Overview
This lecture explains how to define and apply wind loads in ETABS, focusing on differences between rigid and semi-rigid diaphragms, use of wind load patterns, cladding application, and addresses relevant Q&A.
Diaphragm Definition and Assignment
- Diaphragms represent the in-plane stiffness of floor slabs in structural analysis.
- Two types: rigid (infinite in-plane stiffness, no membrane deformation) and semi-rigid (simulates actual stiffness and deformation).
- Assign diaphragms to floor elements using the Select and Assign menus; rigid diaphragms display as a spiderweb image at the centroid.
Wind Load Pattern Definition
- Wind loads can be assigned based on exposure from diaphragms or directly to wall objects.
- Correct wind load type selection allows ETABS to automatically generate load combinations and scale factors.
- Wind pressure coefficients: windward (default 0.8), leeward (default 0.4) adjust wind pressure calculations.
- Exposure width and direction (angle) determine the portion of the structure exposed to wind; usually applied in X or Y directions.
- Selecting "create all sets" generates all code-specific load cases (e.g., ASCE 7-16 creates 12 sets based on wind angles and eccentricities).
Wind Load Analysis and Output
- Rigid diaphragms apply lateral loads to the center of mass; semi-rigid distribute loads along the perimeter to avoid stress concentrations.
- Output includes joint load displays, story response plots (displacement, shear), and exportable reports detailing calculation variables and load distribution.
Cladding and Manual Wind Pressure Application
- Cladding consists of shell objects with no structural weight, facilitating manual application of wind loads.
- Cladding can be generated automatically based on floor or beam perimeters; manual adjustments are possible for irregular geometry.
- Wind pressure coefficients can be assigned to cladding objects for both windward and leeward directions.
Q&A Highlights
- Different parapet heights can be assigned using separate wind load patterns for each direction.
- ETABS can calculate in-plane stiffness of semi-rigid diaphragms using material and thickness.
- "Depth" in wind load patterns refers to plan dimension, not story height.
- Gust factor calculation is automated for some codes; check user's manual for applicability.
- Rigid diaphragms apply loads at center of mass—including torsional moments.
- Load patterns track different applied loads; load cases combine patterns for analysis.
- Manual wind loads can be applied without diaphragms if desired.
- Wind loads should generally not be applied to both diaphragms and cladding simultaneously.
Key Terms & Definitions
- Diaphragm — horizontal element (usually floor slab) distributing lateral loads to vertical elements.
- Rigid Diaphragm — idealized with infinite in-plane stiffness; no in-plane deformation.
- Semi-Rigid Diaphragm — models actual in-plane stiffness and deformation.
- Cladding — non-structural shell objects used to apply wind pressure coefficients.
- Wind Pressure Coefficient — scalar modifying wind pressure for different exposed surfaces.
- Load Pattern — individual set of load definitions for bookkeeping.
- Load Case — combination of load patterns analyzed together.
- Gust Factor — multiplier accounting for wind speed fluctuations.
Action Items / Next Steps
- Review and assign diaphragms correctly for your ETABS models.
- Define wind load patterns with appropriate exposure and direction.
- Apply and adjust cladding where necessary for manual wind load application.
- Export and review structural response and load distribution reports.
- Prepare for the next webinar on seismic auto lateral load application.