Wind Load to TCVN 2737 and Where It Parts From EC1

Two codes, two starting quantities

Ask an engineer trained on Eurocode what the wind map gives them and they will say a velocity: the fundamental basic wind velocity $v_{b,0}$, in m/s, which then gets converted into a pressure through the air density and a chain of factors. Ask an engineer trained on TCVN and they will say a pressure: $W_o$, in daN/m², read straight off the map in Phụ lục D or looked up by province in Phụ lục E.

That difference is not cosmetic. It decides where the conversion assumptions live. In EN 1991-1-4 the air density, the terrain roughness, the turbulence intensity and the gust factor are all explicit and individually adjustable. In TCVN 2737 several of them are already baked into the mapped pressure before the engineer sees it. The TCVN route is shorter and less exposed to user error; the EN route is more transparent and more adjustable. Neither is a translation of the other, and treating one as the other is where mistakes get made.

The TCVN 2737 route

The standard's Mục 6 builds the wind load from a mapped pressure and two multipliers:

$$W = W_o \, k \, c$$

where:

• $W$ - characteristic wind pressure on the surface considered (daN/m²)

• $W_o$ - basic wind pressure for the region, from the map in Phụ lục D or the province and district table in Phụ lục E (daN/m²)

• $k$ - factor for terrain type and height above the datum

• $c$ - aerodynamic coefficient for the surface, from Bảng 6

Three things about that equation are worth drawing out.

$W_o$ is a pressure, already. It carries a return period and an averaging assumption inside it, which is why Phụ lục F exists separately: it gives design pressures at mountain and island meteorological stations for return periods of 5, 10, 20 and 50 years, for cases where the standard 50-year map value is not the right basis.

$k$ combines two effects that EN keeps apart. It rises with height above the ground and it varies with terrain category, so it is doing the job that EN 1991-1-4 splits between the roughness factor $c_r(z)$ and its underlying terrain parameters. Vietnam uses three terrain types, conventionally labelled A, B and C, running from open exposed ground to dense urban. The height $z$ is measured from a datum defined in Phụ lục G, which matters on a sloping site where "ground level" is ambiguous.

$c$ is a shape coefficient, not a pressure coefficient pair. Bảng 6 tabulates it by structural form. For enclosed buildings the windward and leeward faces get separate values, and the standard also addresses friction along surfaces and internal pressure, which is easy to overlook on a building with large openings.

The dynamic component, and when it wakes up

For a stiff, low structure the wind load is the static value above and nothing more. For a tall or flexible one, the standard adds a dynamic component that accounts for the structure responding to gusts rather than simply resisting a steady pressure:

$$W_{total} = W_c + W_d$$

where:

• $W_c$ - the static component, $W_o k c$ as above (daN/m²)

• $W_d$ - the dynamic component, dependent on the structure's fundamental frequency, damping and mode shape (daN/m²)

The trigger is the structure's fundamental frequency compared to a limiting frequency. Above the limit the structure is stiff enough that it does not resonate meaningfully with the gust spectrum and only the static component applies. Below it, the dynamic component must be computed, and for a slender tower it can be a large fraction of the total.

This is the same physical idea as the structural factor $c_sc_d$ in EN 1991-1-4, but packaged differently. EN multiplies a single combined factor onto the peak pressure and gives a detailed Annex procedure to compute it. TCVN adds a separate dynamic term. The two arrive at comparable places by different bookkeeping, so a direct comparison of intermediate quantities between the codes is meaningless. Only the final pressure on the surface is comparable.

Side by side

The row that causes the most trouble in practice is shape. EN 1991-1-4 divides a facade into zones A, B, C, D and E with different external pressure coefficients, and a corner zone can carry two or three times the pressure of the middle of the same wall. That zoning is what governs cladding and fixings. An engineer moving from TCVN to EN who applies a single coefficient across a whole facade will under-design the corners, and the failure will show up in the cladding rather than the frame.

Which return period, and why it is a decision

Both codes are built on a 50-year return period as standard, but both allow it to be changed, and the change is not symmetric with consequence. A temporary works structure standing for one wet season does not need a 50-year wind. A structure whose failure would be catastrophic may deserve more. TCVN handles this through the design-life adjustment factor noted in Mục 6 and, for the stations covered, the shorter return periods in Phụ lục F. EN handles it through the probability factor $c_{prob}$.

The engineering point is the same in both: the return period is an explicit decision about acceptable risk, and it should be recorded in the design basis rather than inherited silently from a template.

Worked example (assumed inputs, clearly labelled)

The two site-dependent inputs below are assumed values used to demonstrate the arithmetic. For a real project, read $W_o$ from Phụ lục D or E for the actual site, and $k$ from the terrain and height table for the actual terrain type and height. The structure of the calculation does not change.

Structure: an enclosed building, windward wall, height $z = 20$ m above the Phụ lục G datum, terrain type B. Assumed inputs: $W_o = 95$ daN/m², $k = 1.14$ at 20 m in terrain B. Shape: windward face $c = +0.8$, leeward face $c = -0.6$ from Bảng 6. The structure is stiff enough that only the static component applies.

Windward pressure:

$$W_{windward} = 95 \times 1.14 \times 0.8 = 86.6\ \mathrm{daN/m^2} = 0.866\ \mathrm{kN/m^2}$$

Leeward suction:

$$W_{leeward} = 95 \times 1.14 \times (-0.6) = -65.0\ \mathrm{daN/m^2} = -0.650\ \mathrm{kN/m^2}$$

Net horizontal pressure driving the frame:

$$W_{net} = 0.866 + 0.650 = 1.516\ \mathrm{kN/m^2}$$

Note the sign handling, which is where arithmetic errors cluster. The leeward value is a suction, so it pulls the building away from the wind. On the overall frame it therefore adds to the windward push rather than cancelling it. Subtracting instead of adding here would under-estimate the total base shear by 43 percent in this example.

Applied over a 6 m bay across a 20 m height, the total horizontal force on that bay is:

$$F = 1.516 \times 6 \times 20 = 182\ \text{kN}$$

That is the number that goes into the load combinations, and from there into the frame analysis.

Practical notes for working across both codes

• Do not convert $W_o$ into $v_{b,0}$ and feed it into an EN calculation, or the reverse. The mapped quantities carry different embedded assumptions and the conversion silently double-counts or drops them.

• If a project is designed to TCVN but a client or lender asks for a Eurocode check, run the wind load from the EN map for that location if one exists, or from measured site data. Do not translate the TCVN pressure.

• Internal pressure is easy to forget in both codes and matters most for buildings with a dominant opening. A large roller shutter left open changes the design case for the roof.

• Record which terrain type was assumed. It is the input most likely to be challenged in review and the one most likely to change during the project as the surroundings develop.

Key points

• TCVN 2737 maps a pressure; EN 1991-1-4 maps a velocity. Everything downstream follows from that.

• $W = W_o k c$ is the whole static calculation: mapped pressure, terrain and height factor, shape coefficient.

• The dynamic component appears only when the structure is flexible enough to respond to gusts; TCVN adds it, EN multiplies an equivalent factor.

• EN zones a facade and TCVN does not, so corner cladding pressures are the biggest practical divergence.

• Leeward suction adds to windward pressure on the overall frame. Sign errors here are expensive.

• The return period is a recorded design decision in both codes, not a default to inherit.

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