Thursday, February 23, 2017
Reinforcement detailing of a slab is performed on the basis of its support conditions. Support to slab is generally provided through walls or beams or columns.
A slab is called one way if the supported is provided from two opposite edges. So, the prime load is transmitted onward the spanning direction. Here, the proportion of longer extent to shorter extent is over 2. In one way slab, one side is greater than the other one. In one way slab, the bending occurs mainly in one direction only (spanning direction).
Consequently, the main reinforcement is necessary to withstand the moments produced and designed for the same. The main reinforcement is arranged at the bottom of the slab that is usually described as the tension face.
In one way slab, as one side is larger as compared to the other one, the greatest load is carried by the larger side. To give more support on the larger side, main reinforcement is set perpendicular to that side or parallel to the shorter direction. Distribution steel is arranged in the extended direction that will not be very useful for carrying any load.
In one way slab, the main reinforcement is calculated with a formula (In limit state design) and to obtain the result the comparison is made between the comparing compressive force and tensile forces.
Ast = 0.5 Fck/Fu[1-√1-2.6Mu/Fck.b.d]b.d
and the distribution steel is calculated as
0.15% of Ag, for mild steel.
0.12% of Ag, for tor steel.
Where, Ast denotes Area of the steel in tension.
Fu denotes Ultimate strength of steel.
Mu denotes Ultimate moment of resistance.
b denotes Breadth of the slab section.
d denotes Depth of the slab section.
Ag denotes Gross area of the section.
Article Source : www.dailycivil.com
Wednesday, February 15, 2017
Balanced section refers to the section in which the quantity of steel is just adequately arranged so that the concrete in compression zone and steel in tension zone attains its acceptable stresses instantaneously.
In this section, the critical depth is identical to its actual depth. i.e n = Na = Nc
UNDER REINFORCED SECTION:
In this section, the quantity of steel is insufficient to produce the highest concrete fibers in the compression area to be compressed with their highest allowable stress.
In this section, the quantity of steel is insufficient to arrange the concrete for being compressed in compression area to their maximum allowable value. It signifies the steel is arranged below that a balanced section needs. In under reinforced section, the depth of actual Na is under the critical Na.
i.e; Na < Nc
OVER REINFORCED SECTION:
In this section, in tension zone, the quantity of steel remains in excess of the quantity of steel necessary to arrange compressive zone concrete to be compressed to their highest permissible value. Alternatively, if the highest compressive stress in concrete attains its permissible limit, the comparing tensile stress in steel will not surpass its allowable value.
So in over reinforced section, the depth of actual Na is over and above the critical Na.
i.e; Na > Nc
Tuesday, February 14, 2017
This construction video is recorded on the detailed process for obtaining the quantity of 1.226 cft cement in 1 bag.
Here density of cement is taken as 1400 kg per m3 (m = millimeter)
Therefore, the quantity of cement for 1m3 = 1440 Kg
It is assumed 1 bag cement = 50 kg
Therefore, cement required for 1m3 = 1440/50 = 28.8 bags (This is taken as equation 1)
It is assumed 1m3 = 35.3147 CFT (Cubic Feet) (It is taken as equation 2)
By combining both equation, the following result is obtained :-
28.8 bags similar to 35.3147 CFT
Therefore, 1 bag is similar to 35.3147/28.8 = 1.226 CFT
Friday, February 10, 2017
Tie beams are constructed to support the differential settlement among the isolated footings other than the vertical loads of the block works.
With reference to ACI 318-08 section 220.127.116.11, grade beams which are built up to function as horizontal ties among pile caps or footings should be proportioned in such a way so that the smallest cross-sectional dimension is identical to or exceeding the clear spacing among associated columns divided by 20 but should not be over 18 inches. Closed ties should be arranged at a gapping not surpassing the lesser of one half the lowest orthogonal cross-sectional dimension and 12 inches.”
But, it becomes difficult to understand that what force these tie beams are required. In this regard, IBC 2009 section plays an important role.
IBC2009 section 1809.13 for shallow foundations and 1810.3.13 for deep foundations.
For SDC C, D, E or F, ties have the capacity to bear, in tension or compression, a force similar to the smaller of the product of the larger pile cap or column design gravity load times the seismic coefficient, Sds, divided by 10, and 25 percent of the smaller pile or column design gravity load.
In brief, Tie beam force; FT = Larger of ( Pu_large x Sds /10 , Pu_small x 25%)
Wednesday, February 8, 2017
This construction video shows the variation among working stress method and limit state method.
The basic variance among Working state method (WSM) and Limit State method (LSM) is that WSM stands for an elastic design method while LSM stands for a plastic design method.
In elastic design method, the design strength is estimated in such a manner that the stress in material is controlled with its yield limit, under which the material adheres to Hooke's law, and therefore, the word "elastic" is utilized. This method is used to produce cost-effective design of simple beam, or other structural elements where the standard for evaluating the design is stress (static). But, in case of changing the evaluation standard to other factors like fatigue stress, both the methods will give similar design. Also, WSM substantially reduces the calculation efforts.
In plastic design method the stress in material is permitted to go outside the yield limit and move into the plastic zone to attain extreme strength. Therefore, the "moment-rotation" power of beam, as for instance, is applied for creating more cost-effective design. However, because of the deployment of the non-linear zone, this method comprises laborious calculation.
Given below, some other general differences:
1) Serviceability check if LSM is necessary since after the elastic region strain is greater, it leads to more deformation, therefore a check is required.
2) LSM refers to strain based method while WSM is stress based method.
3) LSM refers to non-deterministic method while WSM is deterministic approach.
4) Partial safety factor is applied in LSM while Safety factor is applied in WSM.
5) Characteristic values (resulting from probabilistic approach) are applied in case of LSM while Average or statistic values are applied in WSM.
Monday, February 6, 2017
This construction video offers detailed guidelines for measuring the steel quantity in circular column, piers, and piles.
Some vital information on circular column, pier and pile :-
Circular columns are uniform about any centroidal axis. The circular column contains a smaller cross section area (0.785 times the square column) but greater second moment of area (1.178 times the square column). There is less susceptibility for Circular column to buckling and it needs little more reinforcement to bear the identical load like a square column. Besides, circular sections contain unvarying torsion characteristics. Least number of bars necessary in a circular column is 6. There does not exist any weak corners as well as stress concentration in circular columns.
A pier refers to an elevated structure usually supported with well-spaced foundation system. The foundation system ranges from a Pile, Caisson or simple brick pillar that is formed on the basis of the sub-soil condition. The Pile and Caisson are set of deep foundations.
Saturday, February 4, 2017
This construction video is recorded on the topic of how to organize stirrups for a column that contains 12nos of vertical bar. Learn to apply various structural drawings for organizing stirrups.
Stirrup can be defined as follows :-
1. A reinforcement useful for withstanding shear and diagonal tension stresses in a concrete structural member.
2. A steel bar that is curved into a "U" or box shape and arranged perpendicular to, or at an angle to the longitudinal reinforcement, and perfectly anchored.
3. Lateral reinforcement that is developed with separate units, open or closed, or of uninterruptedly wound reinforcement. The word stirrups is generally used with lateral reinforcement in flexural members and the term is linked with lateral reinforcement in vertical compression members.
Concrete beams contain different depth. If the beam contains more depth, the shear strength will be increased. If the depth is insufficient, steel stirrups should be included to enhance the shear strength of the beam. These stirrups generally refer to one piece of steel that is curved into a quadrilateral shape. Generally small diameter steel like #3 and #4 rebar is applied. The stirrup normally covers around the bottom and top bars of the beams.
As the stirrup is built up from two pieces having insufficient lap splice, it is recommended to set up a stirrup simultaneously when the horizontal reinforcement is being set up.