Seismic Load Calculator DPT 1301/1302-61
Get Started in 5 Minutes
- Sign inOpen the Member Portal and log in — until you are logged in, the input fields stay locked
- Open the Calculator pageClick the Calculator menu at the top of the program
- Specify the building siteSelect the province, district and soil class (if you have no soil boring results, select D)
- Describe the buildingSelect the importance and structural system, then enter the number of stories, weight W and story height
- Read the results and export a PDFCheck the V value on the orange card, then click Generate report · Export PDF
1What this program does
This program calculates seismic loads for building design according to standard DPT 1301/1302-61 of the Department of Public Works and Town & Country Planning. It has built-in spectral response acceleration data for every district in the country, so users do not need to look up SS and S1 in the tables of the standard themselves.
ADesign values by district
SS and S1 values for every district nationwide, plus the dedicated spectra for the 10 zones of the Bangkok basin, which are used instead of the usual Fa/Fv values
BEquivalent lateral force method
Calculates the base shear V = Cs·W, distributes it into story forces Fx, and reports the seismic design category (A–D)
CDynamic method
Modal response spectrum analysis (MRSA) with 2–6 modes, checking the ΣWm ≥ 90% criterion, combining modes by SRSS and applying the scale factor SF
DA4 calculation report
Produces a calculation report with the spectrum graph, step-by-step tables and signature boxes for the design engineer and reviewer, ready to print or save as PDF immediately
2Access rights
The program is open to everyone to view its screens and sample data, but entering real data and exporting calculation reports are reserved for logged-in members
| User status | Can do | Cannot do |
|---|---|---|
| Not signed in | View the dashboard, graphs and standard data tables on every page | Enter building data · switch calculation methods · generate calculation reports (an opaque overlay appears with a Sign in / Register button) |
| Logged-in member | Full use of everything in this program, including PDF export | — |
| Lifetime VIP member | Same as above, and the report header shows your own company logo and details instead of the Chang Pile logo | — |
No account yet? See the step-by-step sign-up video guide
3Tour of all 4 screens
The menu bar at the top of the program has 4 buttons for switching freely between pages; the data you have entered is not lost when you switch pages
Dashboard — nationwide risk overview
- Ranking of the top 10 provinces by peak acceleration (using the highest district SS value within each province, in g)
- Share of districts nationwide by design category A / B / C / D (based on soil type D and importance II buildings)
- Graph of SS and S1 for all 10 zones of the Bangkok basin at 5% damping
- Summary chips for the highest-risk areas (the northern and western regions — highest at Wang Chin district, Phrae) and the lowest (the lower northeast — lowest at Na Chaluai district, Ubon Ratchathani)
Use this page to explore the overall picture and to explain to project owners why a given area must be designed for earthquakes
Calculator — the main work page
It is divided into 2 input boxes (1 · Building location and 2 · Building characteristics & structural system), followed by the result card, the spectrum graph, the calculation steps table and the story force table
Dynamic — checking the procedure of section 4.2
Uses the same site, soil class, importance and structural system as the Calculator page (to change these values you must go back to the Calculator page). This page only adds the dynamic parameters, namely the 1st-mode period and the number of modes
Data tables — the full set of standard tables
- SS / S1 values by district with risk-level labels — with a search box (shows 60 rows at a time; type to filter)
- Coefficient tables Fa (by SS at the 0.2-second period) and Fv (by S1 at the 1.0-second period)
- Table 2.3-1 structural system factors — R, Ω0 and Cd for every system
4Equivalent lateral force (ELF) method, step by step
This is the main and most frequently used procedure. Follow the order below from the top to the bottom of the screen. Results update instantly every time you change a value — there is no calculate button to press
-
Choose the area type
In the 1 · Building location box there are two buttons to choose from first
- Nationwide (by district) — for general sites; the system takes the SS, S1 values of the district and adjusts them with the soil coefficients Fa/Fv
- Bangkok basin (10 zones) — for buildings located on the soft soil of the Bangkok basin; the system uses a zone-specific spectrum instead of multiplying by Fa/Fv, because soft soil amplifies seismic waves differently from ordinary soil
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Specify the site
Nationwide mode: select the Province, then the District — both fields can be searched by typing, so there is no need to scroll through long lists
Bangkok basin mode: select the Bangkok basin zone 1–10; each zone is labeled with the provinces/districts it covers
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Select the site soil class
The choices are A (hard rock) · B (rock) · C (dense soil) · D (normal soil) · E (soft soil)
No soil boring results yet? The standard allows class D as the default, which the program has already selected for you -
Select the building importance category
This value sets the importance factor I, which multiplies the design force directly
- I — low (agricultural/temporary buildings) · I = 1.0
- II — normal (general buildings) · I = 1.0
- III — high (assembly of more than 300 people, schools) · I = 1.25
- IV — very high (hospitals, fire stations) · I = 1.5
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Select the lateral force-resisting system
The list is grouped per Table 2.3-1 of the standard — Bearing wall · Building frame · Moment-resisting frame · Dual system · Interactive. Once selected, the system fills in R, Ω0, Cd automatically
The system choice also determines the period formula — steel systems use T = 0.03H while RC uses T = 0.02H. The program switches formulas automatically; choose the wrong system and the period, and everything that follows, will be off -
Enter the building size and weight
- Number of stories — used both to calculate the height and to distribute the story forces
- Weight W (tonnes) — the effective building weight used for the base shear
- Story height — choose Equal for all stories and enter a single value, and the system immediately computes H = number of stories × story height; or choose Varies by story to enter the height of each story (e.g., when the ground story is taller than the others)
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Read the results immediately
Scroll down to the Seismic base shear card, the design category, the spectrum graph, the calculation steps table and the story force distribution table — how to read them is explained in section 6
5Reference table for every input field
Keep this table open next to the screen whenever you are unsure what a field means
| Field | Meaning | Value to enter / effect on the calculation |
|---|---|---|
| Area type | Choose between nationwide district data and the Bangkok basin-specific spectra | If the site is within the Bangkok basin, use basin mode; otherwise the forces will be lower than reality |
| Province / District | The building site, used to look up SS and S1 | You can type part of the name to search; it must match the actual project site |
| Soil class | Soil class A–E, used to select the coefficients Fa and Fv | The softer the soil, the higher the coefficients and the larger the design force · if no data, use D |
| Bangkok-basin zone | Zones 1–10 of the Bangkok soft-soil basin | Select the zone according to the provinces/districts labeled in the list |
| Importance category | The building's importance level per the standard | I = 1.0 / 1.0 / 1.25 / 1.5 for I–IV respectively; multiplies the design force directly |
| Lateral force-resisting system | The structural system that resists lateral forces | Sets R, Ω0, Cd and the period formula · the higher R, the lower the design force |
| Number of stories | Number of stories above the base | Used to compute the height H and the number of rows in the story force table |
| Weight W (tonnes) | The effective building weight | Base shear is directly proportional to W · generally the dead load plus the portion of live load specified by the standard |
| Story height (m) | All stories equal, or entered story by story | H = sum of all story heights · H directly affects the period T |
| Design engineer / Reviewer | Names shown in the signature boxes of the calculation report | Enter the full name and license number · may be left blank, in which case the printout has a blank space for a handwritten signature |
6Reading the results
The seismic base shear card
The largest value on the card is V = Cs·W, shown in both tonnes and kilonewtons. This is the total lateral force that must be distributed to the building's lateral force-resisting system
Seismic design category
| Category | Meaning |
|---|---|
| A | No seismic design required |
| B | Basic-level design |
| C | Intermediate-level design |
| D | Most stringent design |
The category does not just indicate the size of the force; it indicates what level of reinforcement detailing and ductility requirements is needed, which is a separate matter from the V value, so the two must always be read together
The design response spectrum graph
The blue line is the design spectrum at the building site. The orange dot is your building at the period T estimated by the program — the Sa value at this point is the value used to compute Cs
The calculation steps table
Shows line by line where Fa, Fv, SDS, SD1, T, Sa, Cs and finally V come from. Use it directly to check the work or to understand the formulas; it is the same content that is printed in the calculation report
The story force distribution table Fx
Distributes V according to the height and weight of each story using the exponent k, which depends on the period T. This table assumes that weight is distributed equally over all stories; if the actual building has an unusually heavy story (e.g., a mechanical floor), you must distribute the forces yourself from a real model
7Dynamic method (MRSA)
Use this page to check the procedure of the modal response spectrum method per Chapter 4 of the standard, in particular its three criteria: a sufficient number of modes, SRSS combination, and the scale factor SF
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Check the site and building data first
The top box on this page shows the data carried over from the Calculator page as read-only. To change it, click Edit on calculator page →
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Enter the 1st-mode period from the model T1
If you have a value from real structural analysis software, enter it. If left blank, the system estimates it as 1.25·Ta
If the T1 entered exceeds 1.5·Ta, the standard (section 4.2.4) requires 1.5·Ta to be used in calculating V by the equivalent lateral force method. The program enforces this cap automatically and shows both values in the steps table -
Select the number of modes considered (2–6 modes)
Check the displayed sum of modal effective weights ΣWm; it must be ≥ 90% per the standard. If it is not reached yet, increase the number of modes one at a time
-
Read the response table for each mode
Each row is one mode, giving the period Tm, the effective weight Wm and the elastic base shear Vb,e = Sa(Tm)·Wm
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Check the scale factor SF
The program compares Vt (the SRSS-combined modal shear multiplied by I/R) with 85% of V from the equivalent lateral force method
- If Vt ≥ 0.85V → no scaling needed (SF = 1.0)
- If Vt < 0.85V → all internal forces must be multiplied by SF = 0.85·V / Vt
8Exporting the calculation report as PDF
-
Enter the signatories' names
Fill in the Design engineer and Reviewer fields with license numbers before generating the report
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Click the generate report button
Click Generate report · Export PDF — the program opens a full-screen document preview (the Calculator page and the Dynamic page each have their own button and produce separate reports)
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Check the document contents
The calculation report contains building and site data · calculation steps · the reference spectrum figure · the story force table (or the mode table in the dynamic version) · scope notes · signature boxes
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Print or save as PDF
Click 🖨 Print / Save as PDF, then in the browser's print dialog choose Save as PDF as the printer · When done, click ✕ Close to go back and continue editing values
9Worked example
Try entering this example to check that your understanding matches — a 5-story RC office building in Mueang Chiang Mai district, Chiang Mai province, with no soil boring results yet
| Field | Value entered |
|---|---|
| Area type | Nationwide (by district) |
| Province / District | Chiang Mai / Mueang Chiang Mai |
| Soil class | D — normal soil (because there is no soil data yet) |
| Importance category | II — normal (I = 1.0) |
| Lateral force-resisting system | Intermediate/limited RC moment frame |
| Number of stories | 5 |
| Weight W | 1,200 t |
| Story height | Equal for all stories · 3.20 m |
What you should see after completing the entries
- Building height H = 5 × 3.20 = 16.00 m, updated automatically below the story height field
- Fundamental period by Method A for an RC building T = 0.02 × 16 = 0.32 seconds, and the orange dot on the graph sits at this period
- The result card shows V in both tonnes and kilonewtons, together with the design category for this area
- The story force table has 5 rows, with the largest force Fx always at the top story
10Engineering cautions
- Results are preliminary estimates per Method A of the standard, for study and concept checking — permit submissions must be calculated from a real structural model
- Certification by a licensed engineer is required — a calculation report printed from this program is not a certified document in itself
- Formula periods are usually shorter than the real ones, which is conservative for forces, but underestimates displacements if used for them; use the period from a real model instead
- Buildings in the Bangkok basin must not use nationwide mode, because soft soil amplifies seismic waves so much that the spectrum is very different
- Design categories C–D carry additional detailing requirements for ductility, reinforcement and building regularity checks, which are outside the scope of what the program checks
- Always check the full standard — the program refers to DPT 1301/1302-61, Revision 1 (Department of Public Works and Town & Country Planning, B.E. 2564)
11Frequently 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 input fields are gray, covered by an opaque overlay, and nothing can be clicked
This means you are not signed in yet. Click the Sign in / Register button on the overlay to go to the Member Portal, log in, then come back and reopen this page — the input fields unlock immediately
I don't know the soil class at the project site. What should I do?
Use class D, as the standard specifies when there is no soil data (the program already sets this for you). Once you have actual boring results, come back, correct the value and compare the results again
The building is in Pathum Thani. Which mode should I use?
Use Bangkok basin (10 zones), because the whole of Pathum Thani province lies within the basin, as do Bangkok, Samut Prakan, Nonthaburi, Samut Sakhon and Samut Songkhram, as well as some districts of Ayutthaya, Nakhon Pathom, Ratchaburi, Phetchaburi, Nakhon Nayok, Prachinburi, Chachoengsao and Chonburi — the zone list in the program specifies the area covered by every zone
Why does the period T change when I change the structural system?
Because Method A of the standard uses different period formulas depending on the material — steel structures use T = 0.03H and RC uses T = 0.02H. The program selects the formula automatically according to the system you choose
I set the number of modes to 6 but ΣWm still has not reached 90%
This can happen with very tall buildings or buildings with irregular mass distribution, which are beyond the scope of the uniform shear-building model the program uses for its estimates. In this case you must use periods and mode shapes from a real structural model
After printing, the background color of the table headers and the graphs disappeared
Turn on the “Background graphics” option in the browser's print dialog (it is often hidden under “More settings”)
The report header still shows the Chang Pile logo even though I am a VIP member
You must first upload your logo and enter your company details in Member Portal → User Profile. After saving, come back and refresh the program page, and the header will change accordingly
Are the values I entered lost when I switch between pages?
Not while the page stays open — you can switch freely between Dashboard, Calculator, Dynamic and Data tables. But if you close the tab or refresh the page, the values return to their defaults, so you should export a PDF before closing
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
12Glossary of program terms
| Symbol | Full name | Short meaning |
|---|---|---|
| SS · S1 | Spectral response acceleration of the maximum considered earthquake | Values at the 0.2- and 1.0-second periods respectively, read from the district data |
| Fa · Fv | Site (soil) coefficients | Multipliers that adjust values for the stiffness or softness of the site soil |
| SDS · SD1 | Design spectral response acceleration | Values already adjusted for the soil class, used to build the design spectrum |
| I | Importance factor | 1.0 / 1.25 / 1.5 depending on the building importance category |
| R | Response modification factor | Reflects the ductility of the structural system · high R, low design force |
| Ω0 | Overstrength factor | Used to design members that must not fail in a brittle manner |
| Cd | Deflection amplification factor | Used to convert displacements from the analysis into actual values |
| T · Ta | Fundamental period / formula period | Ta = 0.02H (RC) or 0.03H (steel) |
| Cs | Seismic response coefficient | Ratio of lateral force to building weight |
| V | Seismic base shear | V = Cs·W, the total lateral force at the base |
| Fx | Story force | The portion of V applied at each story according to its height and weight |
| ΣWm | Sum of modal effective weights | The standard requires ≥ 90% of the building weight |
| SRSS | Square Root of Sum of Squares | Method of combining the responses of multiple modes |
| SF | Scale factor | = 0.85·V / Vt when Vt is below the criterion · applies to internal forces only |