Connecting a W-Shape to a Rectangular HSS

Has the whole thing been analysed as a three dimensional structure? Why would you input a fixed end if you don't know how to provide one.

You need a mentor or you need to hire a structural engineer, failing that, just note these values on the drawing and let the detailer handle it.

I'd be interested to know what holds the trusses upright and capable of resisting thje moment. It sounds as though this moment could cause buckling in the top chord.

This is his Senior Project.

True, probably schoolwork. But if so, he's just asking for methods, not calculations to be done for him. I reckon that can slide.

Something seems wrong here. Why would you have a fixed end moment at each end of the beam? Shouldn't it be a simple span? How can you have a moment of 21.4 k-ft and a shear of only 1.7 k? Have you checked the moment capacity of the HSS? Are you attaching the beam to the 5" wall or the 3" wall? A sketch would help.

I do not use AISC documents as I come from Canada, but I believe Chapters K and J deal with connections.

I don't have the manual on hand, but I'd just like to cast some question on the design. Fixed ends butting up against the HSS doesn't sound ideal.

As far as attachment, you have a choice of bolted plates in conjunction with welded-on and gusseted brackets also bolted, or welding. Depends.

Backing off from #1, above, and addressing the question.

That moment sounds high for a small beam. 22x12"/8"=33kips top and bottom. A fillet weld (going from memory) is worth 0.9K per inch per sixteenth. On the HSS you will only get 2.5" of weld on the 3" side.

33/2.5 = 13k/inch 13/0.9= 15 1/16s" fillet weld, obviously simple plates on the flanges don't work.

I will give it some more thought but I do dislike this kind of connection.

I was looking at the beam, I should have looked at the vertical HSS.

The section modulus of the 5x3x1/4 is 4.29ins³. 22x12/4.29=62ksi. You need to revisit this. Even if you could split it half up and half down, it would still look too high for combination with its axial forces as a truss member.

Can you show us a sketch, I still have the problem visualizing this bridge. A cross section is most needed along with the spacing between beam/vertical frames. A longitudinal section would be almost as useful and an elevation would help too but is not as important as the cross section.

Why not pierce the HSS with a connection plate of appropriate dimensions, weld it to both sides, and bolt it to the WF? I'm not a structural engineer, but I do have a manual from AISC which is very helpful in HSS connection details, "Hollow Structural Connections / AISC Design Guide 24". You can purchase it and then download the document.

I am somewhat following a design example from LRFD Guide Specifications for the Design of Pedestrian Bridges, 2009. In their example, they design a half-through truss bridge. The lateral support is given by a transverse U frame consisting of the truss verticals and the floorbeam. I may have a bad design for this lateral support, but I am still in the designing phase (as this is my first bridge design for my senior project). Pertaining to the fixed end moment, my assumption would be to apply Hf (minimum lateral force from AASHTO ) concurrently with the horizontal wind force to be applied at the top of the truss verticals, which was 2.139 kips. The truss verticals are 10' tall, thus causing a moment of 21.39 k-ft at the bottom if it is fixed to the floor beam. This is where I may need guidance because I would think the connection at the floor beam would have to be able to resist this moment to keep the truss from overturning??

The reason for the high moment and low shear is due to how the slab is connected. The reinforced concrete slab is supported by a two W10X12 beams spanning between truss panels, and these in turn are connected to the floor beam. I know this is confusing but I will post a sketch below.

Should I design a through truss to provide more lateral support to the top chord? As mentioned previously, this is my first bridge design so any input is very helpful.

Thanks to all

wiljhard,

In your sketch, the W8x13 is flush with the bottom of the truss which means it runs into the bottom chord. Why not raise the cross beam to somewhere near the mid height of the truss? That reduces the magnitude of moment at the connection and leaves room for diagonal bracing between the beam and truss. It also puts the top chord at about the correct level for a handrail.

As part of my design requirement, I have to design a roof system covering the bridge so I will need adequate head room. The horizontal force of 2.1 k has included in it the max roof truss horizontal contribution.

Well, why didn't you say so? That simplifies things considerably. Good luck with your project.

Additional information I failed to include:

1.Spacing of floorbeams=7.5' (which is also the panel length). 10 total panels=75' span

2.Factored moment capacity of HSS at 10' =22.1 k-ft which is ok.

3. Factored moment capacity of W8X13 at 10' =23.3 k-ft which is ok.

4. No local buckling occurs in W8X13 and the HSS is a compact section.

wiljhard,

• Have you been giving us factored loads from the start? I thought they were service loads.
• Your trusses seem a bit high, although not bad. Maybe 7.5' or 8' would work and the panels would be nearly square.
  
• Check your HSS capacity again. You are bending about the minor axis.
• Have you considered using wood deck instead of concrete to make the bridge lighter?
• If you are using concrete on steel deck, why not span the deck parallel to the trusses and eliminate the longitudinal beams?
• Are you anticipating prefabricating the structural steel portion of the bridge in the shop, including steel deck?
  
• Even if your HSS was adequate, I don't think it will be easy to connect the WF to HSS as shown because the wall is too weak. Perhaps an easier connection would be to weld the HSS to the top of the beam and reinforce the beam with a split HSS between flanges so that they aligned with the HSS above. This could mean choosing a slightly wider beam flange.
• Don't forget to provide a fence or similar barrier to prevent children from falling over the edge or from climbing up the truss.
  

I have contemplated using a steel deck rather than a concrete. I was amazed when I calculated the weight reduction by using steel as opposed to concrete. It was 45.9 kips lighter. Considering the wood deck you have mentioned, would it be even lighter than the steel deck? The steel deck I have designed for weights apprx. 13.8 psf to give you an idea.

Not much lighter...64mm (2.5") D. Fir deck weighs 340N/m² (7.1psf) while 89mm (3.5") D. Fir deck weighs 480N/m² (10psf).

Hello All,

I should have responded to this thread a few days ago, but I've been tied-up riping apart our kitchen floor....***UGHHHH***

After taking a look-see at your sketch, the entired framing scheme appears to be overly complicated to me. Just my opinion. I'd definitely would revise it somewhat....

First, I suggest that you get rid of the W10's paralleling the trusses, and span the concrete slab between the floor cross beams, and therefore let them carry the floor loads. This configuration will save weight and money. Of course you'll have to redesign the cross beams ending up with a larger beam size and depth. Deeper floor beams will help immensely in reducing the magnitude of the top & bottom flange forces due to the Fixed-End-Moments, if you choose to have end fixity at all.....I believe that installing secondary framing (truss-like) in the plane of the floor beams will help you out by carrying the wind loads back to the bridge abutments. The floor beams will have to be designed to carry the gravity loads + axial loads due to wind loadings (a simple beam-column design). Just make sure that your roof columns frame into the main truss bottom cord opposite the floor beam end connections (or extend the truss verticals up to the roof framing and provide fixity in the roof framing + a secondary framing horizontal wind truss much like that down in the bridge floor framing plane).

You may want to use hollow core prestressed concrete planks to span between the floor beams instead of cast-in-place concrete floor slab to save weight, material costs, and labor costs. You'll need to have steel welding plates embedded into the bottom ends of each plank @ 24" o/c. If your truss has any sort of positive (upwards) camber or is arched, then this config will help you in weigh savings, fabrication & erection costs, and erection labor time.

To save even more weigh, yes, go with a galvanized steel composite floor deck, at least 1 1/2" deep or more, and a lightweight reinforced concrete slab possibly 3.5" or 4" total depth...final design all depends on what your Design Service Loads are (which I do not know). Are snow removal vehicles or equipment allowed on this bridge? If yes, use must consider it's moving wheel loadings + impact factor!

One thing I'd do would be to design the cross floor beams as composite beams in conjunction with the metal floor deck & Nelson Shear Studs welded to the top flange of the floor beam. The composite beam would help you realise some more weigh savings. Make sure you provide rebar grouted inside the hollow cores w/ building paper stuffed part way down the cores to prevent filling-up the cores entirely. Bar length will depend on the bar size and the required minimum embedment length + 3" min. end cap at each end. These will be need to tie together all of the components + provide better rigidity.

I honestly think that the floor beams can be designed as simply supported rather than have fixed-end connections...so go with a simple web shear tab plate that is welded to the face of the TS 5x3 vertical tube and bolted in the web of the floor beam...make sure you provide short horizontal bolt holes in either the tab plate or the ends of the floor beams for ease of field erection and any encountered misalignments.

If you must go with a fixed-end design for the floor beams, use the shear tab plate together with what is typically called a Type 2 Moment Connection....ie, top and bottom steel angles that would be welded to the TS5x3 verticals and bolted to the floor beam T&B flanges.....pretty easy to design! Just make sure you check the prying forces on the angles as well as the weld group!!!!

That should be good for now..some more food for thought....

==Signed,

CaptMoosie, LPE / BSE, MSE, PhD

Civil, Structural & Environmental Engineer

CaptMoosie,

First snow shoveling, now ripping up kitchen floor. Don't you New Yorkers ever rest?

I agree with much of what you say. There should be essentially four trusses, two to carry gravity loads and two to carry wind loads. The truss in the plane of the roof will need a substantial cantilevered column or frame at each end to take its reactions down to the foundation.

If the steel is pre-fabricated or site assembled as a 10' x 10' x 75' unit, I would think that precast concrete planks would be difficult to place, having to push them in from the ends, but it is certainly possible. I tend to favor cast-in-place concrete on galvanized composite steel deck. Another possibility is wood deck spanning 7'-6" between cross beams.

Before doing extensive calculations, to clarify his thinking, the OP should draw a Roof Framing Plan, a Floor Framing Plan and an Elevation. These may be simple line diagrams. The cross section should be modified to suit the design concept.

Hey Ba, howzit going?

Yeah, we New Yorkers never stop! hahahaa And CHIT happens too!

Actually I'm not alone in this area with house damage due to the ice dam build-up problems. We experienced roughly $50K USD worth of water damage to the house, mainly in the kitchen, living room, dining room, basement, and kid's bath room....in the northern exterior wall, the ceilings, interior walls and the floor. I was ready to bulldoze the place into the ground at one point!

Right now I'm removing the very expensive granite countertop tiles and backsplashes and the equally expensive Italian porcelean floor tiles. Couldn't save the gorgeous solid Oak kitchen cabinets due to all of the wood swelling and mold that was starting to form on the undersides and backsides....the walls will even have to come down. Would you believe that even our top of the line 3-year old GE Arctica dishwasher motor and controls got trashed and now we have to buy a brand new one (of course it's backordered!!!! Would ya know!!???? ). My wife is always crying over her pride and joy kitchen having been totally wiped out. We spent a fortune on the room 9 years ago. It was my Wedding Present to her as she loves to cook and bake as a hobby.

Thank goodness for the insurance coverage we had! Unfortunately, the insurance didn't cover personal belongings! Some of my plastic military aircraft collection, mainly 1/32 scale, that is down in the basement on metal shelving suffered the worse as I lost around $10K there alone. It could have been worse and I lost the entire collection (worth around $60K). Now they're not fit for trading or selling now as the boxes, instructions and decal sheets got soaked and totally ruined! I guess I'll have to build them now IF I only can find the time to do so!! Next big hit was all of my metal working equipment w/ AC motors & 3 welders. Totally shot and frozen-up motors.

Definitely will be installing electrical heating cables along the roof eves this spring....about 800 LF or the stuff and 6 auto sensors to operate it all......lots and lots of electrical work for me to do......then I have to install roughly 450 LF of 6-foot wood stockade fence that I didn't get around to last Fall!!!! YIKES!!!!!

Back to this guy's bridge...the planks, which are 40" wide would fit perfectly between the trusses....would need 3 side-by-side rows of them if the 10-foot truss clearance is maintained. They can be placed with a crane that's equipped with a specialized plank vacuum device. Standard fare around here. I imagine he'd can get away with 4 or 6 inch deep planks with a 2-inch thick lightweight concrete topping & WWM

As far as the horizontal roof trusses go, it'd be fairly easy to case the wind load reactions down to the top of the abutment or piers....just provide an additional diagonal member in the last girder truss panels between the roof framing and the abutments.....or provide a vertically-oriented truss at each end. I'd say it'd be far better than awkward framing and introducing moments (due to wind) everywhere.

Just a thought...

I had forgotten that there is a roof on this bridge. I agree with CaptMoosie on most of his comments, and then the differences are only my preference versus his.

Starting a project like this, I go for simplicity. I think of boxed trusses, and then consider substituting, say the floor as a diaphragm to replace the bottom truss, or cellular deck for the roof to replace the roof bracing. Honestly, I try to keep the bracing anyway to hold everything true during construction. I would rely on the floor and roof bracing to carry the loads to the ends and there build special frames to carry the shears and moments to the abutments. I prefer a reinforced poured concrete, over metal deck, but not not composite deck because the bottom has exterior exposure. I agree on studs on the cross beams. I like cellular metal deck for the roof, it looks good and can be used to run cable for lights etc. Roof slope can be made with insulating board.

This keeps all of the field connections simple and bolted. You might consider cambering out the dead load deflection.

Sorry it has taken me so long to reply! Schoolwork has been keeping me busy busy! But I have read each of your post several times to get an understanding of your ideas and I appreciate each of you taking the time out of your day to respond to this thread. I will certainly look into each of the ideas put forth in the coming days as I dive in a little deeper, and I may have more questions depending which design I go with.

One question I do have, (which is a little off subject for this thread but still dealing with the same bridge) is about the end supports. I am spanning a four lane highway and the land on each side of the road is relatively flat; therefore, a bridge abutment built into a bank is out of the question. From the bottom of the truss to land is roughly going to be 18'. Any general ideas on this?

Once again thanks to everyone!