Confined Masonry · Two Stories
Torsional eccentricity reached three times the permissible code limit. Discovered inside the FEA numerical model, not on site during construction.
01 The Problem
When the center of mass diverges from the center of rigidity, seismic force doesn't just push: it twists. Building codes limit this offset to 10% of plan dimension. Two of the structure's four calculated eccentricities violated this ceiling—one by nearly triple.
Eccentricities in meters measured on the numerical model. The code limit is 0.10 × plan dimension per axis. Resolution was achieved by adding strategic shear walls rather than oversized sections: four walls in total across both building wings.
02 Structural Walls
Fig. 01 — Exploded Axonometric View by Story
Fig. 02 — Ground Floor · 27 Walls
Fig. 03 — Upper Floor · 36 Walls
These are not screenshots. 239 nodes and 15 cm solid panels are parsed from the calculation model, projected to planar coordinates, and classified by elevation range. 1-meter background grid.
03 Structural Regularity Criteria
An irregular structural layout is not prohibited: it is penalized. The geometric re-entrant corners exceed 20%, so the seismic behavior factor is penalized with α = 0.8, elevating design seismic demand accordingly.
Plan symmetry of mass and stiffness
CompliesHeight to narrowest plan dimension ≤ 2.5
0.42Aspect ratio (length / width) ≤ 2.5
CompliesRe-entrant corners ≤ 20% of floor plan
irregular · α = 0.8Rigid floor diaphragm at each level
losa V+B 25 cmDiaphragm slab openings ≤ 20%
15.9 %Story mass variation within permissible limits
Complies04 Building Code, Article by Article
Our calculation reports don't say «complies with building code». They cite the exact article, what is required, and what was engineered.
Additionally: CFE MDOC 2015 for seismic and wind actions, and NTC-Masonry 2023 for confinement detailing. Tie-column spacing on parapets complied with the special provision allowing spacing s ≤ 4.0 m when height H ≤ 1.5 m.
05 Seismic Design Criteria
Zona B del Golfo, margen pasivo, suelo firme. La aceleración del terreno es modesta — pero la mampostería confinada tiene un factor de comportamiento de apenas Q = 2, así que la fuerza de diseño no baja tanto como parecería.
And with the irregularity penalty factor, lateral design demand increases proportionally.
06 Modal Response Spectrum Analysis
El primer modo, con periodo de 0.213 s, moviliza el 96 % de la masa en Y y apenas el 0.2 % en X. En X hacen falta tres modos para pasar el 90 % que exige la norma. Esa asimetría es la misma que aparecía en la excentricidad — y por eso los muros que se añadieron fueron en esa dirección.
Fig. 04 — Cumulative Modal Mass · Response Spectrum Analysis
La deriva de entrepiso queda en la mitad del límite en ambas direcciones. Método: modal espectral, con verificación estática por tratarse de un Grupo B2.
El cortante dinámico salió bajo. El espectral daba 5.27 t en X y 5.08 t en Y, contra un umbral de 7.50 t — el 70 % del estático. La norma obliga a escalar el sismo de diseño hasta ese 70 %, y así se hizo.
El peso hubo que reconciliarlo. El modelo reportaba 454.7 t y la bajada manual 412.8 t: 9.2 % de diferencia. Dentro del 10 % aceptable, pero sólo se sabe después de hacer las dos cuentas.
07 Gravity & Dead Loads
In load-bearing masonry, live load is rarely decisive: dead load governs. The mortar topping, slope fill, and roof finishes weigh nearly as much as the slab itself—and that mass is what the seismic acceleration accelerates laterally.
Values in kg/m². Roof live load reflects an inaccessible roof deck; second floor reflects residential occupancy. Slab weights correspond to the 25 cm precast joist and block system specified in the project.
If everything passes on the first run, you probably didn't look closely enough.
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