Lateral Restrain At MidSpan 1

[astonchamber]

I'm confused with condition of lateral restrain.

Can the following cases be considered a lateral restrain?

1. Secondary beam connected at midspan between 2 parallel Primary beams via web-side plate. (See Diagram 1 below)
(Would it be regarded as Unrestrain?)

2. Secondary beam connected at midspan between 2 parallel Primary beams via full depth web-side plate (acting as stiffener) (See Diagram 1 below)
(Would it be regarded as Partial restrain?)

| |
| |
|----|
| |
| |

(Verticals are Primary beams, horizontal is secondary beam)

Diagram 1
==========




3. Equal Angle bracing connected to Primary beam via Flexible Cleat Connection at 150mm below top of flange to Primary beam (See Diagram 2 below)
(Would this be considered restrained at top flange only? and unrestrained at bottom flange)

-------- <<(Primary beam - Top flange)
|
|--l=========== <<(Bracing)
|
|
|
|
--------

Diagram 2
==========



Thanks guys!
Aston

 

[apsix]

By my interpretation of AS4100 Clause 5.4.2

1. Unrestrained (beams will deflect laterally together)

2. Unrestrained (only "partially restrained if the restraint ... effectively prevents lateral deflection..."

3. Possibly restrains top flange in some situations eg. for deep beams (obviously top flange not restrained for 200mm deep beam for flexible connection). A stiff connection between brace and beam will help of course.

 

[dbuzz]

I agree with apsix's judgement.

BTW, there are two excellent AISC (now SIA) journal papers that address the restraint afforded to beams and cantilevers by various connections:
- Trahair NS, et al (1993), Design of Unbraced Beams, Steel Construction, Vol 27, No 1
- Trahair NS (1993), Design of Unbraced Cantilevers, Steel Construction, Vol 27, No 3
(

http://www.steel.org.au)

They should be in every structural engineer's bookshelf. Unfortunately my copies are in storage!

 

[csd72]

I would suggest you get your hands on one of the 2 references that dbuzz has given you (if your office does not already have one). They are only about $9 each and will explain exactly what you need.

csd

 

[astonchamber]

Have registered for Steel.org.au and waiting for membership. Hopefully that two references will answer it all.
Thanks!
Aston

 

[SayGoodDay]

I am a little bit disagree with apsix, I will consider case 2 as partially restrained if each primary beam's minor axis capacity is enough to resist the 0.025 X critical flanges force in primary beam.
Marco

 

[apsix]

I can understand your reasoning, I'm just not convinced that it is valid.
If only one beam was loaded then, yes, I would probably agree. But with both beams loaded you would need to condider the restraint stiffness and non-linear effects.

AS4100 does not stipulate a restraint stiffness but its American counterpart AISC 360-05 does. In this case I would say stiffness of the restraint is obviously very important.
Another thing, if the beam has the required minor axis capacity then LT buckling might not be much of an issue, even without the midspan 'restraint'.

 

[dbuzz]

See this extract from the commentary to AS 4100.

C5.4.2 Restraints at a cross-section
The case of a cross-section that is restrained against twist rotation but free to deflect
laterally is not included because of the difficulty of providing simple guidance. Some very
stiff torsional restraints may induce buckling modes in which the cross-section acts as if
fully restrained (Clause 5.4.2.1), while in other cases torsional restraints may be
comparatively ineffective, and the restrained cross-section may deflect laterally.

I think it's best to take the conservative case and assume that 'Case 2' is unrestrained.