Bracing Design Length

[ship4885]

When designing braces in your lateral system, is it acceptable to apply a length reduction factor to account for the shortening of the braces due to large gusset connections rather than using the work point to work point distance? With some of the gusset connections in the building I am working on, the buckling lengths of the columns could be reduced quite a bit and brace sizes ultimately trimmed down.

I would assume this is not common practice as it is not conservative, just was looking for some input. Any and all is appreciated. Thanks.

 

[dgkhan]

I think do not take clear span between gusset plate but take from center of connections.

 

[ishvaaag]

In reality some assumptions of this kind are allowed even in the codes. For in-plane members in trusses it is usual to take factors lesser than one in account of a variety of factors, some of them based on geometrical aspects of the joints. For example from (in this case not a code -yet there are- but a good manual) p. 41 of

Design Guide for Hollow Structural Section Connections
Packer, Henderson
Canadian Institute for Steel COnstruction
Universal Offset Limited
Markham, Ontario, 1992

for HSS chord members in the plane of the truss AND in the plane perpendicular to the truss KL=0.9L, where for in plane action L is the length between panel points and for out of plane, the length between effective bracing points

for HSS web members, in and out of plane, KL=0.75 L
where L is measured at panel points.

 

[sandman21]

When designing SCBF the distance used is from hinge zone to hinge zone. For OCBF we use end of tube to end of tube.

 

[JAE]

I agree with sandman21.

Out of plane - the gussets don't do much for you at all as they are bending in the weak axis of the plate.

 

[hokie66]

I would never use a reduced length based on where the member is connected to the gusset. The gusset is frequently the weakest link if not stiffened for out of plane bending.

 

[ishvaaag]

Well I do not remember to have applied K less than 1 but at some cases where the old Spain's steel code gave such lesser values, old MV-103 and then EA-95. Yet the figures showing the specific cases and K (see for example Table 3.2.4.2 of EA-95) were there. The Packer and Henderson is another text admiting such thing... there always have been people needing and wanting the utmost economy.

Other thing is that these K less than 1 might apply in generic terms to every kind of connection, say, with gusset plates. Surely risky statement.

 

[ChipB]

I do end of brace to end of brace. In Staad, I assign a K of 0.9 to the braces to account for it in the design.

If it is in a seismic zone, who ever is doing the connection design, will be very appreciative of a smaller brace.