Wind Load Calculator DPT 1311-50
Get Started in 5 Minutes
- Sign inLog in at the Member Portal first, otherwise the input fields stay locked
- Select the structure typeMedium-rise building · Gabled factory building · or Signboard
- Select the site and conditionsProvince · limit state ULS/SLS · main/secondary structure · openings · terrain
- Enter the dimensionsWidth–depth–height and number of stories (or gable span / sign size, depending on the type selected)
- Read the results and export a PDFCheck the wind pressure and base shear, then click Generate report · Export PDF
1What this program does
The program calculates design wind pressures and the wind loads acting on structures according to standard DPT 1311-50 of the Department of Public Works and Town & Country Planning, with built-in reference wind speed data for every province in the country
AMedium-rise building
Calculates wind pressure by height, story by story, giving the story forces, cumulative shear, base shear and overturning moment, together with a section view showing pressure–suction
H/Ds ≥ 1BGabled factory building
Uses the simplified method for low-rise buildings, calculating wind pressure on surfaces 1–6 for both wind perpendicular and parallel to the roof ridge, including edge zones E
H/Ds < 1 · H ≤ 23 mCSignboard
Calculates wind pressure on the sign using Cpa from Table B-1, for both perpendicular and 45° oblique wind, giving the total force F and the moment at the post base
Cg = 2.35DA4 calculation report
Produces a calculation report with wind pressure diagrams, per-story or per-surface tables, and signature boxes for the design engineer and reviewer
Medium-rise buildings · signboards:
p = I_w · q · C_e · C_g · C_pLow-rise buildings (simplified method):
p = I_w · q · C_e · (C_p C_g)
2Access rights
The same as the seismic load calculator — everyone can view it, but entering data, switching calculation methods and exporting calculation reports require signing in first
| User status | Can do | Cannot do |
|---|---|---|
| Not signed in | View all screens, graphs and explanations | Enter data · switch between simplified/detailed methods · export reports |
| Logged-in member | Full use of everything | — |
| Lifetime VIP member | Same as above, and the report header uses your own company logo and details | — |
No account yet? See the step-by-step sign-up video guide
3Choose the right structure type first
The buttons at the very top of the page are the most important decision point, because they determine the entire set of input fields, the formulas and the format of all results
| Type | Use when | What you get |
|---|---|---|
| Medium-rise building | Buildings with H/Ds ≥ 1 (taller than the narrow-side width) | Wind pressure by height · per-story force table · base shear · overturning moment |
| Gabled factory building | Low-rise buildings with H/Ds < 1 and H ≤ 23 m | Wind pressure on surfaces 1–6 · Cases 1 and 2 · edge zones E |
| Signboard | Advertising signs, freestanding walls and directional signs | Wind pressure on the sign · total force F · moment at the post base |
Simplified method vs. detailed method
For medium-rise buildings there is an additional button to switch methods
- Simplified method (Chapter 2) — uses ready-made factors from the standard; suitable for general buildings that are not very tall
- Detailed method (Chapter 3) — calculates the gust factor from the building's own natural frequency and damping ratio
4Setting the site and conditions
The Site data box is shared by all structure types
-
Select the site province
You can type to search for the province name · Once selected, the system sets the Reference wind-speed group and the V50 value automatically. Some provinces are subdivided by area, e.g., Tak (Umphang) is separate from the rest of Tak, and Nakhon Si Thammarat / Surat Thani separate the city–coastal areas from other areas
-
Select the limit state
- Strength (ULS) — used to design member strength
- Serviceability (SLS) — used to check deflection and vibration; the importance factor is reduced to Iw = 0.75
-
Select the building/sign importance category
There are 4 levels — temporary · general · public · highly essential to public welfare. Each level shows example buildings and its Iw value below the option
-
Select the structure type
- Main structure — rigid frames and shear walls that resist the wind load of the whole building · uses Cg = 2.0
- Secondary structure or exterior cladding — purlins, girts/framing, walls, glazing, roofing · uses Cg = 2.5
💡 You need to calculate twice One building usually requires 2 reports — one for the main structure and another for walls/glazing/roofing — because the gust factors differ -
Select the building opening amount
This sets the internal pressure coefficient Cpi
Option Cpi Meaning No large openings −0.15 to 0 Fully enclosed building Large openings, but closeable −0.45 to +0.3 Can be closed during a storm Large openings, doors cannot resist storms −0.7 to +0.7 High internal wind pressure; walls and roof need special checking -
Select the terrain type
A · open — B · suburban/scrubland — C · city center · This value sets the exposure factor Ce, which varies with height; the more open the terrain, the higher the wind load
5Medium-rise buildings
Enter the building dimensions
- Width W (the windward side) · Depth D · Height H
- Number of stories and Story height — choose Equal for all stories to enter a single value (the system automatically sets H = number of stories × story height) or Varies by story to enter each story
Detailed-method parameters
These appear when the detailed method is selected
- Natural frequency nD (Hz) — the default is nD = 44/H for RC buildings; if you have a value from a real model, enter it to override
- Damping ratio β — typically 0.005–0.015 for steel and RC structures
Reading the results
| Displayed item | How to read it |
|---|---|
| Factors used | Go through q, Ce, Cg, Cp, Iw one by one with the source of each value; use it to check the work immediately |
| Wind pressure by building height | Windward side Cp = +0.8 (referenced to height z) · leeward side Cp = −0.5 (referenced to height 0.5H) per Fig. B.9 |
| Section view showing pressure | Orange = windward pressure · blue = leeward suction; the solid line increases with height |
| Per-story wind load table | Each row gives z, p windward, p leeward, p total, the story force F and the cumulative shear — the values to carry into the structural model |
| Base shear | The sum of all story forces; compare it with the seismic load to see which case governs the design |
| Base overturning moment | Used to check overturning stability and to design the foundation |
6Gabled factory buildings
Enter the dimensions
- Gable span width B · Building length L · Eave height · Roof angle θ (degrees)
The program calculates the ridge height and the reference height h (the roof mid-height, which must not be less than 6 m) automatically, then uses this h to find Ce
Two load cases
1Wind perpendicular to the ridge
The CpCg values vary with roof slope; the program interpolates linearly for the angle θ entered · Surfaces 1–4 are the windward wall, windward roof, leeward roof and leeward wall
2Wind parallel to the ridge
Uses the same values for all slopes 0°–90° · Surfaces 1–6 include both side walls and the windward and leeward gable walls
Edge zones (E)
The edge strips of the roof and walls experience much higher suction than general areas. The program calculates the edge zone width y = the greater of 6 meters and 2z and shows CpCg edge and p edge in separate columns
Plus and minus signs
- Positive values = force acting toward and normal to the surface (pressure)
- Negative values = force acting away from and normal to the surface (suction) — this governs the fastening of roof sheets and purlin connections
7Signboards
Enter the dimensions
- Sign width b · Sign height d · Sign top height above ground h
The Cpa value comes from Table B-1, which depends on the ratios d/h and b/d · The case d/h = 1 means a sign or wall standing directly on the ground · Ce is taken at the height of the sign top
Two wind directions to check
- Perpendicular wind (θ = 0°) — the wind pressure is uniformly distributed over the whole sign
- Oblique wind (θ = 45°) — the wind pressure at the windward edge increases to about 1.9 times and decreases linearly to nearly zero at the other edge, producing torsion at the base. The program shows both the left-edge and right-edge windward cases
Results obtained
| Value | Source |
|---|---|
| Net wind pressure p | p = I_w · q · C_e · C_g · C_pa where Cg = 2.35 |
| Sign area A | b × d |
| Total wind force F | F = p · A acting at the center of the sign |
| Moment at post base M | M = F · (h − d/2) |
8Input field reference
| Field | Meaning | Effect on the calculation |
|---|---|---|
| Province | The project site | Sets the reference wind-speed group and V50 → reference wind pressure q |
| Limit state | ULS or SLS | SLS uses Iw = 0.75, giving lower forces than ULS |
| Importance category | The importance level of the building or sign | Sets the importance factor Iw |
| Structure type | Main structure, or secondary structure/exterior cladding | Cg = 2.0 or 2.5 |
| Opening amount | How the building is open or closed during a storm | Sets the range of Cpi for internal wind pressure |
| Terrain type | A open / B suburban / C city center | Sets Ce by height · the simplified method supports only A and B |
| W · D · H | Building width · depth · height | Sets the ratio H/Ds and the wind-exposed area |
| Number of stories / story height | Used to split the force by story | H = number of stories × story height (equal-stories mode) |
| nD · β | Natural frequency and damping ratio | Used only in the detailed method · default nD = 44/H |
| B · L · eave · θ | Gabled factory building dimensions | θ sets CpCg for Case 1 · the eave and θ set the reference height h |
| b · d · h | Signboard dimensions | d/h and b/d set Cpa · h sets Ce and the moment arm |
9Exporting the calculation report as PDF
- Enter the signatories' names
The Design engineer and Reviewer fields, with license numbers
- Click Generate report · Export PDF
The program opens a full-screen document preview
-
Check the sections in the document
The contents change according to the structure type selected
- Medium-rise building: project data · factors · summary of results · wind pressure diagram by height · per-story wind load table
- Gabled factory: Case 1 and Case 2 design wind pressure per surface, with Fig. B.1
- Signboard: wind pressure diagram on the signboard and signboard wind summary
- Click 🖨 Print / Save as PDF
Choose Save as PDF as the printer · turn on the background graphics option so the colored bands and graphs are included · when done, click ✕ Close
10Worked example
An 8-story RC office building in Bangkok, requiring wind loads for designing the main structure
| Field | Value entered |
|---|---|
| Structure type | Medium-rise building |
| Method | Simplified method (H = 28 m, not over 80 m) |
| Province | Bangkok |
| Limit state | Strength (ULS) |
| Importance category | General building |
| Structure type | Main structure (Cg = 2.0) |
| Opening amount | No large openings |
| Terrain type | B · suburban |
| Building dimensions | W = 24 m · D = 16 m |
| Number of stories / story height | 8 stories · equal for all stories 3.50 m |
What you should see
- Building height H = 8 × 3.50 = 28.00 m and H/Ds = 28/16 = 1.75, which is ≥ 1, so the medium-rise building type is correctly applicable
- The per-story wind load table has 8 rows; windward p increases with height while leeward p is constant (referenced at 0.5H)
- The cumulative shear is largest in the bottom row = the base shear
11Engineering cautions
- Results are for preliminary design and should be checked again against the full standard and certified by a licensed engineer
- Don't forget wind from every direction, especially for gabled buildings and signboards, where oblique wind causes torsion at the base
- Internal wind pressure matters for walls and roofs — buildings whose doors and windows cannot resist storms (Cpi up to ±0.7) need special checking of wall and roof sheet fasteners
- Always compare with seismic loads — low-rise and wide buildings are usually governed by wind, while heavy and tall buildings are usually governed by earthquakes; calculate both and design with the larger value
- Never neglect the edge zones — suction at the roof edges is much higher than at mid-roof and is one of the leading causes of storm damage
- Reference: DPT 1311-50 Standard for Wind Load Calculation and Building Response · Department of Public Works and Town & Country Planning, Ministry of Interior, B.E. 2550
12Frequently asked questions
After logging in, it says this account is currently in use on another device
The system allows one account on one device at a time · If it really is still open on another device, go and click Log out on that device first, then come back and log in again · If it is your own old device where you closed the browser without logging out, simply click the Log out the other device and sign in here button below the warning message — no need to wait · The message also tells you what kind of device is still signed in (e.g., Windows · Chrome) so you can be sure it is your own
I was logged out on my own while in the middle of using it
This means someone logged in to the same account from another device and confirmed taking over the session · If it was not you, change your password immediately in Member Portal → User Profile and notify our team, because it means someone else knows your password · Every takeover is logged, and administrators can review it afterwards
The program warns that the detailed method must be used. What should I do?
Click the method switch button at the top to change to Detailed method, and two more fields appear: natural frequency nD and damping ratio β · If you don't have values from a model yet, you can use the defaults the program fills in for now, and come back to correct them once you have real analysis results
I selected terrain type C and the system changed it back to B on its own
This is the correct behavior — the simplified method per Chapter 2 supports only terrain types A and B. If the building really is in a city center, switch to the detailed method, which supports type C
Can I use the gabled factory building type for a 25-meter-tall factory?
No, because it exceeds the H ≤ 23 m condition for low-rise buildings per Fig. B.1. Switch to the Medium-rise building type; the program shows a warning automatically when the values entered are out of scope
What is the difference between ULS and SLS, and which should I use?
Use ULS when designing the strength of columns, beams, footings and connections · Use SLS when checking lateral deflection and the vibration perceived by building occupants. The SLS importance factor is reduced to 0.75, so the forces are lower — a single project usually has to be checked for both states
Why is the leeward wind pressure constant over the full height?
Because Fig. B.9 of the standard specifies a single leeward value Cp = −0.5 referenced to the height 0.5H, unlike the windward side, where Ce increases with height z
Can a 35-meter-tall sign be calculated with this program?
It can be calculated and the program will give numbers, but it shows a warning that Cg = 2.35 applies only to signs not taller than 30 m with a natural frequency above 1 Hz. Taller signs require an additional dynamic response analysis before being used for actual design
After refreshing the page, all the values I entered were lost
This program does not save the entered values on your device, so you should export a PDF every time before closing the page, and keep the PDF file as supporting evidence for the calculation report
A cookie notice bar appears at the bottom of the screen. What should I click?
Choose whichever suits you — Accept all, Reject all, or Cookie settings to choose category by category · If you reject, the program still works fully, because sign-in, language and the data the program needs to run are in the Necessary category, which is always on · Your choice applies to every page on that device for 12 months; you can change your mind via the Cookie settings link at the end of this manual or at the bottom of the home page · Details are in the Privacy Policy, section 6
13Glossary of program terms
| Symbol | Full name | Short meaning |
|---|---|---|
| V50 | Reference wind speed | The 50-year return period wind speed for that province |
| q | Reference wind pressure | The wind pressure derived from the reference wind speed |
| Iw | Importance factor | Depends on the importance category and the limit state (SLS = 0.75) |
| Ce | Exposure (terrain) factor | Increases with height and with the openness of the area |
| Cg | Gust factor | 2.0 main structure · 2.5 secondary structure/cladding · 2.35 signboards |
| Cp | External pressure coefficient | +0.8 windward · −0.5 leeward (buildings with H/Ds ≥ 1) |
| Cpi | Internal pressure coefficient | Depends on the amount of building openings |
| Cpa | Net pressure coefficient for signs | From Table B-1 by the ratios d/h and b/d |
| p | Design wind pressure | Wind force per unit area acting on a surface |
| H/Ds | Ratio of height to narrow-side width | Used to classify a building as low-rise or medium-rise |
| Edge zone E | Edge strip of the roof/walls | Width y = the greater of 6 m and 2z · subject to higher suction than general areas |
| nD · β | Natural frequency · damping ratio | Used in the detailed method · default nD = 44/H |