Steps for Designing Steel Bracing to Australian Standards

This design guide follows AS 4100:2020 Steel Structures [1]. 
Step 1. Select an Appropriate Bracing TypeTo start, identify the loading conditions imposed on the structure. This could include: 
Dead loads 
Live loads
Wind loads
Seismic loads 
Temperature effects etc. 
Select the appropriate bracing type based on the structure and loading conditions and determine their location in the structure. Consider the general layout and arrangement of the bracing elements.  
Step 2. Calculate Design Loads and Design ActionsCalculate the design loads in accordance with AS/NZS 1170.1 and AS/NZS 1170.2 and use AS/NZS 1170.0 to determine your combination of actions (ULS and SLS). 
Then, determine the resulting design actions (forces and moments) in the bracing members. 
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Resolved forces in a braced bay from lateral load, F
Analysis for lateral loads 
For statically determinant triangulated braced frames the axials forces for each member can be found with 'Method of Sections', more details in our Truss Analysis guide. 
For moment-resisting frames characterised as statically indeterminant a basic lateral load analysis of rigidly jointed frames can be achieved with these methods: 
Portal Method 
Cantilever Method 
Factor Method 
For more accurate results classical techniques (e.g. slope deflection, moment distribution methods) or computer methods (e.g. with stiffness matrices) are your best bet. 
If a rigid diaphragm is created there will be no axial force at the member in that location.
For further explanation check out this document: 
Design of Steel Structures - 3.4 Analysis for lateral loads
Step 3. Select Bracing Member SectionSelect steel sections for the first design reiteration of the bracing members. Some steel sections that are often used in braced frames: 
I-Sections: commonly used for steel beams and columns for it's ability to resist both bending and shear forces.
Angle Sections: L-shaped cross sections commonly used for diagonal bracing members.
Hollow Sections: circular or square hollow cross sections commonly used for compression members
Here is the link to the Australian steel sections catalogue: One Steel - Hot Rolled and Structural Steel products 
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UB = Universal Beam, CHS = Circular Hollow Section, RHS = Rectangular Hollow Section, SHS = Square Hollow Section, EA = Equal Angle
Step 4. Check Axial Tension Capacity Design for axial tension in accordance with clause 7.2 of AS 4100 
Criteria provided tension members by AS 4100: 
Yield 
Ultimate strength 
The member subject to design axial tension force must satisfy:  
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N∗≤ϕ NtN^*\leq\phi\,N_tN∗≤ϕNt​
Where: 
φ = capacity factor (taken as 0.9 from AS 4100 Table 3.4)
N_t = nominal section capacity in tension   
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Nt=min(Agfy,  0.85 ktAnfu)N_t=min(A_gf_y,\;0.85\,k_tA_nf_u)Nt​=min(Ag​fy​,0.85kt​An​fu​)
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Nt=AgfyN_t=A_gf_yNt​=Ag​fy​
Based the global yield
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Nt=ktAnfuN_t=k_tA_nf_uNt​=kt​An​fu​
Based on tensile fracture 
Here, the 0.85 is an additional safety factor to account for actual fracture, not yield, governed by the equation.
Where: 
A_g = gross cross-section area 
f_y = yield stress used in design 
k_t = correction factor for distribution of forces as per AS 4100 Table 7.3.2
A_n = Net area of cross-section, which includes the deduction of all penetrations and holes, including fastener holes made in accordance to Clause 9.1.10 of AS 4100
f_u tensile strength used in design 
Step 5. Check Compression Buckling Resistance Members capacity under axial compression in accordance with Clause 6.3 of AS 4100 
In general members in compression are more complex to design since they can fail under: 
Yielding 
Inelastic buckling 
Elastic buckling (depending on the slenderness ratio) 
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Elastic (aka "Euler" or "Flexural") Buckling
Here, the default effective length factor in each axis is 1.0
The concentrated loaded members subject to a design axial compression force must satisfy the following
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N∗≤ϕ Nsand N∗≤ϕ NcN^*\leq\phi\,N_s\quad\text{and}\quad\,N^*\leq\phi\,N_cN∗≤ϕNs​andN∗≤ϕNc​
Where: 
N_s = nominal section capacity i.e. the squash load at which a very short column of the considered cross-section will fail
N_c = nominal member capacity determined by the member slenderness reduction factor a_c 
The nominal section capacity is given by: 
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Ns=kfAnfyN_s=k_fA_nf_yNs​=kf​An​fy​
Where: 
A_n = net area of the cross-section 
Deduction of all penetrations and holes, including fastener holes made in accordance to Clause 9.1.10 of AS 4100. 
k_f = form factor 
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kf=AeAgk_f=\frac{A_e}{A_g}kf​=Ag​Ae​​
Where: 
A_e = effective area
A_g = gross area of the section 
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The slenderness reduction factor a_c is calculated. However it is usually obtained as a function of "modified slenderness"  λ_n interpolated from a table of a_c. Here the form factor k_f considers the effect of local buckling of the plate elements the make up the cross section. 
The slenderness must be modified to consider yield strength since it is used to calculate N_s and elastic buckling is not dependent on yield strength.  The modified slenderness λ_n is given by:
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λn=Lerkf(fy250)\lambda_n=\frac{L_e}{r}\sqrt{k_f}\sqrt{\left(\frac{f_y}{250}\right)}λn​=rLe​​kf​​(250fy​​)​
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The modified equation to obtain the nominal capacity is given by: 
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Nc=αcNs≤ NsN_c=\alpha_cN_s\leq\,N_sNc​=αc​Ns​≤Ns​
Where: 
α_c = member slenderness reduction factor
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αc=ξ[1−[1−(90ξλ)2] ]\alpha_c=\xi\left[1-\sqrt{\left[1-(\frac{90}{\xi\lambda})^2\right]}\,\right]αc​=ξ[1−[1−(ξλ90​)2]​]
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ξ=(λ90)2+1+η2(λ90)2\xi = \frac{\left(\frac{\lambda}{90}\right)^2 + 1 + \eta}{2\left(\frac{\lambda}{90}\right)^2}ξ=2(90λ​)2(90λ​)2+1+η​
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λ=λn+αaαb\lambda=\lambda_n+\alpha_a\alpha_bλ=λn​+αa​αb​
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η=0.00326(λ−13.5)≥0\eta=0.00326(\lambda-13.5)\geq0η=0.00326(λ−13.5)≥0
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αa=2100(λn−13.5)λn2−15.3λn+2050\alpha_a=\frac{2100(\lambda_n-13.5)}{\lambda_n^2-15.3\lambda_n+2050}αa​=λn2​−15.3λn​+20502100(λn​−13.5)​
α_b = appropriate member section constant AS 4100 Tables 6.3.3 (A)&(B)
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for k_f = 1.0 [6]
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for k_f < 1.0 [6]
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See our Standard Steel Section Designer to AS4100 which includes this axial compression calculation.
Step 6. Design Connections As they say, a chain is only as strong as its weakest link. There's no point in creating a steel bracing system without adequate connections. Requirements for connections are outlined in Section 9 of AS 4100. This includes:
Connection Components: 
Brackets 
Connecting plates 
Cleats 
Gusset plates 
Connectors:
Bolts
Pins
Welds
Of course, the design capacity of each connection element should not be less than the calculated design action effects. 
In AS4100:2020 Clause 9.1.4 the connections at the steel bracing, which have been defined either tensile or compression members in this analysis, must be designed to transmit the greater of: 
The design action in the member 
A force of 0.3 times the member design capacity (except for threaded rods acting as a bracing member with turn buckles, where the minimum tensile force must be equal to the member design capacity)
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References[1] Standards Australia. (2020). Steel structures (AS/NZS 4100:2020). SAI Global. https://www.saiglobal.com/ 
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