Structural design to ASCE 7, AISC 360 and ACI 318

In the United States, structural design is split between a loading standard and material standards: ASCE 7 tells you which loads to apply and how to combine them, AISC 360 covers structural steel and ACI 318 covers reinforced concrete. Choosing AISC / ACI as the design code in Struxt3D applies all three with load and resistance factor design (LRFD). This tutorial lists what the app uses and checks some of its numbers by hand.

#Standards used

#Load combinations

These are the ASCE 7-22 strength combinations, with wind and seismic at strength level. Wind combinations are repeated for each of the four directions; seismic ones for ±(E_X + 0.3E_Y) and ±(0.3E_X + E_Y) (ASCE 7 12.5.3).

Roof live, snow and rain terms (L_r, S, R) are not generated, and the vertical seismic effect E_v = 0.2S_DSD is not added. The seismic weight includes 25 % of the floor live load.

#Live loads

Live load reduction is not applied, which is conservative for columns and footings carrying large areas.

#Wind

ASCE 7 uses a 3-second gust basic wind speed V (50 m/s ≈ 112 mph by default). Struxt3D computes the velocity pressure with exposure C and a gust factor of 0.85:

Take V from the ASCE 7 wind maps (or the ASCE Hazard Tool) for your risk category and type it in Wind speed.

#Seismic

Enter S_DS and S_D1, the design spectral accelerations at short periods and at 1 s, and pick the structural system. Struxt3D builds the ASCE 7 11.4.6 spectrum (T_L = 8 s) and divides it by R; modes are combined by CQC.

The importance factor I_e is taken as 1.0. For risk category III or IV, multiply S_DS and S_D1 by I_e before you enter them; that has the same effect as R/I_e. Read the response spectrum tutorial for how the modes combine.

#Materials and sections

Beams come from W12×26 to W40×249 and columns from W8×31 to W14×257. Steel uses φ = 0.9 for flexure and yielding, with the AISC column curve (E3). Concrete uses ACI φ factors (0.9 tension-controlled, 0.65 tied compression, 0.75 shear) and the 0.85f′_c rectangular stress block.

#Member and element checks

• Steel: AISC 360-22 LRFD, compactness, compression (E3), flexure with lateral-torsional buckling (F2), shear (G2) and interaction H1-1. See steel column design.
• Concrete: ACI 318-19 flexure, shear, column interaction with slenderness, two-way slab shear (punching), shear wall piers and footings. See RC column design.
• Composite beams: AISC 360 Chapter I with shear studs, unshored, plus DG11 walking vibration. See composite beam design.
• Connections: AISC 360 Chapter J bolts and welds; base plates and anchors. See steel connection design.

#Worked example: a four-storey office frame

1. Open the Multi-storey frame template.
2. In Materials, set Design code to AISC / ACI. Sections become W shapes and steel A992.
3. In Supports & loads, choose Office, wind 50 m/s, and turn on Earthquake with S_DS = 0.5 g, S_D1 = 0.25 g, ductile moment frame.
4. Press F5 then F6. Each design row shows the governing AISC or ACI clause; Report of the selection… prints the full calculation for one member.

#Check 1: factored floor load

Dead load with a 200 mm slab and 1.5 kN/m² finishes is 6.41 kN/m²; office live load is 2.40 kN/m².

#Check 2: velocity pressure at 10 m

#Check 3: spectrum ordinates

#How AISC / ACI changes the design

Compared with the Eurocode, the US gravity combination 1.2D + 1.6L puts more weight on the live load and less on the dead load. On a heavy concrete floor (D much larger than L) the factored load is lower than the Eurocode 1.35D + 1.5L, so concrete members often come out slightly lighter; on a light roof with a large live load the difference narrows. Wind and seismic are at strength level with a factor of 1.0, so the pressures themselves are larger (3-second gust) while the factor is smaller.

For steel, the AISC column curve and the F2 lateral-torsional buckling equations give capacities close to EN 1993 curve b for rolled W-shapes. For concrete, ACI ties its φ factor to the strain in the extreme tension steel: a beam with too much reinforcement drops from φ = 0.9 towards 0.65. If a beam fails in flexure with heavy reinforcement, a deeper section is usually cheaper than more bars. Auto-sizing (Design › Size members automatically) handles this by trying the next section up.

The deflection limit defaults to span/250 under D + L for every code. US practice often uses L/360 under live load for floors with brittle finishes; set the limit you need in Design settings….

#What you still need to check

• Risk category and importance factors (I_e, wind map choice).
• Scaling the modal base shear to 100 % of the equivalent lateral force base shear (ASCE 7 12.9.1.4), accidental torsion, drift limits and Ω₀ load combinations.
• Snow, roof live, rain, ice and flood loads, and live load reduction.
• Seismic detailing of special moment frames and special walls (AISC 341, ACI 318 Chapter 18): Struxt3D checks strength, not ductile detailing.
• Wind exposure other than C, topographic effects and components and cladding pressures.

Struxt3D gives a sound first design quickly. Confirm the items above before you issue drawings; accuracy and limitations lists the rest.