Accuracy, assumptions and limitations

Struxt3D is built to give a quick, sound first answer: the right structural behaviour, sensible loads from the code and a design you can trust to the level of a preliminary or scheme design. It is not a substitute for a qualified engineer's judgement. This chapter lists what the analysis and design assume and what they leave out, so you know where the numbers are reliable and where you need to look further.

#The analysis

#Linear elastic, first order

• Materials are linear elastic: stiffness does not change with load, and nothing yields or cracks in the analysis. Concrete members use their gross section stiffness.
• The analysis is first order: equilibrium is written on the undeformed shape. P-delta effects — extra moments from loads acting on the deflected structure — are not in the analysis. For slender, sway-sensitive frames, the member design's buckling and slenderness checks cover part of this; a second-order analysis may still be needed.
• Each load case is solved separately and combinations are added up afterwards, which is only valid because the analysis is linear.
• There is no time-history, nonlinear, staged-construction, creep or temperature analysis. For nonlinear work, export the model to OpenSees.

#The model

• Members are 3D beam elements on their centrelines. Member offsets and rigid end zones are not modelled: a beam spans centre to centre of its columns, and eccentric connections are not represented.
• Joints are rigid unless you release a member's end or the group is pin-jointed. Real joints are somewhere in between.
• Slabs, walls and plates are four-node shell elements. The mesh is kept coarse for speed — about two elements per bay — which gives good overall behaviour and member forces, but local peaks in slabs (at column heads, re-entrant corners, openings) are smoothed. See Slabs and walls as plates.
• Soil under a raft is a bed of independent (Winkler) springs. Elsewhere supports are rigid: settlement and soil-structure interaction are not modelled.
• Cladding carries no load itself: it passes its load to the members around it.

#Earthquake

• The response-spectrum analysis uses up to 12 modes, combined by CQC, in X and Y separately. If those modes reach less than 90 % of the mass in a direction, a message warns you; the response is then underestimated.
• Results of a response spectrum are peak magnitudes without sign; the seismic combinations take them with both signs.
• The two directions are combined in the seismic combinations as E_X + 0.3E_Y and 0.3E_X + E_Y, each with both signs.
• Accidental torsion, vertical seismic action and scaling of the spectrum to a static base shear are not applied.
• With NSCP 2015 the importance factor is taken as I = 1.0.

#Wind

Wind is simplified: a pressure that grows with height from the code family's exposure formula, applied as windward and leeward pressure on cladding, or as drag with one force coefficient on the members of open structures. Pressure coefficients by zone, internal pressure, dynamic response, topography and shielding are not modelled. Use it for overall stability and member sizing; check cladding and connections to the code. See Wind loads.

#The design

#One set of rules per code family

Each code family has one set of rules: its load combinations, live loads, resistance factors, grades and section tables. The factors and default values are quoted from the codes, but national annexes and local amendments can differ, and the defaults (live loads, wind speed, seismic inputs) are typical values that you must check for your site and use. NSCP 2015 adopts ACI 318-14 and AISC 360-10; Struxt3D checks its members with the ACI 318-19 and AISC 360-22 equations, and says so in the results.

#What the checks cover

• Member checks cover strength, buckling, lateral-torsional buckling and deflection. Buckling and unbraced lengths are taken from the nodes where other members or supports frame in; check them where bracing is indirect.
• The deflection check uses D + L against your span limit (span/250 by default). Long-term effects such as creep and cracking of concrete are not included.
• Composite beams are designed simply supported and unshored. AS/NZS composite beams are checked to EN 1994-1-1.
• Slab punching is checked, but punching reinforcement and drop panels are not designed: a failing slab is reported so you can thicken it or add them.
• Pad footings are designed under column supports only — not under walls, on rafts or offshore.
• Seismic detailing — capacity design, strong-column/weak-beam, joint shear in ductile frames, confinement beyond what the column and wall design give — is not complete. Treat the seismic design as a strength check at the reduced force level.
• Steel connections cover the common types (base plates, end plates, fin plates, gussets, splices). Unusual geometry is designed but may not get a drawing.

#Auto-sizing

Auto-sizing picks the lightest section in each group's list that passes. It does not optimise the layout, balance cost against fabrication, or consider anything outside the checks — fire, durability, robustness, constructability. Review the sections it picks.

#How Struxt3D is verified

• The solver is tested against closed-form results: beams, frames and trusses against hand formulas, and plates against Timoshenko's plate solutions.
• The concrete, steel, foundation and connection design reuse engines that have been checked against published worked examples and the design manuals of established software.
• Every design check prints its clause, and the calculation report shows the formulas and numbers, so any result can be checked by hand.

#Checking your model

1. Check the loads. Show each load case on the model in the Supports & loads step, and compare the total reactions in the Reactions table with a hand estimate of the weight and loads.
2. Check the deflected shape. Under dead load the structure should sag where you expect; under wind it should sway the right way. A part that moves strangely is often badly connected.
3. Check the diagrams. Moments should be largest where you expect — at midspan and over supports for beams, at the base of a cantilever.
4. Check a member by hand. Pick the most utilized member and work through its element report.
5. Read the messages. Every warning in the Messages tab and the report's Notes is there for a reason.

The tutorials How structural analysis works and Reading results show what to look for.