Choosing Your Structural Soulmate

February 6, 2020

By: Emily Dunham, PE and Win Bishop, PE

At the start of every project, when the architect’s floor plans are only beginning to take shape, structural engineers are making game changing decisions about the building. They are deciding the structural framing system that will bring the architect’s vision to life and make it stand.

Determining the structural system for a new building requires the compilation of all kinds of facts regarding the building, all of which must be considered before selecting the system. Specific considerations start with where the building will be located in order to understand the environmental conditions the structure will have to withstand. This includes the likelihood of the structure to experience extreme wind, earthquakes, or the accumulation of snow. This information is also coupled with the overall size and shape of the building to understand the impact of these environmental factors. Additional considerations include the intended use of the building and the functionality required by the future occupant. This leads to developing the gravity and lateral load requirements that the structure will be required to resist.

Other building considerations that impact the choice of a building system include the likelihood of vibration, required fire ratings, allowable defections, the building self-weight, and building geometry (floor-to-floor heights, building height, overall building dimensions). The cost and availability of different building materials can also play a role in the selection of the building system.

All of this information is used to determine a cost effective, constructible structural system for the building, which is confirmed by collaboration with the architect and contractor.

The four main building systems include concrete, steel, masonry, and lightframe construction (wood and lightgage steel). Each have distinct advantages and challenges.

Concrete systems tend to be heavy structures. This is advantageous when dampening against vibrations is needed, like in hospitals or schools. The high self-weight can also be a detriment as it increases the seismic demands on the structure. Thankfully, concrete is also very stiff, which helps control horizontal drift due to wind and seismic events. Another benefit of concrete is that the fire resistance of concrete is provided with the concrete itself. Additional applied fireproofing is not necessary with properly dimensioned concrete framing. A concrete system can also be advantageous in situations where floor to floor heights are limited as concrete floor systems can often be thinner than a steel system. Cast-in-place concrete framing systems tend to be more economical in structures taller than 3 floors due to the opportunity to reuse the forming system for multiple concrete pours.

Steel systems are light compared to concrete. While the lighter self weight is beneficial in many aspects, this reduces the inherent vibration dampening capacity of the system and specific measures may be needed to minimize the vibrations felt by the building occupants. Steel itself is not fire resistant, but fire resistance can be achieved with applied fire-retardant materials. Steel structures are more flexible than concrete structures and utilize steel frames to resist lateral loads. X-braces are the stiffest and most economical braces, but these can conflict with doors or windows. As an alternative, moment frames can be used which eliminates the possibility of conflicts with doors or windows. The use of moment frames also provides more flexibility for future renovations. Unfortunately, moment frames are much less stiff than x-braces and can be expensive due to the labor involved in the fabrication and installation of moment connections.

Masonry, often in the form of CMU (concrete masonry units), has great bearing and shear capacity and behaves similarly to a concrete system. While CMU has less strength than a concrete system, masonry tends to be cheaper than concrete and the units allow for assembly in situations in which placing concrete would be more difficult. Masonry also does not require formwork and the reinforcement is significantly simpler than a concrete system. As with concrete, masonry does not require additional fireproofing measures to be added. Masonry is limited in that it is only available in discrete sizes which must be considered and worked around during the design process.

Light frame construction is advantageous as it is lightweight, quick to erect, and cheaper than the more robust structural systems. By its inherent nature of being a load bearing wall system, coordination of the structural system and the floor plan is simplified as the walls of the floor plan become the structural system. Lightframe is limited in multi-story buildings as the wall details become more complicated and the additional weight becomes challenging to the material. Fireproofing is also an issue and care must be taken to ensure the details are appropriate to achieve the required fire rating.

There are many factors in choosing a structural framing system, but through the process, one system tends to rise to the surface as being the most logical for the structure being evaluated. Also, as technology advances and buildings are becoming more unique, we find ourselves using multiple systems in a single structure and even hybrid systems in order to utilize the best aspects of each system.

No matter what the intended structure is, we will find a way to frame it and make it stand.