Clause/Table/Figure Renumbering

There is some very minor renumbering.

Cl 7.2 – ASCE Design Snow Load Geodatabase

The new ASCE Design Ground Snow Load Geodatabase (https://asce7hazardtool.online/) gives the geocoded design ground snow load values for all four risk categories.

There are three new exceptions, snow loads need not be considered if

• p_g < the factored L_r (roof live load)

• P_g ≤ 10 lb/ft² and upwind roof from any upwind drifting locations l_u ≤100 ft

• P_g ≤ 5 lb/ft² and upwind roof from any upwind drifting locations l_u ≤300 ft

While you can manually elect not to consider snow loads in the above situations, Tekla Structural Designer cannot do this automatically as the l_u values are not known.

Cl 7.3 – Flat Roof Snow Loads

Since p_g comes from the Geodatabase it now incorporates I_s x p_g.

Hence in Tekla Structural Designer there is no longer a requirement to input the Snow Importance Factor I_s.

Cl 7.3.3 – Minimum Snow Load for Low-slope Roofs

The minimum snow load, p_m is defined as:

p_m = p_m,max (previously this used to be 20 x I_s)

Where

• Risk category I - p_m,max = 25 lb / ft2

• Risk category II - p_m,max = 30 lb / ft2

• Risk category III - p_m,max = 35 lb / ft2

• Risk category IV - p_m,max = 40 lb / ft2

As before, p_m is only applied to roofs of slope > 15 degs.

When p_g > p_m,max then p_m = p_m,max and this is applied as a uniform load not in combination with drift, unbalanced or partial loads.

Cl 7.4.1 and Figure 7.4.1 – Slope Factor Cs

Figure 7.4.1 is referred to in clause 7.4.1 - Figure 7.4.1 has been updated from the previous version of the code. The revision of Figure 7.4.1 says (note all Roof Slope angles in degrees in the equations below)

• Warm Roofs with C_t = 1.1

  • All surfaces

    • 0 deg ≤ Roof Slope ≤ 30 deg - C_s = 1

    • 30 deg < Roof Slope ≤ 70 deg - C_s = 1 - ((Roof slope - 30) / 40)

    • Roof slope > 70 deg - C_s = 0

• Warm Roofs with 1.1 < C_t < 1.2

  • Unobstructed slippery surfaces

    • 0 deg ≤ Roof Slope ≤ 10 deg - C_s = 1

    • 10 deg < Roof Slope ≤ 70 deg - C_s = 1 - ((Roof slope - 10) / 60)

    • Roof slope > 70 deg - C_s = 0

  • All other surfaces

    • 0 deg ≤ Roof Slope ≤ 37.5 deg - C_s = 1

    • 37.5 deg < Roof Slope ≤ 70 deg - C_s = 1 - ((Roof slope - 37.5) / 32.5)

    • Roof slope > 70 deg - C_s = 0

• Cold Roofs with C_t ≥ 1.2

  • Unobstructed slippery surfaces

    • 0 deg ≤ Roof Slope ≤ 15 deg - C_s = 1

    • 15 deg < Roof Slope ≤ 70 deg - C_s = 1 - ((Roof slope - 15) / 55)

    • Roof slope > 70 deg - C_s = 0

  • All other surfaces

    • 0 deg ≤ Roof Slope ≤ 45 deg - C_s = 1

    • 45 deg < Roof Slope ≤ 70 deg - C_s = 1 - ((Roof slope - 45) / 25)

    • Roof slope > 70 deg - C_s = 0

Cl 7.6.1 – Unbalanced Snow Loads for Hip and Gable Roofs

Previously, h_d was determined from fig 7.6-1, whereas it now comes from the following NEW equation:

h_d = 1.5 x √( p_g^0.74 x l_u^0.7 x W₂^1.7 / γ )

where

• W₂ is NEW input

A limit on the calculation for I_u has been removed.

• Previously when W < 20 ft then W = I_u = 20 ft.

• Now I_u always = W (there is no 20 ft min.)

Cl 7.7.1 – Lower Roof of a Structure

Previously, h_d was determined from fig 7.6-1 whereas it now comes from an equation as per the section above.

A NEW requirement for windward drifts, the drift width is taken as = 6h_d.

Cl 7.10 – Rain-on-Snow Surcharge Load

Previously this was applied to situations where p_g < 20 lb/ft² (but not zero) and applied as a 5 lb/ft² surcharge load. Now it is applied to situations where p_g ≤ p_m,max (but not zero) and applied as a 8 lb/ft² surcharge load.

Site Class

There are now 9 different site classes of which 8 are permitted for selection in Tekla Structural Designer as below:

• A – Hard rock

• B – Medium hard rock

• BC – Soft rock

• C – Very dense sand/hard clay

• CD – Dense sand/very stiff clay

• D – Medium dense sand/stiff clay

• DE – Loose sand/medium stiff clay

• E – very loose sand/soft clay

“Structure Irregularities”

The structure irregularities have been updated and the new concept of Torsional Irregularity Ratio (TIR) has been introduced – so there is now only one plan irregularity for torsion.

“Seismic Force Resisting System”

There are a number of additions to the listed Seismic Force Resisting Systems.

Calculation of design spectral factors

When using the curve defined for “Site Specific Spectra (user defined – generic curve)" the following calculations are amended for ASCE 7-22

• S_DS = 0.9 x max(S_a at T = 0.2s ≤ T ≤ 5s)

• S_D1 =

  • 0.9 x max(S_a at T = 1 ≤ T ≤ 2s) with v_s;30 > 1,450 ft/s

  • 0.9 x max(S_a at T = 1 ≤ T ≤ 5s) but ≥ S_a at 1s with v_s;30 ≤ 1,450 ft/s

• S_MS = 1.5 x S_DS

• S_M1 = 1.5 x S_D1

Rules for Irregularities, SDC, Height etc

The rules for ASCE 7-22 areas follows;

Annex 1 – Calculation of C_s (Seismic Response Coefficient) (Cl 12.8.1.1)

If the Design Spectrum being used is either of

• Multi-Period Design Spectrum

• Site Specific Spectrum (user defined – generic curve)

then the calculation for C_s is as follows

C_s = max(min(S_a / (R/I_e), S_DS / (R/I_e)), 0.01, 0.044 x S_DS x I_e, if(S1 >= 0.6, 0.5 x S₁ / (R/I_e),0))

Where

S_a = the value from the defined Design Spectrum curve at the Fundamental Period T of the structure – linear interpolation is used between defined points. If T < T for S_amax value then S_amax is used in the above in place of S_a

Note S_DS can be set equal to max(1.0, 0.7 x S_DS) provided certain criteria are met . In ASCE 7-22 there is an additional criteria to those in ASCE 7-16, that is R ≥ 3.0.

If the Design Spectrum being used is either of

• Two-Period Spectrum

• Site Specific Spectrum (User defined based on SD and T)

these remain exactly as for ASCE 7-16 for the calculation of C_s other than the additional criteria for R ≥ 3.0.

Annex 2 –Torsional Irregularity Ratio (TIR) (Cl 12.3.2.1.1)

Torsional Irregularity Ratio (TIR) is a new concept in ASCE 7-22.

TIR is calculated for each story for each seismic combination in both Dir1 and Dir2 (with A_x = 1.0 for the seismic torsion cases in these)

TIR_floor = Δ_max/ Δ_avg

Note there is an accidental torsion loadcase in both Dir1 and Dir2.

Where

Δ_max is the maximum story drift in a floor at the building edge with a rigid or semi rigid diaphragm. As the building edge values are calculated when there is a torsional irregularity. Δ_max can be determined as the maximum of these.

Δ_avg is the average story drift at two opposing edges in a floor with a rigid or semi rigid diaphragm. As the building edge values are calculated when there is a torsional irregularity. Δ_avg can be determined from summing the Design Story Drift for all the edge columns and walls and dividing the total by the number of columns and walls used in the sum.

The building TIR_building is the maximum TIR_floor for all floors in the building for both directions Dir1 and Dir2 (i.e both torsion loadcases).

For the purpose of calculating TIR, while it is permitted to assume semi-rigid diaphragms are rigid – in Tekla Structural Designer we do not do this, instead a consistent model with rigid and semi-rigid diaphragms is kept.

Annex 3 – Accidental Torsion (Cl 12.8.4.2)

In ASCE 7-16 Accidental Torsion is applied if

• SDC = B with Type 1b horizontal irregularity exists

• SDC = C, D, E or F with Type 1a or 1b horizontal irregularity exists

In ASCE 7-22 then Accidental Torsion is applied if

• SDC = B with Type 1 horizontal structural irregularity and TIR > 1.4

• SDC = C, D, E, or F with Type 1 horizontal structural irregularity

Annex 4 – Amplified Accidental Torsion (Cl 12.8.4.3)

In ASCE 7-16 Amplified Accidental Torsion is applied if

• SDC = C, D, E, or F with Type 1a or Type 1b horizontal structural irregularity

In ASCE 7-22 then Accidental Torsion is applied if

• SDC = C, D, E, or F with Type 1 horizontal structural irregularity

Annex 5 – Displacement and Drift (Cl 12.8.6)

Design Earthquake Displacement

There is a Design Earthquake Displacement at every point in a structure. Design Earthquake Displacement is defined as

δ_DE = C_d x δ_e / I_e + δ_di

δ_di is the displacement due to diaphragm deformation corresponding to the design earthquake, including section 12.10 diaphragm forces.

Design Story Drift Δ

The Design Story Drift is defined as the difference between the Design Earthquake Displacements in any two adjacent stories (points vertically aligned).

So Design Story Drift is defined as

Δ = δ_DE2 - δ_DE1

Where δ_DE2 is the Design Earthquake Displacement at the higher story point of interest and δ_DE1 is the Design Earthquake Displacement at the lower story point vertically below the upper story point.

While previously in Tekla Structural Designer seismic drift was determined by assessing drifts at column/wall nodes, an option is now provided to allow it to be calculated automatically at the Center of Mass / edge of the structure. This new method is made available in Tekla Structural Designer for all ASCE 7 code versions.

When using the new automatic option seismic drift is determined

• for a torsionally irregular structure with SDC C, D, E or F at the edges of the structure

• for all other structures at the Center of Mass in a story

The former accounts for diaphragm rotation whereas the latter does not.

Note that in Tekla Structural Designer each rigid diaphragm in a floor has a Center of Mass - so there can be several Centers of Mass in a given floor. Tekla Structural Designer assesses all of these and reports the one with the worst design story drift.

Calculation of θ

NEW for ASCE 7-22

θ = P_x x Δ_xe / (h_sx x V_x)

Calculation of θ_max

NEW for ASCE 7-22

θ_max = max (min(0.5 / (β x C_d), 0.25), 0.1)

Where

β can conservatively be taken as 1.0 but must be ≥ 1.25 / Ω₀