Select ALL CORRECT option(s) which can be considered to check whether the flexural stresses in a prestressed concrete beam are within the allowable stresses at the transfer and the service stages.

The correct options are:
- Limiting zone for prestressing
- Magnel’s graph
- Load balancing method

To check whether the flexural stresses in a prestressed concrete beam are within the allowable limits at both the transfer stage (when the prestress is applied to the green concrete, and only self-weight acts) and the service stage (under full working loads), several analytical and graphical methods are used:

1. Limiting Zone for Prestressing:
The limiting zone (or safe cable zone) defines the region along the span of the beam within which the prestressing tendon must be placed. The boundaries of this zone are derived directly from the allowable stress limits (tensile and compressive) at both the transfer and service stages. If the eccentricity of the prestress tendon profile lies entirely within this limiting zone at every section, the resulting flexural stresses are guaranteed to remain within the allowable limits.

2. Magnel’s Graph:
Magnel's method is a graphical approach used for the design and check of prestressing force and eccentricity. By setting up stress inequality equations at a critical section (two equations for the transfer stage and two for the service stage), we obtain relations between the eccentricity ($e$) and the reciprocal of the prestressing force:

$\frac{1}{P}$

Plotting these four relations on a graph of $\frac{1}{P}$ vs. $e$ yields a safe zone bounded by four straight lines. Any coordinate combination of $\left(e,\frac{1}{P}\right)$ chosen inside this zone ensures that the allowable stress limits are satisfied at both stages.

3. Load Balancing Method:
Introduced by T.Y. Lin, the load balancing method is an analytical design tool. In this method, the transverse thrust exerted by the curved or draped prestressing tendon is calculated to balance a specific portion of the external load (typically the self-weight and a portion of the live load). Once the balanced load is calculated, the net stresses in the beam are determined under the unbalanced load system and verified to ensure they do not exceed the allowable stresses at either the transfer or service stage.

Why "Hoyer effect" is incorrect:
The Hoyer effect refers to the expansion of prestressing steel wire at the ends of a pretensioned concrete member upon the release of tension. This lateral expansion creates a wedge action, which enhances the bond strength between the steel and concrete within the transfer length zone. It is a local anchorage mechanism and is not used to check overall flexural stresses at the transfer and service stages of the beam.