Eaves tie in portal frames

[Vinny7]

Can someone point me in the direction of information relating to the position where longtitudinal eaves ties can be used in portal frame buildings? (is there a limit of say within d/2 of the bottom of the haunch). If an eaves tie is provided at a higher level (say mid depth of the rafter member) would a stiffener suffice at the bottom of the haunch without struts back to cladding rails?

Thanks for any help.

Vinny

 

[Vinny7]

I am designing using British Standards.

 

[valleyboy]

Hello Vinny

What is the purpose of the eaves tie? If you are using it to act as a strut to transfer gable wind loads into the vertical bracing system, then the plane of the tie should correspond with the plane of the horizontal rafter bracing to avoid inducing bending into the minor axis of the portal columns.

If you are designing the frame plastically, and are using the eaves member to restrain the plastic hinge at the underside of the haunch, then you will need to modify the arrangement of the vertical bracing - excellent guidance is given in SCI Publication P252 - see figure 9.9 on Page 57.

It is also worth noting that this document recommends that a torsional restraint is provided at the underside of the haunch whether the frame is desined elastically or plastically - see Section 8.2 on Page 51. I do not think that a stiffener in the compression zone of the eaves moment connection provides the torsional restraint required.

I either provide a hot rolled CHS or position a sheeting rail at this location and use cold rolled angles connected back to the sheeting rail sleeve fixings to provide torsional restraint.

VB

 

[Vinny7]

The frame is being designed elastically and the eaves tie is being used to transfer wind from the gable so I intend to put it at a higher level (mid-rafter depth. As you stated I also think it would also be prudent to provide restraint to the column at the bottom of the rafter haunch. The walls will be built up in infill blockwork so the option of using an angle tie/struct won't work. This is why i was wondering if the eaves tie i any intending to use would also provide the column restraint?

Any thoughts?

Vinny

 

[hokie66]

Generally, the eaves tie would not restrain the bottom of the haunch to column connection without a lot of stiffening. But you could use a diagonal brace from the eaves tie to the haunch. Where are your roof purlins? Another way of bracing the rafter/column connection is to provide a purlin to coincide, and use a brace from the purlin. By the way, I think it not only prudent but essential to brace this point, however you do it.

 

[valleyboy]

Hello VInny

P252 suggestes that if repositioned, and the vertical bracing arrangement is modified, you can achieve restraint with the eaves tie.

Have a look at P252 and it will all become clear !!

VB