Seismic Design of Foundations: The 2015 Canadian Building Code Provisions
Notice bibliographique
Résumé
The concept of allowing foundations to rock as a way of limiting the seismic forces that are induced into an existing structure is well known. Currently, designers of new buildings in Canada are permitted to allow their foundations to rock at force levels as low as 50% of the elastic demand without having to consider the increased displacements in the structure, which is a concern. Also, the current rocking foundation provisions are often misused by designers of high-rise buildings having numerous floor levels below grade with shear walls interconnected by numerous stiff diaphragms connected to large foundation walls supported on separate foundations. The results of a study on rocking footings has been used to develop new design provisions for the Canadian building codes - both the loading code and the code for design of concrete foundations. Nonlinear analyses of rocking footings were done using OpenSees. Nonlinear fibre elements were used to model concrete shear walls and the QzSimple1 model was used for the nonlinear cyclic behaviour of the soil. The initial stiffness of the soil springs were calibrated to give an initial rotational stiffness of the foundation consistent with Gazetas’ well-known equation. Clay, three different sands (loose, medium and dense) and rock were studied. Three different building heights (5, 10 and 20 story) and two different mass ratios, which is equal to axial load on the footings as a ratio of the seismic mass, (0.4 and 0.6) were examined. A large variety of each type of building with very different strength shear walls and very different size footings were studied. Ten ground motions scaled to the 2500-year UHS for Vancouver BC, which has similar seismicity to Seattle, were used for each of the buildings. The new Canadian design provisions require designers of all foundations that are not restrained from rotating freely to consider how the movements of foundation will influence the seismic-force-resisting-system (SFRS) and most importantly, the gravity-load resisting structure surrounding the SFRS. The requirement to consider foundation movements is for foundations that are both not capacity-protected by the SFRS, i.e., “rocking foundations,” as well as for foundations that are capacity-protected. Significant movements may occur in the latter case because of the assumed uniform bearing stress in soil or rock that is used to calculate the overturning resistance of the foundation. Simplified procedures were developed to determine the rotation of a foundation in static equilibrium using the concept of an equivalent rectangular bearing stress block as commonly used for bending of nonlinear concrete sections. This simplified hand-calculation approach gives very accurate estimates of foundation rotations when compared with the nonlinear analysis. In addition, procedures were developed for estimating the effective stiffness of a rotational spring that can be used in a linear dynamic analysis to determine the increased displacements of a structure with rocking foundations. The effective linear stiffness accounts for the reduction in stiffness due to footing uplift and nonlinear soil compression, and reduces with increased rocking as a function of the ratio of footing overturning resistance to elastic demand on a fixed-base structure.
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Comment cette classification a été obtenuedéplier
Prédiction machine sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Le volet Gemma est une étiquette directe du modèle pour chaque travail de la base, lue sur la notice réduite au titre. Le volet Codex est un classifieur appris des 10 348 étiquettes directes de Codex et calibré sur les taux pondérés de l'échantillon; les champs sans appui suffisant ne portent aucun appel Codex. Le mode candidate est l'union des deux volets; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont pas des étiquettes humaines.
Scores du classifieur distillé par catégorie (deux têtes)
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,001 | 0,002 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,001 | 0,001 |
| Études des sciences et des technologies | 0,001 | 0,001 |
| Communication savante | 0,001 | 0,000 |
| Science ouverte | 0,001 | 0,001 |
| Intégrité de la recherche | 0,001 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,006 | 0,001 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule source (Gemma direct ou Codex distillé), pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».