Concrete Properties
1. Temperature and shrinkage causes tensile forces in concrete, due to the interaction of reinforcement and concrete; Cracking levels depend on,
a) Tensile strength of concrete.
b) The cover thickness.
c) The diameter of rebar
d) Rate of corrosion.
e) Modulus of elasticity of concrete and reinforcement
f) Spacing of reinforcement
f) Cement content (or factor) of concrete
g) Water-cement ratio of concrete
h) Curing method and length of curing
i) Aggregate gradation and type (high absorption coarse aggregate increases shrinkage)
j) Coefficient of expansion for aggregate, least for lime stone highest for granite
2. PoissonÆs ratio: A value of about 0.15-0.2 is usually considered for design.
3. Shear strength: The strength of concrete in PURE SHEAR has been reported to be in the range of 10 to 20% of its compressive strength.
4. Factors influencing creep:
Creep increases when,
a) Cement content is high,
b) w/c ratio is high,
c) Aggregate content is low,
d) Air entertainment is high,
e) Relative humidity is low,
f) Temperature (causing moisture loss) is high,
g) Size / thickness of the member is small,
h) Loading occurs at an early age &
i) Loading is sustained over a long period.
j) Coarse aggregate type and gradation
k) Magnitude of loading
5. Effect of creep:
Detrimental results in RC structures due to creep:
a) Increased deflection of beams and slabs.
b) Increased deflection of slender columns (possibly leading to buckling)
c) Gradual transfer of load from concrete to reinforcing steel in compression members.
d) Loss of prestress in prestressed concrete.
6. Symmetrical arrangements of reinforcement will aid to avoid the differential restraint.
7. Reduction of moments on account of moment redistribution is generally NOT APPLIED TO COLUMNS.
8. Reinforcement availability:
Standard diameter sizes (mm): 6, 8, 10, 12, 16, 20, 24, 32, 40
Standard lengths: > 12mm diameter: 12 metres
< 12mm diameter: from a coil
If youÆre not going to inspect everything keep the difference in bars sizes greater than 3mm (1/8 inch).
9. These values are approximate and should be used only as a check on the total estimated quantity:
Slabs - 80 - 110 kg/m3 (50-70lb/ft3) (flat slab120-220kg/m3 (75-140lb/ft3))
Columns - 200 - 450 kg/m3 (125-280lb/ft3)
Walls - 60 - 100 kg/m3 (25-65lb/ft3)
R/C footings 70-90 kg/m3 (45-60lb/ft3)
Pile caps - 110 - 150 kg/m3 (70-95lb/ft3)
Rafts - 60 - 70 kg/m3 (40-45lb/ft3)
Beams - 150 - 220 kg/m3 (95-140lb/ft3) (edge 180kg/m3)
Transfer slabs 150kg/m3 (95lb/ft3)
Retaining walls-110kg/m3 (70lb/ft3)
Stairs û 135kg/m3 (85lb/ft3)
Note: The actual reinforcement quantity in the element will vary according to detailing practice and efficiency of the concrete element. For jobs where a provisional quantity of reinforcement is part of the contract documents, the rates should be determined by measurement of the typical elements plus allowance for non-typical and laps.
Reinforcement estimates
In order for the cost of the structure to be estimated it is necessary for the quantities of the materials, including those of the reinforcement, to be available. Fairly accurate quantities of the oncrete and brickwork can be calculated from the layout drawings. If working drawings and schedules for the reinforcement are not available it is necessary to provide an estimate of the anticipated quantities.
In the case of reinforcement quantities the basic requirements are, briefly: for bar reinforcement to be described separately by: steel type, diameter and weight and divided up according to:
a) element of structure, e.g. foundations, slabs, walls, columns, etc.
b) bar æshapeÆ, e.g. straight, bent or hooked; curved; links, stirrups and spacers. for fabric (mesh) reinforcement to be described separately by: steel type, fabric type and area, divided up according to a) and b) above. There are different methods for estimating the quantities of reinforcement; three methods of
varying accuracy are given below.
Method 1
The simplest method is based on the type of structure and the volume of the reinforced concrete elements. Typical values are, for example:
warehouses and similarly loaded and proportioned structures: 1 tonne of reinforcement per 10m3
offices, shops, hotels: 1 tonne per 13.5m3
residential, schools: 1 tonne per l5m3.
However, while this method is a useful check on the total estimated quantity it is the least accurate, and it requires considerable experience to break the tonnage down.
Method 2
Another method is to use factors that convert the steel areas obtained from the initial design calculations to weights, e.g. kg/m2 or kg/m as appropriate to the element.
If the weights are divided into practical bar diameters and shapes, this method can give a reasonably accurate assessment. The factors, however, do assume a degree of standardization both of structural form and detailing.
This method is likely to be the most flexible and relatively precise in practice, as it is based on reinforcement requirements indicated by the initial design calculations. Reference should be made to standard tables and spreadsheets available from suitable organisations (e.g. The Concrete Centre).
Method 3
For this method sketches are made for the ætypicalÆ cases of elements and then weighted. This method has the advantages that: the sketches are representative of the actual structure the sketches include the intended form of detailing and distribution of main and secondary reinforcement
an allowance of additional steel for variations and holes may be made by inspection.
This method can also be used to calibrate or check the factors described in method 2 as it takes account of individual detailing methods.
When preparing the reinforcement estimate, the following items should be considered:
Laps and starter bars û A reasonable allowance should be made for normal laps in both main and distribution bars, and for starter bars. This should be checked if special lapping arrangements are used.
Architectural features û The drawings should be looked at and sufficient allowance made for the reinforcement required for such ænon-structuralÆ features.
Contingency û A contingency of between 10% and 15% should be added to cater for some changes and for possible omissions.
10. In normal circumstances and where N-grade (normal) concrete is used, forms may generally be removed after the expiry of the following periods:
Type of Form Work (Location) Min period before striking
a) Form Work Vertical formwork to columns, Walls, beam 16 - 24 hrs
b) Soffit formwork to slabs (props to be re-fixed immediately after removal of formwork) 3 days
c) Soffit formwork to beams (props to be re-fixed immediately after removal of formwork) 7 days
d) Props to slabs: Spanning up to 4.5m (16 ft) 7 days Spanning Over 4.5m (16ft) 14 days
e) Props to beams & arches: Spanning up to 6m (55ft) 14 days Spanning Over 6m (55ft) 21 days
11. CONCRETE MIX RULES OF THUMB
ò ADDING 3L OF WATER TO ONE CUBIC METER OF FRESHLY MIXED CONCRETE WILL:
a. Increase slump about 25mm (1 inch)
b. Decrease compressive strength about 1 to 2 mPa (200 to 300 psi)
c. Increase shrinkage potential about 10%
d. Waste as much as 1/4 bag of cement
ò IF FRESHLY MIXED CONCRETE TEMPERATURE INCREASES 10 DEGREES:
a. About 3L (1 gallon) OF WATER TO ONE CUBIC METER (per cubic yard maintains) maintains equal slump
b. Air content decreases about 1%
c. Compressive strength decreases about 0.5 to 1.2 mPa (150 to 200 psi)
ò IF THE AIR CONTENT OF FRESHLY MIXED CONCRETE:
a. Increases 1%, then compressive strength decreases about 5%
b. Decreases 1%, then slump decreases about 10mm 1/2 inch
c. Decreases 1%, then durability decreases about 10%
12. The main components of cast-in-place concrete floor systems are concrete, reinforcement (normal and/or post-tensioned), and formwork. The cost of the concrete, including placing and finishing, usually accounts for about 30% to 35% of the overall cost of the floor system.
13. Where post-tensioned reinforcement is used, a concrete compressive strength of at least 40mPa (5,000) psi is usually specified to attain, among other things, more cost-effective anchorages and higher resistance in tension and shear.
14. Having the greatest influence on the overall cost of the floor system is the formwork, which is about 45% to 55% of the total cost.
Three basic principles govern formwork economy for site-cast concrete structures:
ò Specify readily available standard form sizes. This is essential to achieve economical formwork. Most projects do not have the budget to accommodate custom forms, unless they are required in a quantity that allows mass production.
ò Repeat sizes and shapes of the concrete members wherever possible. Repetition allows forms to be reused from bay to bay and from floor to floor, resulting in maximum overall savings.
ò Strive for simple formwork. There are countless variables that must be evaluated and then integrated into the design of a building. Economy has traditionally meant a time-consuming search for ways to reduce the quantities of materials. For example, it may seem appropriate to vary the depth of beams with the loading and span variations, providing shallower beams where the loads or spans are smaller. This approach would result in moderate savings in materials, but would create additional costs in formwork, resulting in a substantially more expensive structureùquite the opposite effect of that intended. Providing a constant beam depth while varying the amounts of reinforcement along the span length is the simplest and most cost-effective solution.
15. ABRASIVE RESISTANCE of concrete increases with compressive strength and use of aggregate shall have low abrasion loss under standardized testing or high abrasion resistance
16. For steel bars to lose one mm diameter due to corrosion, it takes about 12.5 years. For 6mm dia to corrode completely it takes about 75 years in good conditions. But due to practical reasons the number of years reduces due to hostile corrosive environment. In coastal applications, poor consolidation of concrete, insufficient cover, use of chloride admixtures, 6mm rebar corrode completely in 5 years or less.
17. The most significant impact that admixtures have on concrete is usually a positive increase in the ability to consolidate the concrete. Air entraining admixtures, low and high range water reducing admixtures, and set retarding admixtures all contribute to better consolidation, which provides better durability and more consistent properties of the concrete throughout its cross section.
18. Tests have shown that the cracked torsional stiffness can be as low as 1/10 of the un-cracked torsional stiffness (AS5100 limits this to 20%)
19. concepts:
1. Increase Temperature change through a wall doesn't cause tensile or compression loads. A wall with perfectly pinned supports will only bow ( with small temperature differences say up 40deg (even thought this is very unlikely to happen since concrete is a heat sink). Note in below 1 deg concrete changes it behavior and you can have other problems with regard to freezing ect, but that is a topic for another day.
2. Restraints cause environmental loadings (will be referred to as service loadings from now on). Ie if you have a piece of wire and you lay it on the ground and heat it up, it will expand but no compressive force can develop due to no restraints. Now if we get this same piece of wire and restraint it at both ends and cool it down it will induce a tensile load in the wire.
3. Concrete is normally designed for two different states, 1. limit, 2. service limit. in limit state it is conservative to assume that the wall is pin pin restraints and worst bending moment from the wall loads is found. for service limits ie temperature and shrinkage, restraints are all that matter, the more restraint the worse the case for these service loads, Just think of a slab and restraint due to walls and shrinkage.
4. In really high temperature situations you can get spalling cause by expansion of the concrete; this is not in accordance with basic first principles and develops from gas pockets and internal bonds.
The area's you need to look at are: top, bottom and between walls corners restraints. These will cause restraint moments, note however if you are tanking the structure these items are not as critical.
Modelling
To model concrete to any real effect you must understand a few parameters first. Effect sections Ieff, concrete sections crack during bending hence you do not get full Ig for preliminary analysis the following IeffÆs are suggested, these do not include the effects of creep as mentioned above.
Suggested Effective Member Properties for Analysis
Member Effective moment of inertia for analysis
Structural Members / Service / Ultimate
Beams / 0.5EcIg / 0.35EcIg
Columns / 1.0EcIg / 0.70EcIg
Walls /
Uncracked 1.0EcIg / 0.70EcIg
Cracked 0.5EcIg / 0.35EcIg
Flat plates / 0.35EcIg / 0.25EcIg.
Flat plates (equivalent slab-beams consideration)/1.0EcIe / 0.70EcIe
This is basically according 10.11.1 and R10.11.1 x1.43 for service as per ACI 318.
Note: Ig is the gross uncracked moment of inertia. Use gross areas for input of cross-sectional areas.
Compatibility torsion:
Please do extra research on this as you will need to know it limitations such as box beams, curved beams ect.
AS3600- 8.3.2 Torsion redistribution
Where torsional strength is not required for the equilibrium of the structure and the torsion in a member is induced solely by the angular rotation of adjoining members, it shall be permissible to disregard the torsional stiffness in the analysis and torsion in the member, if the torsion reinforcement requirements of Clauses 8.3.7 and the detailing requirements of Clause 8.3.8 are satisfied. C8.3.2 Torsion redistribution The concept behind this Clause has been derived from compatibility torsion and incorporated in the ACI 318 Code. In a statically indeterminate structure, where alternative load paths exist and the torsional strength of a member is not required for equilibrium (i.e. compatibility torsion), the torsional stiffness of the members may be disregarded in analysis and torsion may be ignored in design.
However, minimum torsional reinforcement in accordance with Clause 8.3.7 must still be provided to avoid serviceability problems.
ACI: Two conditions determine whether it is necessary to consider torsional stiffness in the analysis of a given structure: (1) the relative magnitude of the torsional and flexural stiffnessÆs, and (2) whether torsion is required for equilibrium of the structure (equilibrium torsion) or is due to members twisting to maintain deformation compatibility (compatibility torsion). In the case of compatibility torsion, the torsional stiffness may be neglected.
Within that code the ACI explains that if you have secondary beams framing into a primary edge beam, as long as you design the secondary beams for pinned ends, you can then design the primary beam for a minimum torsion loading instead of a full analysis which would assume fixed ends and calculated torsion.
They even have a couple of 3D sketch-views of two structures showing the difference between a structure which doesn't need the torsional resistance for stability and one that does need the torsional resistance for stability. For a typical exterior bay of a building, where the interior joists or beams are designed with assumed pinned exterior ends, then the exterior beam's torsional resistance is not theoretically needed for the structural stability of the floor.
For a beam that has a single cantilevered slab hanging off the edge of it, the torsional strength of that beam is essential for the cantilevered slab to remain cantilevered...thus for that sort of case you must include the torsional aspects of the design.
Formwork
According to the Concrete Reinforcing Steel Institute (CRSI), ôformwork and its associated labor is the largest single cost segment of the concrete structural frameùgenerally more than 50%.ö
ò maintaining constant depth of horizontal construction
ò maintaining constant spacing of beams and joists
ò maintaining constant column dimensions from floor to floor
ò maintaining constant story heights
Standard Forms
Since most projects do not have the budget to accommodate custom forms, basing the design on readily available standard form sizes is essential to achieve economical formwork. Also, designing for acctual dimensions of standard nominal lumber will significantly cut costs. A simplified approach to formwork carpentry means less sawing, less piecing together, less waste, and les time. this results in reduced labor and mated costs and fewer opportunities for error by construction workers.
Repetition
Whenever possible, the sizes and shapes of the concrete members should be repeated in the structure. By doing this, the forms can be reused from bay to bay and from floor to floor, resulting in maximum overall saving. The relationship between cost and changes in depth of horizontal construction is a major design consideration. By standardizing the size or, if that is not possible, by varying the width and not the depth of beams, most requirements can be met at a lowered cost, since the forms can be reused for all floors. To accommodate load and span variations, only the amount of reinforcement needs to be adjusted. Also, experience has shown that changing the depth of the concrete joist system from floor to floor because of differences in superimposed loads actually results in higher costs. Selecting different joist depths and beam sizes for each floor may result in minor savings in materials, but specifying the same depth for all floors will achieve major savings in forming costs.
Simplicity
In general, there are countless variables that must be evaluated, and then integrated into the design of a building. Traditionally, economy has meant a time-consuming search for ways to cut back on quantity of materials. As noted previously, this approach often creates additional costs-quite the opposite effect of that intended. An important principle in formwork design is simplicity. In light of this principle, the following questions should be considered in the preliminary design stage of any project
1) Will custom forms be cost-effective? Usually, when standard forms are used, both labour and material costs decrease. However, custom forms can be as cost-effective as standard forms if they are required in a quantity that allows mass production. Class A1 concrete finish
It is firstly imperative that you understand that this type of formwork is normally only specified for monumental surfaces of relatively small area. For just what to expect and when it should be specified we suggest reading AS3610 and commentary. We can fax you some extracts if need be. The Cement and Concrete Association also has some useful publications on the subject.
Some information that you wonÆt find in the above literature but that you may find useful is given below.
Rebates
It is important to specify what materials shall be used to form rebates as standard rebates used to be ripped from maple but are now often ripped from miranti or other cheaper timber. The trouble with some of these is that the surface left after ripping is often not smooth and this has a deleterious effect on the final product. Consider specifying clear strips of radiata pine but also check and approve all rebate timber prior to being used in the forms.
Plastic rebates leave a beautifully smooth finish but can cause serious problems where you have high daily temperature variations. The thermal coefficient of plastic can be as much as 20 times that of timber.
Nails are generally used to hold the rebates in place and the specification may call for these to be punched filled and sanded smooth. Covering nail holes is also a little more difficult with plastic rebates.
Curing
If you are looking for colour control A, we do not recommend wetting intermittently or covering with plastic or leaving the formwork on too long as all of these will tend to lead to a colour variation. Consider the use of a water based acrylic curing compound complying with AS3799 such as Masterkure 404 but remember that some of these do leave a milky film on the surface that can take a while to vanish especially if it is rolled on rather than sprayed.
If your concrete is coloured forget these water based acrylic and go for a solvent based alternative such as Masterkure 402 and make sure you spay it. It does not quite meet the Australian Standard for water retention but it will give you a superior finish.
If you are doing coloured precast work we normally recommend using CCS same day sealer and curing out of the sun and wind.
Bar Chairs
Give serious thought as to how you can keep the reinforcement in position without the need for bar chairs and take extreme care during the placement and vibration operations. Make the contactor aware that you have a cover meter and will reject any elements found with reinforcement cover outside the specification tolerances.
Formwork
For this class of finish the formwork needs to be very stiff and often has a reduced allowance for form tie holes or a detailed setout of the requirements. If you need to have the formwork specifically designed or require an structural engineerÆs certification
2) Are deep beams cost-effective? As a rule, changing the beam depth to accommodate a difference in load will result in materials savings, but can add considerably to forming costs due to field crew disruptions and increased potential for field error. Wide, flat beams are more cost-effective than deep narrow beams.
3) Should beam and joist spacing be uniform or vary with load? Once again, a large number of different spacingÆs (closer together for heavy loads, farther apart for light) can result in material savings. However, the disruption in work and the added labour costs required to form the variations may far exceed savings in materials.
4) Are formed surface tolerances reasonable? The suggested tolerances for formed castin-place surfaces are shown in, The following simplified guidelines for specifying the class of formed surface will usually minimize costs:
a) Class I finish should be specified for surfaces prominently exposed to public view,
b) Class II finish should be specified for surfaces less prominently exposed to public view,
c) Class III finish should be specified for all noncritical or unexposed surfaces, and
d) Class IIII finish should be specified for concealed surfaces or for surfaces where roughness is not objectionable. If a more stringent class of surface is specified than is necessary for a particular formed surface, the increase in cost may become disproportionate to the increase in quality
Floors and the required forming are usually the largest cost component of a concrete building structure. The first step towards achieving maximum economy is selecting the most economical floor system for a given plan layout and a given set of loads. This will be discussed in more detail below. The second step is to define a regular, orderly progression of systematic shoring and re-shoring. Timing the removal of the forms and requiring a minimum amount of re-shoring are two factors that must be seriously considered since they can have a significant impact on the final cost.
Figures 1-5 and 1-6 show the relative costs of various floor systems as a function of bay size and superimposed load. Both figures are based on a concrete strength f: = 4000 psi. For a given set of loads, the slab system that is optimal for short spans is not necessarily optimal for longer spaas. Also, for a given spaa, the slab system that is optimal for lighter superimposed loads is not necessarily optimal for heavier loads. Reference 9.3 provides material and cost estimating data for various floor systems, It is also very important to consider the fire resistance of the floor system in the preliminary design stage (see Chapter 10). Required fire resistance ratings can dictate the type of floor system to specify in a particular situation.
The relationship between span length, floor system, and cost may indicate one or more systems to be economical for a given project. If the system choices are equally cost-effective, then other considerations (architectural, aesthetic, etc.) may become the determining factor. Beyond selection of the most economical system for load and span conditions, there are general techniques that facilitate the most economical use of the chosen system.
Slab Systems
Whenever possible, avoid offsets and irregrrlaxities that cause a ôstop and startö disruption of labour and require additional cutting (and waste) of materials (for example, breaks in soffit elevation). Depressions for terrazzo, tile, etc. should be accomplished by adding concrete to the top surface of the slab rather than maintaining a constant slab thickness and forming offsets in the bottom of the slab.
Cross section (a) in Fig. 9-2 is less costly to form than cross section (b).
Figure 9-2 Depressions in Slabs
When drop panels are used in two-way systems, the total depth of the drop h should be set equal to the actual nominal lumber dimension plus + in. for ply form (see Fig. 9-3). Table 9-2 lists values for the depth hl based on common nominal lumber sizes. As noted above, designs which depart from standard lumber dimensions are expensiveô Keep drop dimensions constant
Figure 9-3 Formwork for Drop Panels
Whenever possible, a minimum 16ft (plus 6 in. minimum clearance) spacing between drop panel edges should be used (see Fig. 9-3). Again, this permits the use of 16ftlong standard lumber without costly cutting of material. For maximum economy, the plan dimensions of the drop panel should remain constant throughout the entire project.
9.3.3 Beam-Supported Slab Systems
The most economical use of this relatively expensive system relies upon the principles of standardization and repetition, optimal importance is consistency in-depth, and of second~ importance is consistency in width.
These two concepts will mean a simplified design, less time spent interpreting plans and more time for field crew to produce.
ECONOMICALASPECTSOFVERTICALFRAMING
9.4.1 Walls
Walls provide an excellent opportunity to combine multiple functions in a single element; by doing this, a more economical design is achieved. Whh creative layout and design, the same wall can be a fire enclosure for stair or elevator shafts, a member for vertical support, and bracing for lateral loads. Walls with rectangular cross-sections are less costly than non-rectrmgular walls.
9.4.2 Core Area
Core areas for elevators, stairs, and utility shafts are required in many projects. In extreme cases, the core may require more labour than the rest of the floor. Standardizing the size and location of floor openings within the core will reduce costs. Repeating the core framing pattern on as many floors as possible will also help to minimize the overall costs.
Core Areas
Core areas for elevators and stairs are notoriously cost-intensive if formwork economies are neglected.
In extreme cases, the core alone may require more labor than the rest of the floor, on a per-foot basis. Formwork economy here is achieved through a simplification strategy: eliminate as much complexity from the core configuration as possible. The core will cost less to build, if the design follows the principles listed below and illustrated
in Figure 27:
ò The shape is symmetrical, rectilinear, without acute angles.
ò The number of floor openings is minimized.
ò Floor and wall openings are constant in size and location within the core.
ò The core framing pattern for walls and floors is repeated on as many floors as possible.
Columns
Although the greatest costs in the structural frame are in the floor system, the cost of column formwork should not be overlooked. Whenever possible, use the same column dimensions for the entire height of the building. Also, use a uniform symmetrical column pattern with all of the columns having the same orientation. Planning along these general lines can yield maximum column economy as well as greater floor framing economy because of the resulting uniformity in bay sizes.
Use the same shape as often as possible throughout the entire building. Square or round columns are
the most economical; use other shapes only when architectural requirements so dictate. Columns must be sized not only for adequate strength but also for constructability. For proper concrete placement and consolidation, the engineer must select column sizes and reinforcement to ensure that the reinforcement is not congested. Bar laps splices and locations of bars in beams and slabs framing into the column must be considered. Columns designed with smaller number of larger bras usually improves constructability.
Concrete is more cost effective than reinforcing for carrying compressive axial loads; thus it is more economical to use larger columns sizes with lesser amounts of steel.
Reuse of column forms from story to story results in significant savings. It is economically sound to use the same size column or the entire building and to vary only the logintuduical reinforcement and concrete strength.
Walls
Wall Thickness Trade-offs must be evaluated when designing wall thickness. Reasons to maintain constant wall thickness include repetitive use of standard forms, tie lengths and hardware. Reasons to change wall thickness include accumulating load. When wall thicknesses are changed, incremental steps of 2" or 4" are most efficient. Further, steps should be designed only on the wall face that intersects the horizontal framing. (Figure 31) It is more efficient to step-in formwork toward an opening or building edge than to step formwork away from these conditions.
Use the same wall thickness throughout a project if possible; this facilitates the reuse of equipment, ties, and hardware. In addition, this minimizes the possibilities of error in the field. In all cases, maintain sufficient wall thickness to permit proper placing and vibrating of concrete. Wall openings should be kept to a minimum number since they can be costly and time-consuming.
A few larger openings are more cost-effective than marry smaller openings. Size and location should be constant for maximum reuse of formwork.
. Brick ledges should be kept at a constant height with a minimum number of steps. Thickness as well as height should be in dimensional units of lumber, approximating as closely as possible those of the masonry to be placed. Brick ledge locations and dimensions should be detailed on the structural drawings.
. Footing elevations should be kept constant along any given wall if possible. This facilitates the use of wall gang forms from footing to footing. If footing steps are required, use the minimum number possible.
. For buildings of moderate height, pilasters can be used to transfer column loads into the foundation walls. Gang forms can be used more easily if the pilaster sides are splayed as shown in Fig. 9-9.
Guidelines for member sizing
-for a continuous beam keep beams size constant and vary the reinforcement from span to span.
-wide flat beams are easier to form than deep beams
-spandral beams are most cost intensive than interior beams due to their location at the edge fo the floor slab or at a slab opening.
Beams should be as wide or wider than the column into which they frame, in addition to formwork economy this also alliavtes some of the reinforcement congestion.
l
OVERALLSTRUCTURALECONOMY
While it has been the primary purpose of this chapter to focus on those considerations that will significantly impact the costs of the structural system relative to formwork requirements, the 10-step process below should be followed during the preliminary and final design phases of the contraction project as this will lead to overall structure
economy:
1. Study the structure as a whole.
2. Prepare freehand alternative sketches comparing all likely structural framing systems.
3. Establish column locations as uniformly as possible, keeping orientation and size constant wherever possible.
4. Determine preliminary member sizes from available design aids
5. Evaluate the sketches and make rough cost comparisons. Consider consulting a formwork office about economic variables relating to formwork, which in turn may influence the basic structural system.
6. Select the framing scheme which best seems to balance structural and aesthetic objectives with economic constraints.
7. Distribute prints of the selected framing scheme to all design and building team members to solicit suggestions that may reduce future changes.
8. Refine the design, placing emphasis on aspects with the greatest economic impact on structural frame cost.
9. Visualize the construction process and the resultant impact on cost.
10. Establish specifications that minimize construction cost and time by including items such as early stripping time and acceptable finish tolerance.
Where pour-strips are used (time-delayed pours to allow for shrinkage in long or posttensioned structures) the backshoring condition may be avoided by designing the slabs adjacent to the pour strips as cantilevers. The pour-strip is designed as simple span, as in Figure 39.
Construction Joint Location A concrete structure normally is built in progressive stages. (Figure 44)
However, to facilitate high-production recycling of equipment and manpower, some latitude in the precise location of construction joints (Figure 45) is desirable. The permissible locations for construction joints should be indicated on the construction drawings, to save time on the job and help ensure a quality structure. The contractor may then select the most efficient sequencing for the construction method to be used. The designer should approve all construction joint locations prior to commencement of the work. Once established, these locations should be communicated to all parties involved in formwork, concrete and reinforcement.
Permanent Slopes For Drainage Four methods are available to design sloped surfaces (typically for drainage).
a. Top-surface slope ù Much preferred due to its considerably lower cost, this method maintains a constant soffit elevation and consequently, is faster to form. It is achieved either by varying slab thickness or with fills. This slope method and method (b) below may require a higher-quality roof membrane than other roof designs. But even with its added cost, the total cost of these methods is much less than methods (c) and (d) below.
b. One-way slope ù top and bottom surfaces fig 41ù To reduce deadload and save permanent materials, bottoms of slabs may be sloped to parallel the top. This is more costly than method (a). Positioning the deck at varying elevations is labor-intensive. (Beams should also be sloped to parallel the slab, to avoid variable beam depth.)
Two-way slopeùtop and bottom surfaces (Figure 42)ùThis design is an extreme-cost option and almost always can be avoided. With ridges and valleys running in two directions, two-way sloping impedes formwork productivity, with stop-start disruption at each change of slope direction.
Warps (Figure 43)ùOf all slope designs, warps are the most extreme impediment to formwork productivity. Forming the curved surfaces requires intricate, expensive carpentry and precision installation. If at all possible, alternative designs should be considered instead.
Camber to Offset Floor Deflection
Typically, cambered slabs are not structural necessities, sufficient stiffness can be designed into floor framing systems to keep deflection within tolerances. This also avoids forming costs associated with camber. If camber is a design imperative, it may be specified much like the sloped surfaces previously discussed: as oneway, two-way, or warped. Again like slopes, costs are progressively higher as complexity increases, with warps at the extreme.
Stripping the formwork and falsework from a concrete Deck
Stripping the formwork under slabs is an issue close to every builderÆs heart because it directly affects when other trades can get onto the floor that is blocked up with falsework and start hanging the various services Considerations that may influence the timing of the strip are:
Are there any unusual heavy loads such as materials stacking or erection cranes that may require support in future construction works? It is sometimes more economical to leave formwork and false-work in place a little longer and strip out in one hit?
Will floors over need to be carried by the false-work and is there a set stripping and re-shoring procedure? If this is the case it is the responsibility of the design engineer to provide sketches showing the minimum amount of shoring required on each floor and the timing of the stripping operation. This information is normally requested by the managing contractor and is influenced by his site management needs. If the projects structural engineer wants to charge you for the advice we suggest you point them to AS3610 section 2.3 and ask when they stopped complying with code requirements.
Will any scheduled building activities be physically in the way of the stripping operation (blocking access etc)? We have seen money wasted on high strength concrete where the stripping could not be completed due to blocked access ways.
Will cranege and manpower be available at the time to take advantage of an early strip time? Systems such as Table-forms will need a crane booked and ready.
Is the formwork needed elsewhere on site or at another site? Sometimes very competitive formwork rates are contingent upon a specified cycle of form reuse.
Is it cost effective to spend the money on higher strength concrete to allow early stripping? This question may be a function of project programming, staff salaries, early finish incentives and/or liquidated damages.
Is the slab stressed? Stressed slabs can often be stripped earlier as concrete strengths form the only criteria if the future loads are within the floors capacity. We have allowed complete stripping of single storey stressed car park decks in four days using high early strength concrete. On a job we did in Asquith in October 2002 we allowed stripping of each floor of a multistorey warehouse 5 days after each floor was poured. The trick is ensuring that the site cured cylinder strengths are truly representative of the concrete in the deck (but that is another storey).
How stiff is the reinforced concrete slab? If the slab is conventionally reinforced it would normally need to be supported after the concrete has reached its design strength because if the props are taken away too early the slab will deflect beyond recommended limits. This additional deflection is known as creep deflection and is more pronounced when the concrete is relatively ôgreenö. Where normal class early age strength concrete is used with reasonable stiffness parameters, guidance can be taken from AS3600 (2001) Tables 19.6.2.4 and Table 19.6.2.5.
