The Structural Design Process of a Building

Originally published by the following source: SBC MagazineJune 10, 2019
by Jess Lohse

   

Editor’s Note: The following article is part of SBCA’s Construction Industry Workflow Initiative, which is exploring how the construction industry currently functions and the role component manufacturers play within that structure. Your feedback is encouraged to help us complete this project, email workflow@sbcindustry.com to provide your input.

Structural engineers, often referred to as an Engineer of Record (EOR), are positioned early in the construction design process ensure the structural viability of buildings designed by Architects or Building Designers. Certain buildings are exempted from the legal requirements for the use of an Architect or Engineer. Generally these buildings are designated as 1 and 2 family residential structures designed within the prescriptive code. Buildings designed under the IBC, exceeding certain provisions of the IRC or exceeding legal exemption requirements will employ the use of an EOR. Structural engineers are typically brought into a project by the architect/building designer and work on behalf of the project owner and remain engaged throughout the construction of a structure to review and accept deferred submittals and RFIs (request for information) for conformance with the structural plans and specifications, monitor construction and perform special inspections as defined on the permit or as contracted to undertake. Once engaged on a project an EOR will typically work through the following processes:

  1. Conceptual Design
  2. System Design
  3. Element Engineering
  4. Iterative Design & Drafting
  5. Construction Administration

Conceptual Design

  • Review Arch drawings
    • Unit types
    • Bearing walls stack?
  • Location requirements
    • Soil report
    • Exposure Category
    • Wind Load
    • Seismic Load
  • Initial design of building elements
    • Roof, wall & floor layout per Arch drawings
    • Footings & Slabs
    • Bearing walls, Beams & Columns
    • Review MEP conflicts

Conceptual Design

An initial step a structural engineer will take is to review the drawings produced by the architect/building designer. The engineer will look at the various types of units on a larger multifamily project or the variety of room uses in a larger single family home or commercial structure to have an idea of the various uses of a structure. Consideration will be given to potential bearing walls, obviously inclusive of the exterior walls, but potentially to utilize interior walls should the need arise to distribute loads through the interior of the structure.  If the structure has multiple levels, the engineer will note if interior walls fall on top of each other, commonly referred to as ‘stacking’, to efficiently transfer loads between levels.  This information will be referred back to the architect to make any necessary adjustments.

The EOR will determine site specific requirements for the structure that are dependent on its location. This will include a soil report to determine potential footings, which exposure category to design to, applicable wind loads as well as seismic considerations. Once the environmental factors have been determined, the engineer will perform an initial design of building elements such as roof, wall and floor layouts per the architectural drawings, footings and/or slabs, bearing walls including beams and columns, and identify potential mechanical, electrical, and plumbing conflicts. 

System Design

  • Understand Load Path
    • Gravity
    • Lateral
    • Uplift
    • MEP conflicts
  • Initial Designs
    • Roof System
    • Walls
    • Floor System(s)
    • Foundation
  • Broad Analysis for construction documents

System Design

Once an initial conceptual design is complete, an engineer will turn their attention to system design in a top down manner. An understanding of the structure’s load path is imperative with specific considerations given to gravity loads, lateral loads, and uplift on the various elements within the structure. Once the engineer has a general idea of the structure’s load path, they will begin initial designs of various structural systems. Working from the top down, engineers will produce initial designs first for the roof system. Then the walls including the gravity and lateral force resisting systems and any required beams and columns will be designed followed by the floor systems and repeated as many times a necessary, dependent on the number of levels and different unit types in the structure. Once the roof, walls and floor system has been designed attention will turn to the foundation and footings, leveraging information from the soil report derived in conceptual design. This broad analysis information is compiled into initial structural construction plans.

Element Engineering

  • Accurate dimensions
  • Specific member analysis
  • Coordinate geometry defined in CAD
  • Analysis model created (SAP 2000, STAAD, RAM, etc)
    • Dead loads
    • Live loads
    • Wind loads
    • Seismic loads
  • Internal forces
    • Axial forces
    • Bending moments
    • Shear force
    • Drag force
    • Combined forces
  • Initial Member Sizing

Element Engineering

The element engineering phase begins with the engineer ensuring accurate dimensions for the various portions of the construction project through geometry coordination as defined in 2D or 3D CAD software. Trusting the dimensions are accurate, the engineer will begin specific member analysis for the defined spans, such as calculating roof loads that are transferred to exterior and interior bearing walls. The lateral force resisting systems generally require the most engineering time combine with window and door perforations that require headers and beams. This analysis combines gravity, wind uplift and lateral loads paths. This load path analysis will also be applied to the floor system and foundations, giving the engineer a general idea of the variety of loads within the structural elements and where additional attention will be required in subsequent design phases. To aid in this analysis, engineers will use specific software applications geared towards structural design such as SAP 2000, STAAD and/or RAM. This analysis software will allow the engineer to apply a variety of loads including dead, live, wind and seismic. As a function of this analysis, the engineer will be able to determine axial forces, bending moments, shear force, drag force and the combined forces. Once the forces have been determined, the initial member sizing can commence, allowing the engineer to establish a ‘rough draft’ to further refine in downstream design steps.

Iterative Design

  • Design to code
  • Redesign Analysis model
    • Incorporate more accurate load paths
  • Fine Tune Final Designs

Drafting

  • Create structural plans
  • Fully detailed

Iterative Design and Drafting

Engineers use an iterative process to fine tune the various elements into final structural element designs. Think of this as repetitive in nature working toward the ultimate goal of an efficient design that meets the variety of requirements the structure’s configuration places on the path that the applied load will need to take to get to the ground. The engineer starts with a broad understanding of the loads on individual elements and narrows the focus until each element and ultimately the entire structure is designed to safely transfer all loads, meet code requirements and provide an acceptable solution that can be signed and sealed. Through this process the load paths are accurate, specific and reliable. With the accurate load paths, drafting can be completed with fully detailed structural plans available for construction. 

Construction Administration

  • Review submittals
  • Obtain approvals
  • Prepare schedules
  • Monitor construction
  • Perform site inspections

Construction Administration

Further in the construction process, the engineer is often called upon to review RFI and deferred submittals, obtain code approvals or prepare construction schedules. Certain products, such as roof trusses, are considered a deferred submittal. This means the engineer allows the designs to be created by others and sealed by a specialty or delegated engineer.  Those sealed designs are reviewed by the EOR and either approved for manufacture or returned for revisions. Beyond review of conformance to the structural design, engineers will also monitor construction progress on behalf of the client and will often perform site inspections to make sure construction process is progressing and installation of products is without errors.