Bow String Roof Truss - Wind Uplift Net Negative Pressure

[FinnB]

I have been looking at a roof truss for the past few hours and I would like to get a few opinions.

The truss is a bow string arch which is popular for school halls where I work, Ireland. There are many example of trusses where I live that use only steel rods for the bottom chord. Some of the trusses I see locally are for example in train stations that were built in Victorian times (c1850). IJ Brunnel even did a stint in Dublin during his life.

Now to my problem. I am designing a curved sloping bow string truss for a school that has a span of 15.6m or approximately 52 feet. I have attached a sketch of the truss. The dims are in metres,just multiply by 3.28 to convert to feet.

I have a light weight metal clad roof on purlins which has very little dead weight. When I look at wind loading on the roof I get net negative pressure on the roof resulting in the bottom chord of the truss, a steel rod, going into compression and failing in buckling.

I cannot see how I can justify using only a steel rod 36mm (1.4 inch) diameter for the bottom chord. Am I missing something? Other engineers are specifying bow string trusses with light weight roofs.

Can anyone throw any light on this? Are other engineers taking big risks on these types of trusses based on these trusses having a history of performing well over a long periods of time historically. These trusses don't seem to stack up on paper.

 

[msquared48]

Without diagonals in the diagram, this seems like a collar tied situation, or a tied arch of sorts.

As for the uplift, I would never ignore it regardless of the history of the structure type. However, I would investigate lateral bracing of the bottom chord and your application of any load combinations.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

If you provide bottom chord bracing at every panel point, L/r = 3210/9 = 357 which is high for a tension member and much too high for a compression member.

The lack of diagonal web members would also prove problematic for unbalanced snow load.

These trusses (they are not really trusses) certainly do not stack up on paper.

BA

 

[peal724]

what if you went ahead and assumed that the bott chord does buckle, then you're free to design the top chord as a beam under net uplift conditions.

 

[msquared48]

That's an interesting thought, but you would also have to consider an additional impact loading on the end connections when the uplift is removed and the rod is retensioned, as well as bracing of the bottom flange of the top beam. 52 feet is a long span.

Is the top chord wood or steel?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dhengr]

FinnB:
I’m not going to shed (throw) any light on the problem, it is already too light.

I suspect that they didn’t worry much about roof truss uplift back in the late 1800's, Brunnel or not, they were quite pleased that they could actually span 15.6m under static loading. But, they probably weren’t working with roofing materials as light as you are either. While other engineers might still be using this design, out of force of habit, because statically it works, you should not ignore the potential uplift. Remember, our design loads, be they 50 or 100 year winds, may happen in two consecutive years/months or not at all in 100 years. And, that might be why some of the new lighter designs are still standing, they haven’t seen a design load from the right direction yet.

Some diagonals and bottom chord members which will act in compression would be a very good idea. The diagonals and bottom chord members might be light structural tubes with very clever pin plates at each end so that they mated as a pinned connection at each bottom panel point. You might also improve the design by increasing the height of the arch (top chord) or by increasing the harping on the bottom chord. Then add some light cross bracing from top to bottom chords of adjacent trusses to stabilize things at each lower panel point. Finally, pay attention to attachment of the metal roof cladding and the purlins for the max. negative pressures which can occur at various locations on the roof. These are different than the gross uplift on the trusses.

 

[FinnB]

In terms of letting the bottom chord buckle leaving the top chord to fend for itself in a heavy wind is something I looked at. It doesn't work. For gravity loads using a steel 254x146x31UB i.e. a light 10"x6" I beam with a 36mm (1.4 inch)tie rod is very effiecient structurallly. In the uplift situation though the 10"x6" beam spanning 52 ft just doesn't cut it.

I agree the beam is essentially a tied curved beam as opposed to a truss.

Attached is a drawing of the 73m (240 ft) tied arch at St. Pancras Station, London built in 1868. This is the kind of effect the architect was after. Something tells me he will be getting a lattice truss whether he likes it or not.

I also did a calc for a curved or segmented deep beam and came up with a 457x191x74 UB (18" x 7.5" I beam)

 

[JLNJ]

Could you pretension the bottom chord so that it doesn't go into compression?

 

[FinnB]

Here's a lattice truss option attached. Top and bottom chord 120x8mm (5x1/3") equal angle with the two middle members of the top chord slightly heavier.

JLNJ.....I suppose the bottom chord could be prestressed to ensure it does not go into tension but it still has no compression capacity so I don't see it helping the top chord to resist the wind uplift load case. I would be interested if there is any prestressing option though.

 

[BAretired]

I suppose you could add prestressing in an upward bow, i.e. following along the top chord such that an initial tension is placed in the bottom chord. This would also put initial compression in the top chord so that under full live load, the top chord compression would be greater than a similar truss without the prestress. Seems a bit off the wall, however.

Better to go with a truss similar to your latest sketch.

BA

 

[hokie66]

It is not surprising that many existing structures would not survive in what we now know to be realistic wind loading situations. Old churches with rods for the bottom chord are one common building type which would fare poorly. And the St Pancras Station.

 

[msquared48]

Used to go to an old church like that - survived a 7.1 and 6.3 and still standing. I don't know why though.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[patswfc]

What is the net uplift pressure?
Could you add some additional dead load to the roof so that uplift doesn't happen?

 

[FinnB]

Pat

The roof purlins and insulated cladding we are generally expected to specify is roughly around half the load of the wind uplift load. I can't even think of something practical that a light weight roof could be weighted down with. Ballast wouldn't be an option on this roof due to the fact that it is sloping.


To take the prestressinng idea a bit further, as discussed by JlNJ and BA, below is a sketch of some prestressed cabled roofs ideas.

http://lh5.ggpht.com/_PR8ZKUe4Yc0/StVc03r07YI/AAAAAAAAKiQ/W6ISNqnxBZU/clip_image011[4][2].jpg

 

[BAretired]

Those are interesting ideas for cable trusses, but if you start with a regular bowstring truss and prestress the top chord from end to end, keeping the prestressing force at the c.g. of the top chord, you will put the top chord in compression with little or no effect on the bottom chord.

If the prestressing force is above the c.g. of the top chord, the chord will try to flatten but will be resisted by an increase in tension in the bottom chord. If that effect is sufficient to balance the uplift pressure of the wind, the bottom chord will not go into compression.

BA

 

[dhengr]

FinnB:
I can’t see your attachment, of 18APR, in the format you transmitted it. Are you running this roof structure on the computer so we could actually see the forces in the various members, the reactions and the various displacements at joints, the loads, etc? Then your 20APR answer to Pat, would have been more meaningful if you had said something like -200lbs./lin.ft. uplift and only about 100lbs./lin.ft. for the roof cladding D.L. on the top chord, some actual values. You guys all want help and answers, but then are very reluctant to share enough actual info. to get to a meaningful discussion started. What’s the secret?

The bottom chord can not be pretensioned as JLNJ suggested, at least not in the regular sense of that term, without causing the top chord to start to go into tension, which is basically what you are trying to guard against. You have already implied that the top chord compression component is reduced sufficiently, by the negative pressure, to cause compression in the bottom chord.

There is a prestressing option; and it is to provide tendons following the arch (the top chord) to keep it in compression even under the uplift or negative wind pressures. Then you will also have larger combined forces and stresses which the top chord must withstand, and the top chord will of necessity become heavier and stiffer to counteract these added prestressing forces. You will also have higher forces in the verts. and diags. and higher tensions in the bottom chord, due to this prestressing. You must also be sure to tie the entire roof down to its bearing points.

You are approaching a double cable tension systems with vertical struts. You have shown this with your last attachment: but, you either need a support structure which can take the lateral thrust of these roof systems, and that’s usually a tension or compression ring or a buttress system; or you need a truss system which imparts only vertical reactions on its support walls and is self equilibrating under the negative wind pressures or the gravity loads. And, the fact is, that if any of these alternatives get too expensive or interfere with the light truss appearance you are trying to achieve, maybe a heavier roof covering system is not so far fetched or impractical. Is the light roof system supplier paying for this bldg., such that you must use his cladding system?

But, you can theoretically prestress this roof system sufficiently to lift the whole bldg. without the bottom chord going into compression, if you can make the arch strong enough to take this prestress and the accompanying stresses.

 

[a2mfk]

Finn- not sure about Ireland, but many US codes only let you use 0.6D + W, which can be an additional bummer in your situation.

 

[FinnB]

dhengr.......thanks for your comment. The concept of tensioning the arch member is interesting. The attachment you can't open is of a very standard lattice truss design that I put forward to the architect.

In terms of there being a "secret" there is none. I am also not really looking for help on how to design standard structures. My query was about how historically many steel trusses used only rods for tie members. I was asked to replicate this design and I found it didn't work on paper so I asked if anyone here had any opinions on this form of structure.

On a final note I would say everything you have written in this thread is good sensible advise but how much of it relates to the topic of the thread. I suppose when you don't know a poster you give a very all encompassing comprehensive answer in case they misinterpret your words.