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The problem is asking for the calculation of the shear reinforcement for the rectangular section indicated, using Model I from the NBR6118 standard. The key details provided are:

- Distributed load (\( q = 6 \, \text{kN/m} \))
- Permanent load (\( g = 20 \, \text{kN/m} \))
- Beam span (\( 6 \, \text{m} \))
- Concrete strength (\( f_{ck} = 25 \, \text{MPa} \))
- Steel type (\( Aço CA50A \))
- Effective depth (\( d_{ef} = 45 \, \text{cm} \))
- Cover to the top reinforcement (\( d' = 2.625 \, \text{cm} \))

### Step-by-step approach for solving the shear reinforcement:

1. **Calculate the total uniform load:**
   The total distributed load on the beam is the sum of the permanent load and the live load:
   \[
   q_{total} = q + g = 6 \, \text{kN/m} + 20 \, \text{kN/m} = 26 \, \text{kN/m}
   \]

2. **Calculate the shear force:**
   The maximum shear force (\( V_{max} \)) occurs at the supports for a simply supported beam. The shear force at the supports is given by:
   \[
   V_{max} = \frac{q_{total} \cdot L}{2} = \frac{26 \, \text{kN/m} \times 6 \, \text{m}}{2} = 78 \, \text{kN}
   \]

3. **Calculate the design shear stress:**
   According to NBR6118, the design shear stress (\( \tau_{d} \)) is given by:
   \[
   \tau_d = \frac{V_{max}}{b \cdot d_{ef}}
   \]
   Where:
   - \( b = 15 \, \text{cm} \) (the width of the rectangular section)
   - \( d_{ef} = 45 \, \text{cm} \) (effective depth)

   Substituting the values:
   \[
   \tau_d = \frac{78 \, \text{kN}}{15 \, \text{cm} \times 45 \, \text{cm}} = \frac{78000 \, \text{N}}{675 \, \text{cm}^2} \approx 0.1156 \, \text{MPa}
   \]

4. **Check the minimum and maximum shear stresses:**
   - The concrete's contribution to shear strength (\( \tau_{c} \)) is based on \( f_{ck} \) and can be obtained from the NBR6118 tables or formulas, which typically give \( \tau_{c} \) values for a specific concrete class. For \( f_{ck} = 25 \, \text{MPa} \), an approximate value can be used:
   \[
   \tau_{c} \approx 0.34 \, \text{MPa}
   \]

   Since \( \tau_d < \tau_c \), shear reinforcement is required.

5. **Determine the shear reinforcement:**
   The shear reinforcement can be calculated by:
   \[
   A_s = \frac{V_{sw}}{f_y \cdot \sin \alpha}
   \]
   Where \( V_{sw} \) is the shear to be resisted by the stirrups, \( f_y \) is the yield strength of the steel (for CA50A, \( f_y = 500 \, \text{MPa} \)), and \( \alpha \) is the angle of the stirrups (usually \( 90^\circ \)).

Let me know if you would like further calculations, such as determining the number of stirrups, spacing, or specific reinforcement area. I can also provide more insights on the distribution of reinforcement and minimal reinforcement areas according to NBR6118. 

### Relative questions:
1. How is the critical section for shear determined in reinforced concrete design?
2. What are the main factors influencing the design of shear reinforcement?
3. How does the NBR6118 standard differ from other international standards in terms of shear reinforcement design?
4. What is the significance of effective depth in reinforced concrete beams?
5. How is the minimum shear reinforcement calculated according to NBR6118?

### Tip:
Always check if the concrete alone can resist the shear forces before calculating additional shear reinforcement.

Para a seção retangular indicada, calcule as armaduras de cisalhamento utilizando o Modelo I da NBR6118. Indique a distribuição de armaduras adotada e a região de armadura mínima.

Calculate the shear reinforcement for a rectangular section using NBR6118's Model I. Given q = 6 kN/m, g = 20 kN/m, beam length 6 m, f_ck = 25 MPa, and CA50A steel, this solution explains how to calculate the design shear stress, shear force, and reinforcement required.

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