Knowing what structural system (floor) is best

[StrEng007]

When you all design the horizontal portion of your structures (particularly floor systems), how do you determine which system is actually the best in terms of cost and performance? We're all taught, or self-taught, how to design different systems out of the most conventional materials... but knowing which one is most appropriate takes a certain amount of seasoning.

Most of the time, I find this decision being guided by the GC/Arch (depending on contract) who has the reigns in appropriate cost and performance for fire ratings etc. A lot of these decisions are also based on past projects, what is deemed acceptable per jurisdiction, and what a contractor is familiar with.

However, I often find myself in situations where I'm not sure which floor system is appropriate, such as pre-engineered floor trusses, concrete slabs 1-way or 2-way, composite joists with topping slab (Hambro). I've used all of these systems on different structures (Note: I don't mess around with post-tensioned slabs).

I'm really interested to know how you all approach this. As a reference, my of my work consists of mid to high end residential (up to 2 or 3 stories) and moderately sized commercial (up to 2 stories).

 

[phamENG]

Mid to high end residential will mostly depend on floor layout. If I can get away with all dimension lumber, I will. But that's pretty rare. So I default to floor trusses. (They're not pre-engineered, by the way...they are engineered for the specific job.) They are more forgiving for installing services and perform well. Also, the contractor doesn't have to modify them on site. And if they do, most are aware that they can't without approval. I-joists, on the other hand...yikes. I-joists have their place in the lower tiers of the "mid residential" category. Tract homes, for instance. Sometimes I'll use them when a house has a very complex floor plan with lots of lengths of joists and not much volume of any one in particular. The trusses get their advantage from quantity - so if you're just making 3 of each truss, that will get really expensive really fast. But they can order a truckload of 16" I-joists and cut them to length.

Commercial...depends on the goals and the architectural design and fireproofing as you say. For types of floor slabs, it'll depend on the available column layout. I let the aspect ratio of slab panels drive it as that will dictate the behavior anyway. Composite systems will come into play when you need to limit floor to make room for other services or maximize lease-able space.

 

[StrEng007]

They're not pre-engineered, by the way...they are engineered for the specific job

Yeah, sorry. I'm in the habit of calling them pre-engineered, when in fact I'm really saying they are a delegated/deferred engineering item. Sometimes I get requests to either design wood trusses, or heavily modify them to which I tell my clients they need to use "pre-engineered"... eh, semantics.

phamENG, your thoughts on Hambro? I know you used to be a FL engineer but not sure if your work ventured down to Palm Beam, Broward/Miami Dade counties.

 

[jerseyshore]

In higher end residential (even middle residential) around here everyone uses I-joists. 2x floor joists are only used in very basic homes these days.

Depending on arch/ GC preference, I've seen more floor trusses used the last few years. Not hard to get and great for utilities. I love when they want to use floor trusses, less for me to engineer.

For wood-framed commercial we usually see I-joists for smaller projects and trusses for 3+ story buildings.

For steel framed commercial, as pham said, there's a lot of factors. I've used the open web Marino Ware joists for smaller projects, but generally fireproofing will dictate slab and construction type and then work backwards from there.

 

[phamENG]

I know you used to be a FL engineer

Not quite. I grew up in Florida - St. Pete, to be exact - but enlisted at 18 and now my wife won't let me move back. I am licensed in Florida, but have not had an opportunity to do much work down there. Just little things for friends/family. Nothing down on the Gold Coast.

Hambro has shown up here in Virginia a time or two, but I have not had the pleasure of designing it myself. I have done Vulcraft's composite joist system for a hotel. It was quite effective in getting the floor thickness down to the absolute minimum necessary to run duct work and plumbing.

 

[Eng16080]

For residential projects, wood is almost always the most common and most economical material. By wood I mean some combination of dimensional lumber, I-joists, LVLs/PSLs, and prefab trusses (mainly roof but also occasionally floor). Most residential contractors, at least in my area, are going to struggle with any other material. I always try to use wood, only specifying steel beams and columns where engineered lumber cannot work.

Just introducing a few pieces of steel on a project is often problematic for a lot of these builders. In fact, something I'm dealing with right now concerns a new house that I designed where the builder substituted every piece of steel on the project with engineered lumber, using the local lumber yard for the design, and not telling us!

Anyway, using the material which the local contractors have the most experience with is a huge factor in the decision, sometimes the only factor.

 

[RPGs]

I've done inspections on Hambro composite floors for mutli-family residential apartments. The shop drawings can be a bit complicated, but nothing insane. It's a pretty efficient system with an impressive record.

I am currently specifying Hambro composite floors for the first time on a high-end residential home with ~38 ft clear spans. The design guide was pretty straight forward. I put in a request on their website for assistance and I was contacted within a day or two by a local contractor that was able to help me with basic details for getting a bid set together. The contractor was very responsive and I really like that I can delegate the design to them; all I've really specified at this point was the joist depth using their span tables and some educated guesses for joist seat heights/assemblies for bearing on a mixed bag of steel framing, CFSF bearing walls, and concrete foundation walls. I assume they'll have some feedback and adjustments for me when they start to dig into my bid set, but I think it will go smoothly even though I have some somewhat unusual details for the huge balconies, massive wall openings, and a drive through car park over a basement.

 

[XR250]

Most of my high-end residential customers prefer dimensional lumber if it makes sense for the layout. Even adding a flush steel beam here and there is OK with them.
They are old school and prefer old-school products - which is fine with me as I feel the same way.
Otherwise, I will resort to I-joists or floor trusses.
I don't work for tract builders but most of the ones I see are using i-joists or floor trusses.

 

[RontheRedneck]

I don't think any one floor system is best. There are advantages and disadvantages to each one.


2X lumber is easy to get. You can make a run to a lumberyard and pick some up any time.

2X lumber is the least material cost. For someone building their own home it might be the best option.


I-joists are not quite as readily available. We have a 2 day delivery policy on our stock (11 7/8" I-joists.

They're straight and consistent. Gives you a good flat floor. Long lengths (typically up to 48') make framing the floor faster.

I try to encourage them in places where there are angled walls and each one is a different length. We can lay floor trusses out on the computer and make them look perfect. But that doesn't always translate into them fitting at the jobsite.


Wood webbed floor trusses offer a lot of flexibility. They're typically the highest material cost, but the lowest labor cost. They offer a lot of room for mechanicals.

Lead times can vary widely. The place I currently work at has been several weeks out on floor truss orders.

If you're building a job where prevailing wages apply, floor trusses can look pretty good. They reduce expensive jobsite labor.


Regardless of what's best, the customer often has their own ideas. Some guys hate floor trusses. Others hate I-joists. They all have their preferences. And we do our best to accommodate them.