Lateral movement at the top of a wall

[oxbridge]

We have a project on site and have noticed and issue with the design. Its a new house and we have a steel roof frame supported on external masonry cavity walls - the horizontal span is approximately 6metres. The 1st floor extends vertically into the roof to form a cathedral ceiling so horizontal ties were not possible as they cros at approx. head level. There are 3 steel roof frames and each takes support from a 1st floor steel beam via a steel column. The floor beams span between the external walls. The roof frames are simply upside down V's with columns approximately centered.

The feet of the frame are modelled as having a release horizontally on one support so that the V frame is designed to minimise spread. This is required for 2 frames where the columns are off centre. The lifting up of one end of the steel roof frame is dealt with by a vertical steel tie. The design allowed for the about 6mm of horizontal movement at the the external walls which we were advised would be acceptable.

The roof frame was modelled independently of the floor beams. In other words although the beam was designed to take the column load the effect of deflection of the beam had not been considered in the roof deflections! Big Problem! The lateral deflection has now jumped to about 40mm.

We have no option but to strengthen the floor beams to deal with this but with what we can do we're still getting 15-20mm lateral deflection. I've checked all the steel members which still work for strength but my conern is all the timber rafters supported by the frame will try and push the walls out.

I'd be grateful for any thoughts or advice?

Also has anybody used slip bearings in a building structure situation. The roof frames sit on concrete padstones on the masonry walls which will not allow much slippage I presume. Again I'd be interested in peoples views on this as I've seen engineers detail steelwork sitting on masonry like this lots of times before and presumably not had a problem. Even the stiffest steel member will still deflect laterally so at what point is it considered not an issue or should slip bearings be used?

I'd be grateful for comments as I have a site meeting tomorrow to discuss the options!

Thanks

ps I'm not able to send a sketch at the moment but can do tomorrow if it helps.

 

[kieran1]

Is it possible to jack up the roof frames and provide inplane bracing between the steel frames to minimise the horizontal deflection. Or perhaps plywood sheeting glued and screwed to the rafters.

Kieran

 

[oxbridge]

Kieran,

Thanks for the quick response.

Might work but I think it maybe a variable of what I'd thought of which was to form a steel ring beam at eaves level to prevent spread. The problem with this is that the 1st floor is then hung from the roof rather than it working the other way around.

 

[BAretired]

A sketch would really help, even if it's freehand.

BA

 

[kieran1]

Would the steel ring beam not need to span between the ends of the building? It could end up a very large size. Is it feasible to stiffen up the roof beams and add in some small diameter diagonal ties from eaves to the mid point of the opposite steel beam to form a scissor truss?

Kieran

 

[BAretired]

I'm having trouble understanding how the beam deflection can result in 40mm of lateral deflection of the frame or how strengthening the floor beam can reduce the 40mm to 15 to 20mm. What am I missing?

BA

 

[msquared48]

BA:

I agree that a sketch would help immensely here.

As I visualize this, the inverted "V"'s are supported by the masonry wall at the ends, and a column that supports the top of the inverted "V". The lower end of the column rests on a floor beam that is deflecting, allowing the top of the Vee to drop and the walls to spread. If that is so, then strengthening the floor beam would reduce the spread.

Is this correct?

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

 

[BAretired]

Mike,

You're probably correct. I thought the inverted V frame spanned 6m with not central support. I don't suppose it would be acceptable to add a column and footing under the beam.

Could add a collar tie above the headroom requirement, but that may not be acceptable either.

BA

 

[BAretired]

Another possibility is to raise the column and provide steel shims between column and beam. Snow load on the roof will still cause the beam to deflect, however.

BA

 

[csd72]

BAretired beat me to it.

Is the 40mm deflection a theoretical deflection or one measured on site? If it is theoretical then under what loads does it occur?