terms of performance and time
– Local weather conditions
• Adequate knowledge of weather is necessary for planner for making allowance in performance for delays due to weather for example new hotel on the island of Zanzibar or the coast of Thailand? Failure in site inspections?
• Inadequate power supply to site
• Inadequate water supply to site