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Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

(OP)
I have an interesting problem. It's something that I haven't dealt with before, but perhaps some people on here who typically deal with commercial wood designs can steer me in the right direction. It's a bit longwinded, but here goes.

First off, my specialty is industrial steel structures. I have done only a very few wood-related designs in my career, and they were pretty basic.

A local volunteer fire department is building an addition. This new addition is roughly 70' x 65' in plan and is a typical gable roof with 18' eave height and 3:12 roof slope. It's a pretty decent size structure. This building has two (2) elevated levels besides the ground floor (slab) where the fire trucks are stored. The first elevated level is for offices, which is supported by wood floor trusses, and a 100 psf live load is being used. The second elevated floor is smaller, located within the wood roof trusses, and is used for minor storage (20 psf LL).

Early on, before I became involved, some people within the fire department procured the services of an architect, who began the "layout" this building. Originally this building was to be wood framed, so the architect got a local wood truss supplier involved. The wood truss supplier "designed" some floor & roof trusses along the way (which only supported some gravity loads), but they limited their design to only those wood items. They did not handle the supporting wood walls, roof rafters, or any other part of the building. (The architect was going to take care of that somehow.) In their design, they assumed a few steel beams with posts that hold up their floor trusses on the first elevated floor.

Since some "minor" steel design was neededm the fire department & architect called the company I work for and ask if we can size a few steel beams and columns for them. Sure, no problem. Once I get involved however, I raise the same question that any structural guy would - what about lateral loads and stability? Who was handling that? This building layout, as large as it was, had no interior shearwalls. Instead, the plywood floor diaphram would have to transmit all the load out to the exterior walls. However, they're not going to provide much help since the large garage doors blow right through them and there's nothing there to act as a shearwall.

At this point, the architect wisely decides that they need to appoint a structural EOR for the project. Long story short, I recommend going with an interior steel frame on the lower level (basically a knee-brace frame), which supports (both vertically and laterally) the wood floor & roof system above.

The plan was to have the wood truss supplier design the individual floor and roof trusses (PE stamped) and I would handle the rest, including the incidental wood walls and roof rafters in the upper area.

Here's the problem - the structural frame of this building is a mix between an upper level wood frame and a lower level steel frame. You can't simply separate the two (2) systems....they act together and need to be analyzed as such. The only person involved in this process who is able to perform an overall frame analysis and determine the forces within both frames is a structural engineer (me). The truss supplier is off to the races already "designing" these trusses, but the question is, with what loads? So.....how do I best convey the internal loading requirements for the wood floor & roof trusses to the truss supplier? Are we talking basic stick diagrams with max tension/compression loadings, or what?

This has been a real headache. Any help or guidance would be really appreciated.

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

(OP)
Correction - the roof is 8:12, not 3:12. Overall peak height is about 43'.

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

While I haven't built one myself, I know of many projects that have used a steel podium to support a wood structure above. I think they are pretty common and typically used to help with fire resistance.

In regards to the wood trusses. You, being the EOR, need tell the truss mfr. what loads need to be applied to the trusses. You can easily do this with loading diagrams on the trusses. When the submittal comes in from the mfr. you need to make sure they followed your design requirements. I think once you are done looking at their submittal you will see how thorough they are with their design (at least the ones I see are).

One major thing to keep in mind is that this structure appears to be a occupancy category IV (if you are in the US) which means you will probably need to amplify your wind, earthquake and snow loads (if you even have snow). This will be the most common thing missed int he submittal from the truss mfr.

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

(OP)
SteelPE

That's exactly what I have.....the lower steel frame supports the entire upper wood system. However, in order to figure out the loading on my steel frame (imposed by the wood system), I need to run through a frame analysis of the combined system.

I was thinking Risk Category 3, but perhaps 4 is warranted.

I take it though that my approach seems reasonable?

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

You might want to look at ASCE 7 to make sure you are using the correct Occupancy Category as I believe fire stations are listed under Category 4 (but then again you may not be located in the United States).

Taking on a wood structure is not an easy task if you are unfamiliar with how things get done. After graduation I went to school for a steel fabricator doing design-build work. Last year they went belly up and I have been out on my own. I have been working with another engineering company doing some wood design (we help each other out) and have gotten much better at it.... but I am no where near as good as they are when it comes to designing wood structures. Oh, and I forgot to mention that I used to help frame houses as a part time job through high school and college so I know how they go together.... but know the little tricks is what puts you over the edge.

Simpson makes a ton of specialty products that may help you out. They make pre manufactured shear walls out of wood and corrugated steel. The also make pre manufactured moment frames that may help as well. If those don't work then I don't see what you couldn't engineer a moment frame or some other system that will. All you need to do is to properly get the forces in and out of the systems.... that is the tricky part.

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

(OP)
I'm certainly located within the US.....and you're right, the commentary within ASCE 7-10 lists fire stations in Risk 4.

The wood framing that I would have to deal with is minimal....I would mainly be used to find the forces within the wood framing to provide the wood truss designer some loads to use.

I'll check out the Simpson products......thanks!

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

From my experience, the truss mfr's will only design the trusses for the loads you provide them. It's up to you to design the rest. This includes truss bearing on their supports, horizontal diaphragms, vertical diaphragms (shear walls), bearing walls, headers, drag struts, hold downs etc.

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

(OP)
That's my thought as well. I'm catching a little heat from the truss supplier because "they haven't had to do this before". Excuse me? You've never had to design trusses to another engineer's specifications or consider what role your trusses play in the lateral stabilty of a frame? Seems a bit surprising!

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

What types of loads are you trying to drive through your trusses? I can see vertical (dead, live and snow) horizontal (wind) and diaphragm drag forces at certain locations(wind and EQ). Are you trying to do anything else? Also, do they realize that it is a Category 4 building?

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

(OP)
Nope, that's it. They don't understand why I have to supply the loads.....they're not seeing why I need to extract the internal force data from a frame analysis and tell them what it is. They think they can apply their own dead, live, snow & wind loads to their localized truss design, without consideration of how the truss acts within the overall frame, and that covers it. I'm looking like the overly complicated engineer right now and I'm left scratching my head since this seems to be pretty straight forward to me. Ever hear of P-delta effects? For goodness sake, I surely can't be the first guy they've dealt with that is actually performing a structural analysis on a frame that involves one of thier trusses.

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

Wait until you need to figure out how to get the gable end to work. Good luck with that.

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

(OP)
lol........thanks for the help

RE: Structural Analysis of Building Frame - Upper Wood Frame and Lower Steel Frame

Quote (andysines)

For goodness sake, I surely can't be the first guy they've dealt with that is actually performing a structural analysis on a frame that involves one of their trusses.

I'm having trouble visualizing the problem. Could you provide a rough sketch showing the frame in question, including the wood truss and the forces to be determined in your frame analysis which affect the wood truss design? If they don't understand why you have to tell them the forces, perhaps you should consider substituting steel or glulam members to complete the frame action, leaving them to design a simple truss for dead, live, snow and wind loads.

BA

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