1.1 – Basis of Design

What is a Basis of Design (BOD) Document?

The basis of design document is an essential part of the performance-based seismic design documentation. The purpose of a BOD Document is to,

• State deviations from code requirements (exceptions/enhancements). For example, if you have utilized the results from a latest research to verify the design methodology of coupling beams, it should be referenced in the BOD document.
• Describe methods justifying deviations from the code requirements by explaining your thought process and approval from the peer review committee.
• Content includes descriptions of structural systems, design procedures, performance objectives, modeling methods, and acceptance criteria.
• Should be a stand-alone document with references to required details.

Scope and Presentation

Document is standalone with all the possible references that were used to carry out design and analysis for this particular project. No presentation of structural engineering results are included in this document.

BOD is included in the design drawings for building owner reference, especially if code exceptions are taken.

Incorporation in Drawings

The engineer should include the BOD in the design drawings for future reference, particularly if code exceptions are made. It is also highlighted in architectural building site permits in some jurisdictions.

Submission and Review

The BOD is submitted for peer review and local governing authorities. The process of review and modification to the BOD occurs early in the design process. Sometimes BOD is also revised through the design process to align with the final design.

Here is the Basis of Design document for the building “Salt Tower” we are designing.

Basis of Design for Salt Tower

1. Project Description

The Salt Tower is a new 42-story mix use building in downtown Salt Lake City, Utah (US). The building will provide retail space on the first two floors, office suites on 15 floors, and residential spaces with a mix of studio, 1-bedroom, 2-bedroom, and 3-bedroom apartments for the rest of the floors. The top 3 floors contain private luxury residences utilizing the entire floor area.

The structural systems for the various building components are summarized below:

1.1 Foundation

The foundation system consists of a single mat foundation supporting gravity and lateral elements. The mat foundation is designed for hydrostatic pressures. Foundation elements are anticipated to range from 36” thick to 84” thick dependent on demands.

To maintain support on shallow foundations, additional over excavation of the site is required to reach bedrock and remove existing Tolman substructure as required. The over excavated area shall be filled with on-site crushed concrete and mixed with 20% on-site native soils as recommended by the geotechnical engineer of record.

1.2 Gravity Framing

The substructure is primarily concrete construction, including concrete mild-slabs spanning between concrete columns and basement walls. Where spans/ loading exceeds the limitations of flat slab, wide-shallow beams are used to supplement. This selection is based on the requirements to retain significant lateral earth pressure from the east side of the site, as well as the need to support significant landscaping loads at Level 1. In addition, the shallower structural depth compared to steel construction will benefit the overall program due to the MEP routing below grade.

The gravity system for the super structure consists of 8” post-tensioned nominal weight concrete slab spanning between concrete columns and the core walls. Vertical columns consist of a combination of 24” square reinforced concrete columns on an approximate 24-ft x 24-ft grid system.

1.3 Lateral-Force-Resisting System

The lateral force-resisting system (LFRS) consists of a concrete core wall system with coupling beams spanning along building Y-direction. The base shear from the core wall system is transferred to the mat foundation at the seismic base level.