Cap. II - P12: ETABS: Estudio de Losa Aligerada Tipo SHELL (Ribbed) y MEMBRANA (Espesor Equivalente)

Overview of Aligerated Slabs

Introduction to Aligerated Slabs

• The discussion begins with a focus on aligerated slabs, emphasizing the importance of understanding the weight of bricks used in these structures.

• A mathematical model is introduced to illustrate basic structural elements, including columns and beams, which are essential for understanding slab design.

Defining Aligerated Slabs

• The speaker defines an aligerated slab as having specific dimensions (20 cm thick), highlighting its unique properties compared to solid slabs.

• Two modeling methods for aligerated slabs are mentioned: "slap section" and "dex section," indicating different approaches to defining these structures.

Modeling Techniques

• The use of software tools like River is discussed for modeling aligerated slabs, allowing users to specify dimensions accurately.

• Standard dimensions for bricks and beams in Peru are referenced, stressing the need for accurate input in software models.

Calculating Concrete Volume

• The process of calculating concrete volume based on standard dimensions is explained, using a 1m x 1m area as a reference point.

• Key variables such as the number of bricks and beams are identified as critical components in determining concrete volume.

Importance of Weight Considerations

• The relationship between brick dimensions and overall slab weight is emphasized; this affects calculations related to concrete volume.

• An expression relating concrete volume to height (H), which remains an unknown variable during calculations, is introduced.

Standards and Normative Guidelines

Understanding Local Standards

• Reference is made to local Peruvian standards that provide guidelines on typical thicknesses for aligerated slabs.

• Common thicknesses such as 17 cm and 20 cm are highlighted, showing their relevance in practical applications.

Adjusting Calculations Based on Standards

• By knowing total slab height and subtracting standard values from regulations, one can determine necessary adjustments for accurate concrete volume calculations.

Weight Analysis of Aligerated Slabs

Comparing Manual Calculations with Normative Values

• A comparison between calculated weights from manual methods versus normative values reveals discrepancies that must be addressed.

Implications of Underestimating Weight

• If brick weight isn't accounted for properly within models, it could lead to significant underestimations affecting structural integrity assessments during seismic analysis.

Shell vs. Membrane Models

Differences Between Modeling Types

• Distinctions between shell-type models (like River type shell), which require additional weight considerations due to unmodeled bricks, versus membrane-type models are discussed.

External Load Consideration

• When using shell-type modeling, external loads representing brick weight must be added manually; failure to do so can result in inaccurate structural assessments.

Transitioning Between Model Types

Using Equivalent Thickness Concepts

• Discussion shifts towards using equivalent thickness when transitioning from aligerated slab models into solid ones while maintaining structural integrity through proper load distribution.

Conclusion on Modeling Practices

• Emphasis is placed on ensuring all relevant weights (including those from bricks within the structure itself or modeled externally as loads), must be considered regardless of whether a shell or membrane approach is taken.

Understanding Lightweight Slabs in Structural Engineering

Key Concepts of Lightweight Slabs

• The speaker emphasizes the importance of a lightweight slab with an equivalent thickness, noting that it corresponds to a weight of 300 kg per square meter for a 20 cm thick slab according to standards.

• In SAP software, modeling lightweight slabs is limited; it does not allow for drawing beams like other programs. Users often resort to using equivalent thicknesses to represent lightweight slabs.

• When using equivalent thicknesses, it's crucial to account for the weight of bricks, which is typically included in the design process.

Connection and Support Considerations

• The connection between lightweight slabs and beams is likened to simple support, while solid slabs may behave more like fixed supports due to their greater steel content.

• The interaction between beams and lightweight slabs can be modeled as simple supports; however, some engineers treat them as fixed supports based on coefficient methods.

Design Implications of Negative Moments

Steel Reinforcement Requirements

• Both positive and negative reinforcement must be considered when designing multi-panel lightweight slabs due to moments generated at intersections with beams.

• Engineers need to include additional reinforcement (bastones) in areas where negative moments occur due to connections between lightweight slabs and beams.

Moment Distribution Analysis

• Generally, solid slabs exhibit higher negative moments compared to lightweight ones because of their monolithic connections with beams.

Modeling Techniques: Shell vs. Membrane Elements

Defining Element Types

• When defining elements in modeling software, users can choose between shell or membrane types for equivalent thicknesses. Each type has implications on how loads are handled.

• Using an equivalent thickness as a shell allows for internal incorporation of brick weight without needing separate load considerations.

Practical Applications

• The choice between shell and membrane impacts how engineers approach design; understanding these differences is essential for accurate modeling.

Manual Design Approaches

Feasibility of Manual Calculations

• Manual design calculations are feasible but require careful consideration regarding the contribution of different elements when comparing solid versus lightweight designs.

Impact on Shear Forces

• Differences in effective thickness impact shear forces significantly; thinner sections may not perform equivalently under load compared to thicker sections.

Addressing Brick Weight Concerns

Importance in Structural Models

• Understanding brick weight's role is critical since most structural models will involve lightweight slabs that necessitate careful attention during design phases.

Behavior Under Load Changes

Effects of Thickness Reduction

• Reducing the depth of a shell element alters its behavior towards resembling that of a membrane under load conditions, affecting overall structural performance.

Diaphragm Functionality

Role in Structural Integrity

• Lightweight slabs serve as diaphragms that help tie together structural elements ensuring stability across floors during loading scenarios.

Summary Insights on Modeling Choices

Comparative Analysis

When modeling light structures such as decks or membranes:

• Deck options only function as membranes without solid counterparts available.

• Users must define dimensions carefully while considering material properties unique to each type (e.g., steel plates).

Deck Modeling Specificities

Unique Features

• Deck systems allow direct input for brick weights without requiring adjustments through equivalent thickness calculations unlike other methods used previously discussed.