Which structural framing systems and materials are most commonly used in the United States for residences? For office buildings?
◦ What factors explain why those dominate?
For residences, wood frames and concrete dominate. Wood is cheap and easy for construction crews to handle, making it the most logical choice for low-rise buildings like single-family residences. Apartment buildings are often larger and taller than building codes allow wood alone to go, so they need a sturdier frame. Often, they have a concrete podium to support upper floors made of wood, or have a reinforced concrete frame throughout.
For office buildings, steel is a more common choice. Steel spans farther and needs fewer, thinner columns than wood or concrete, allowing for wide open floor plans. This is preferable because offices want open floors they can rearrange, instead of being locked into many small rooms.
◦ Why did you choose your system, and what did it cost you?
I chose to have a concrete podium with steel on top. Since my building is so tall (8 stories at the tallest), I needed a stronger structure than wood framing would provide. Concrete at grade gives the strength to carry the transfer girders where the upper columns don’t line up with the lower ones. The steel above keeps the floors lightweight. In order to achieve this, my ceilings had to drop below the framing, my columns had to be shifted off the grid lines to avoid my windows and doors, and my core wall had to be moved to clear the stair. Additionally, steel needs fireproofing and a separate floor system (composite deck), and a joist-and-deck floor gives weaker sound separation between units than a concrete slab.
You sized members by judgment rather than analysis.
◦ What did you base your first guess on?
◦ Why is it safer to guess large and shrink than to guess small and grow?
◦ At what point in a real project does the engineer take over from the guess?
Design teams link and share models rather than working in one file.
◦ What are the advantages of linking?
Linking allows a team to have different models that serve different purposes, such as architectural and structural, that are all linked together. Updates to one model are reflected in the models that link to it. This is a more efficient way to model sub-parts of a structure, such as keeping the beams and columns in their own structural model.
◦ What errors does linking make possible that a single model would not?
If one model gets altered incorrectly, then all of the other models that link to it will update to incorporate that error. Models can also get out of sync if one is changed and the others are not updated to match. There will also be clashes between models where parts overlap, even when this is not an issue in real life.
◦ Why is getting shared coordinates right at the start so much cheaper than fixing it later?
Each linked model knows its location based on a shared reference point that all of the models use. Setting the location correctly at the start means every model lines up, while changing the coordinates later on would mean a user would have to update every model and link.
Not every clash is a problem.
◦ On what basis did you classify a clash as an issue versus not an issue?
Clashes were an issue when they would directly impede the functionality of the building. For instance, when I first created my grid, many columns landed in windows, balcony doors and storefront, and one of the concrete cores cut through a stair. Similarly, on the foundation, the footing under one of the columns overlapped the core’s footing and the elevator shaft. In these cases, I had to re-evaluate my system and find ways to change it to avoid these problems. Other clashes, such as a beam sitting on a partition or a column passing through a ceiling, are not issues. They come from modeling the structural and architectural elements separately. In real life, the building will be one structure, not two models. The exception is when both models drew the same element, like the duplicate core walls, which I kept open as an issue.
◦ Who should have authority to close a clash on a real project?
Stakeholders should come to a consensus about which clashes are and are not issues, but the final authority should lie with the engineers and architects responsible for the elements involved, with the builders helping review them. It is their job to critically evaluate the structure, find errors, and ensure its safety and functionality.
Your beam depths just constrained next week's ductwork.
◦ Where in your building is the ceiling plenum now tightest?
◦ What would you give up to get it back?
◦ Should the structural engineer have consulted the mechanical engineer before setting spans?