As structural engineers, we are often asked about the “constructability” of a project. Yet some of our clients may not completely understand the notion of constructability, or when it is best to consult a structural engineer about the constructability of a project. Let’s take a deeper dive into this issue.
What is constructability?
The Construction Industry Institute (CII) defines constructability as “the optimal use of construction knowledge and experience in planning, design, procurement, and field operations to achieve overall project objectives.” This is why consulting with a structural engineer when renovating or constructing a new structure is so very important.
When should structural engineers be consulted about the constructability of a new project?
The sooner the better. The ability to influence the outcome of a project without a major financial impact is the highest at the beginning of a project and decreases exponentially over time. Conversely, the cost of implementing changes to a project increases exponentially over time.
Constructability versus efficiency
Many aspects of efficient construction overlap with constructability, so for this blog, we will consider that they are one and the same.
Constructability is not value engineering
The major difference between constructability and value engineering is timing. Value engineering is typically performed after substantial design decisions have been made. Constructability is typically performed prior to the establishment of a defined scope and during the early planning and design phases.
Deciding on the best structural system
The biggest impact that a structural engineer has on a potential project is deciding on the best structural system. Let’s take a closer look at two of the major construction systems – concrete and steel – and learn some key constructability take-aways.
Select one framing system and use it throughout the structure – For each framing system used, a separate forming system will be necessary. This means additional costs associated with the formwork and mobilization will accrue, as well as a learning curve for the construction personnel.
Orient one-way structural members to span the same direction throughout the entire structure – Structures that are detailed with one-way structural members oriented in the same direction throughout, tend to be constructed most efficiently because there is less confusion and fewer mistakes.
Use modular formwork – Modular forms have been used to form large areas of walls or floors where the forms can be moved in large sections and reused many times.
Arrange columns in a regular pattern – Arrange columns in a regular pattern throughout each floor level of the structure, as well as vertically. This practice enforces consistency of the structural members, which also dictates that the formwork and reinforcement layout are consistent.
Use a consistent column size – It is more efficient to use the same size column throughout a structure’s height. Rather than varying the column size, varying the number of bars and the concrete compressive strength is more efficient.
Use beams and joists of the same depth – While using beams and joists of the same depth throughout a structure may seem wasteful, just the increased cost of shoring – without a separate set of forms – will exceed the costs of concrete and reinforcing. Also, using the same depth allows all the formwork to be the same, and assists in reducing interferences between the structure and mechanical system.
Use standard-shaped forms – Avoid shapes that must be either fabricated by the form supplier or customized by carpenters in the field.
Use the same floor-to-floor heights throughout the structure – If changes in floor-to-floor heights are necessary, reduce the heights in the upper stories. Cutting the column form down in length is easier than adding to it.
Use shear walls around the elevator and stair shafts for lateral resistance – Shear walls are more economical when column size or beam depth have been constrained, or when column layouts are not on a uniform grid.
Maximize economy with a bay length of 1.25 to 1.5 times the width.
Follow the rules of thumb for minimum depth-to span ratios:
- Beams for typical framing – 1:24
- Beams subject to vibrating activities or equipment – 1:16
- Girts – 1:50
- Purlins – 1:32
Use deeper girders to simplify connections of in-fill beams – This will simplify the connection and eliminate the coping of the bottom flange of the in-fill beams.
For long-span truss framing, follow the rules of thumb for minimum depth-to-span ratios:
- Roof truss > 1:12
- Floor truss > 1:10
For long span truss framing, consider using a Warren truss – Generally a Warren truss will weigh less than a Pratt truss.
For multi-floor framing, use a wide flange (I-shaped) column – Splicing a wide flange column is simpler than splicing a tube section.
For single level framing, use a hollow structural section (HSS) column.
For lateral bracing, use braced frames instead of moment frames – Moment frame members will require deeper and heavier sections to resist the same lateral forces versus a braced frame.
Use the inverted V-brace (chevron) instead of X-bracing – With an X-bracing system, the bracing members are longer, and do not allow a possible aisle space in the middle of a brace frame.
If you have questions about the concept of constructability or would like to speak with one of our engineers about the constructability of your upcoming renovation or construction project, please contact Top Level Engineering at 703-738-9913 to schedule a consultation!
“Constructability of Structural Steel Buildings,” David I. Ruby, AISC Design Guide 23, 2008
“Constructability Maximizing Simplicity,” David I. Ruby, Structure Magazine, June 2006
“Tips to Take Your Team to the Top,” Matthew D. Brady and Cliff Schwinger, Modern Steel Construction, February 2014.
“ETN-C-1-10 Economical Reinforced Concrete Construction,” CRSI
“Current Trends in Economical Concrete Construction,” Jim Delahey and Brad Christopher, Structure Magazine, July 2007