Semi top-down: thiết kế biện pháp từ mô hình Plaxis đến kingpostSemi top-down: designing the method from Plaxis to kingposts

In Top-down or bottom-up? I compared basement construction methods. This article goes deeper into semi top-down: basement slabs are built from the top down and used as the strutting system for the diaphragm wall, but the superstructure is not built in parallel (or only partly). It is a common choice on tight urban sites with soft ground, where wall deflection and settlement of neighbouring buildings are the biggest risks.

The difficulty of semi top-down is not any single calculation. It is that several models have to talk to each other: geotechnical (Plaxis), global structural (ETABS) and slab (SAFE), each taking its input from the one before.

1. Construction sequence

2. Six design stages

Stage 1 — Planning. Survey existing conditions (levels, neighbouring structures), study the site investigation, then sit down with the project director, site team and plant department to agree on three things:

• Excavation stages and levels — ground slab first or B1 first, and whether a working platform is used.
• Basement slab formwork — earth-formed (excavate to the soffit), blinding plus plywood, or excavating about 2 m below the beam soffit to use props (easier to build and inspect, but at the cost of longer unbraced lengths for kingposts and walls).
• Layout and openings — truck routes, muck-removal openings and ramps, and which zones (if any) will build the superstructure in parallel.

This stage has the fewest calculations but drives almost every result that follows. One badly placed opening can force strengthening of the whole slab system.

Stage 2 — Deep excavation (Plaxis). Inputs are the stages from Stage 1 and soil parameters from the site investigation; outputs are wall deflection, wall forces and excavation stability. The key subtlety is slab stiffness as a strut: a slab with large openings is far less stiff than a solid one, so the equivalent axial stiffness must be reduced according to the size and position of the openings — ideally taken from the ETABS model rather than guessed.

This is also the moment to check base stability: basal heave when the formation is in soft clay, and sand boiling when excavating in sand with a high water table.

Stage 3 — Kingposts, beam strengthening, shoring (ETABS). Model the actual beams and slabs, with self-weight, construction loads (much higher in truck zones than elsewhere), fill loads at level changes and lateral loads from Plaxis. A few modelling rules:

• Permanent columns and walls are not yet working: draw them, but reduce their elastic modulus (e.g. E/1000) so they attract no force.
• Kingposts coinciding with columns are converted to point loads; kingpost bases are pinned.
• Use at least three combinations: dead + construction live, dead + earth load, and all three together.

Stage 4 — Slab strengthening (SAFE). Export from ETABS to SAFE with the loads from above, draw strips along the actual reinforcement directions so the forces reflect how the slab really works, then compare required and provided steel to define additional bars — usually around openings and kingpost heads.

Stage 5 — Drawings and review. The calculation report must state all inputs and standards; drawings must be detailed enough for the site team to build without guessing. Two levels of internal checking come before external review.

Stage 6 — Construction and lessons learned. Make sure the approved method is what gets built, follow the monitoring, and record where reality differed from the prediction for the next project.

3. Kingposts — the building’s temporary legs

Kingposts are steel sections (usually H) plunged into bored piles, carrying the whole slab system until the permanent columns are built. Three points are often underestimated:

• Effective length changes with every excavation stage. Check all cases, especially at cores and openings, where a kingpost may be unbraced over one or two storeys.
• Moments from erection tolerance. Besides the modelled moments, add the moment from construction out-of-plumbness (for example N·H/150) where the design requires it.
• Kingpost-to-pile connection. The two common options are with or without shear studs. For small piles (below about 1 m diameter), the no-stud option is usually easier to build; the final choice depends on the project scale, equipment and erection method.

For quick section checks I rebuilt the spreadsheet as an online kingpost calculator to TCVN 5575:2012 — enter the ETABS forces and effective lengths, and the results appear instantly with every step explained. Slabs bearing directly on kingposts also need a punching shear check at the kingpost head.