Planning Reinforced Concrete Beams

This time we will discuss about the planning reinforced concrete beams.


Beams without reinforcement 
Concrete properties are strong against compressive force but weak against gravity. Therefore, the concrete can be fractured if the burden of assuming causing tensile stress exceeding its tensile strength.

Sample Case:If a block of concrete (without reinforcement) The fulcrum by a simple pedestal (joints and rollers), and on top of the beam work concentrated load P and a distributed load q, then there will be moments outside so that the beam will bend down.
On the curved beams down due to external load is basically detained by coupling styles in the form of press and tensile stress. So the fibers of the top edge of the beam will withstand compressive stress, and getting to the bottom of the voltage will be smaller. Instead, the fibers will hold the edges under tensile stress, and getting to the top of its voltage will be smaller too.At midspan (neutral line), the fibers do not experience stress concrete at all (voltage tap and drag = 0).If the load is too large beam above the neutral line the bottom will experience a tensile stress is large enough that can lead to cracking in the concrete at the bawah.Keadaan this happens, especially at the moment a large concrete areas, namely in the field / midspan.

Concrete beams with reinforcement
To withstand tensile force big enough fibers on the bottom edge of the beam section, it is necessary to rebars so-called "reinforced concrete". At this reinforced concrete beams, reinforcing grown such that tensile force required to hold the fractured moment in cross section can be retained by the reinforcing steel. Due to the nature of concrete is not strong tehadap pull, then in the picture above, it appears that the beams that hold appeal (below the neutral line) will be retained reinforcement, while the hold press (at the top of the neutral line) remains on hold by concrete.

The main function of the concrete and reinforcement
From the above description can be understood, that both the steel-reinforced concrete and reinforced concrete structures that have functions or different basic tasks in accordance with the material properties of concrete, namely for the main bersangkutan.Fungsi

The main function of the concrete:

• Withstand load / compressive force
• Closing the reinforcing steel that does not rust

The main function of reinforcing steel

• Withstand tensile force (although it is also strong against compressive force)
• Prevent cracking of concrete in order not to widen  

Safety Factor
In order to be assured that a structure capable of withstanding loads direncankan work, then used structural design safety factor of this security tertentu.Faktor tersdiri of two types, namely:

1. Safety factors acting on the outside of the load acting on the structure, called the load factor.
2. Safety factors relating to the strength of the structure (in style), called the power reduction factor.

 Outside the load factor / factor load
Big load factor given for each load acting on a cross-section of the structure will vary depending on the combination of the load in question. According to Article 11.2 SNI 03-2847-2002, so that the structure and components of the structure are eligible and suitable to be used on a variety of load combinations, it must be filled provisions berfaktor load combinations as follows:

1. If the structure or components only withstand dead loads D (dead) only then formulated: U = 1.4 * D
2. If a combination of dead load and live load D L (live), then formulated: U = 1.2 * 1.6 * D + L + 0.5 (A or R)
3. If a combination of dead load D, L live load and wind load W, then taken a great influence of two kinds of the following formula: U = D + 1.2 * 1.0 * 1.6 * L + W + 0.5 (A or R) and only formula: U = 0.9 * 1.6 * D + W
4. If the effect of earthquake load E taken into account, then taken the larger of the two kinds of the following formula: U = 0.9 * D + 1 * E

Description:
U = Combination factored load, kN, kN / m 'or KNM
D = dead load, kN, kN / m 'or KNM
L = Load life, kN, kN / m 'or KNM
A = roof live loads kN, kN / m 'or KNM
R = Load rainwater, kN, kN / m 'or KNM
W = Wind load, kN, kN / m 'or KNM
E = Expense earthquake (Earth Quake load), kN, kN / m 'or KNM, established by the SNI 03-1726-1989-F, Procedure for Earthquake Resilience Planning for Home and Building, or successor.
For more factored load combinations in the following article:

1. Article 11.2.4 SNI 03-2847-2002, for combination with lateral soil
2. Article 11.2.5 SNI 03-2847-2002, for combination with hydraulic pressure
3. Article 11.2.6 SNI 03-2847-2002, to the influence of shock loads
4. Article 11.2.7 SNI 03-2847-2002, for the effects of temperature (Delta T), creep, shrinkage, settlement.

Strength reduction factor
The uncertainty of the strength of the material to loading on the components of the structure considered a strength reduction factor, which is determined in accordance with Article 11.3 SNI 03-2847-2002 as follows:

1. The structure of bending without axial load (for example: beams), reduction factor = 0.8
2. Axial load and axial load bending

• axial tensile and axial tensile by bending: 0.8
• press axial and axial bending press

1. Component structure with spiral reinforcement or stirrups tie: 0.7

Reinforcing structural components with the usual ties: 0.65

1. Shear and torsion: 0.75
2. On a concrete pedestal,: 0.65

 Strength reinforced concrete

1.    the kind of strength

According to SNI 03-2847-2002, in the calculation of reinforced concrete structures, there are some terms to express the strength of a cross-section as follows

1. Strong nominal (chapter 3:28)
2. Strong plans (Article 3:30)
3. Strong need (chapter 3:29)  

 Strong nominal (Rn)
is defined as the power of a component of the cross-sectional structure is calculated based on the terms and assumptions planning method before multiplied by the appropriate factor power reduction. In the cross-section of reinforced concrete, strong nominal value depends on:

• dimensional cross-section,
• the number and location of reinforcement
• reinforcement layout
• quality of concrete and steel reinforcement

So basically this is the result of strong nominal count of the actual strength of the state of reinforced concrete structures on the state of nominal normal.Kuat is usually written with symbols Mn, Vn, Tn and Pn with the subscript n indicates that the values
M = Moment
V = shear force
T = Torque (torque)
P = axial force (obtained from nominal load of a structure or structural component)

Strong plan (Rr)
defined as the power of a component or structure cross-section is obtained by multiplying the nominal strong Rn and strength reduction factor. Strong plan can also be written with symbols Mr, Vr, Tr and Pr (same description as above except P) = obtained from the load plan that should work on a structure or structural components.

Strong need (Ru)
defined as the strength of a component or a cross-sectional structure necessary to support the weight of the factored or moments and style in relating to the burden of the load combinations U. Strong need can also be written with symbols Mu, Vu, Tu, and Pu.
Because basically strong Rr plan, the strength of the force (being in the structure), while a strong need to Ru is the strength of the external force (outside the structure) that works on the structure, so that the planning of the structure can be secured following conditions must be met:

Strong Plan (Rr)> Strong Need (Ru)

Reinforced concrete matter of principle

Count of reinforced concrete structures basically involves two pieces of matter, ie matter related to external forces and matter relating to the style in.
On the count of external forces, it must be accompanied by a safety factor called load factor in order to obtain a strong need to Ru. While the count of styles in, then accompanied with safe factor called factor of strength in order to obtain a strong reduction plan.  

Rr = Rn x reduction factor

Furthermore, so that the structure can bear the burden of the work on the outside of the structure, then it must meet that strong Rr plan must be at least equal to the strong need to Ru.