Canada - Wind Load Calculations

Hello albogino,

First of all let me state that I'm not familiar with the Canadian NBC, it's intricacies and practical usage. There are several Canadian Structural Engineers in this forum that may be able to serve you better than I.

Please note that I use the NYS Building Code and ASCE-7 Standard for my structural engineering projects. The ASCE 7 Standard may or may not be very close to the Canadian NBC....the terminology may be very similar, whereas the constants and variable notations may not be...

You're asking a lot of important questions, and that is fine. I always encouraged my past students during my teaching years.....there's nothing worse for a student that is afraid t ask questions...."No question is a stupid question" is my motto....and we all learn each and every day of our lives.

But, due to the number, depth and important of the questions that you have posed I strongly suggest that you set up a meeting with your Structural Engineering Professor that handed you this project and ask him/her the questions and the exact methodology you should be employing. This may take a few hours out of your and your prof's already busy schedules.

Q: Didn't your professor already cover with your class the design and analysis methodologies that your design team was to incorporate during this project? IF YES, I strongly suggest that you go back and re-study your class notes.

For me and others here to go through and explain the entire design procedure with you via this forum would take several hours of dedicated time from any one of us here in the Forum......That just not practical because it's quite complicated and there are a lot of variables, much like the ASCE-7 Standard (if you are familiar with that).

===Signed,

CaptMoosie, PE / BSCE / MSCE / PhD

Civil, Structural & Environmental Engineer

Thanks for taking the time to reply captain. No our technical advisor/prof did not explain anything. There no class notes or anything to guide us. For almost anything we ask, we get "it's your company, do what you think is best. Make an assumption" (we have a fake company right, our team). I asked him questions on wind load he intetrupted me and said he didn't want detail, just rough numbers. How can wind load be rough? He didn't let me ask other questions about it. As im, never mind the coefficients. But what do I do with the negative pressures inside. How do the windloads get carried over inside the building. Do they cause extra moment on the beams connected to the perimeter columns? Are they transferred as axial loads through beams all the way inside to the shear wall? When I have a concrete slab roof do I even need to worry about winf uplift? These are only SOME of the questions I had but he didn't want to hear it. It's supposed to be an independent project, we do research and such. BUT if he is supposed to be there as a technical ADVISOR, ot BOSS. He doesn't know the difference. So I'm at a loss. I'm learning by.myself and it's so hard when I also have to worry about my other classes. Yesterday I spent 6 hours going around in circles because I have no proper guide for this stuff. And right now we're only working on a preliminary report. We have only a vague idea of the HOW process for it. He doesn't make it clear. He says he doesn't want detail (for wind loads or snow or seismic loads for example) but then says present me alternatives and give me a cost for them. For cost I need volume, so dimensions, so don't I need moment + shear, and for that loads? Ughhh. Hopefully a canadian engineer will pich in here.

Hello albogino,

I'm sorry to hear that your Technical Adviser/Prof is so unresponsive to your technical queries. He/she should be ashamed of them self for such a piss-poor attitude towards teaching undergraduates......A PROF IS THERE TO TEACH, PERIOD!

Is this guy/gal a Grad Student or a tenured Professor? Credentials?

Let me see if I can find a concise design methodology for you to follow, even if it's a US-based one.....it should be close to the CA NBC. A tutorial found online would be the best. Have you done a Google search for this, even if it's based on the ASCE-7 Standard?

Tell me more about the building...it's height, story heights, planimetric layout (square, rectangular, round, etc), floor beams & girder arrangement, column arrangement, central core(?), materials being utilized(?), and Cladding type and if continuous? Do the windows open, which would influence the interior pressures/vacuum? What type of secondary framing are you envisioning to utilize to take-up the wind forces (K-bracing, X-bracing, girder-to-column moment connections if steel construction)?

Some of your questions regarding how the wind forces are distributed from the shell to the columns and girders can be found if you study how to perform a "PORTAL ANALYSIS" if using structural steel w/ moment connections or concrete construction. Look it up. Also, the wind loads are applied to each column (both interior and exterior ones) at each floor level.....leeward (ie, suction forces) and windward forces are additive.

A very good textbook to learn how to do a Portal Wind Analysis (and other bracing types) of multi-story buildings is (my copy and old standby BIBLE):

"Structural Engineering Handbook", Gaylord & Gaylord, McGraw-Hill Book Company, © 1968, Chapter 19, Part 3.

There are newer editions available, but it'll cost you. You can locate free e-book files of this wonderful textbook on the web. Sorry, I don't have a e-book copy of this text to send to you. You may also find it in your university library.

He is a tenured Structural Engineer. Works for Atomic Energy Canada, teaches at University of Toronto and teaches at my university, Ryerson, grad level. And he is responsible for the undergrad, 4th year projects. He is the technical advisor for 7 projects.

I have gone to Google for this, and maybe I am not searching well but I did not get anything except for a website, jabacus.com that calculates wind loads (I enter parameters). However it's only for LOW RISE buildings (H<20m). My building is 23m and 26.45m at the upper roof (mechanical penthouse).

I have architectural drawings, but not sure how to upload it here.

Storey heights:

Underground parking height = 4.6m

1st story = 5.1m

2nd to 5th = 4.2m

Mech. penthouse = 3.6m

Planimetry --> L=35.9m, W=33m (roughly). It's almost square, but there is an addition, I'm not sure how to write this. I will draw it here:

_____

| |

| -----\

| | |

| |____/

|_____|

That arch section extends from the mechanical penthouse all the way to the ground.

2 way slabs, unless we add other thinner beams (joists? don't know what you call them) accross beams so that L/W>2 (this is how we are taught if it's one way or two way). There are no girders, that I can see, but we are thinking of adding 2.

There is a stairwell almost in the middle of the building, enclosed by a shear wall I guess, or a steel bracing system? (It's up to us). Then next to it there is an elevator shaft enclosed (by something, again, we pick here right?) and adjacent (other side of the shear wall if you will) another staircase. These 3 are right in the middle of the building. So I am guessing these will be used for lateral support against Wind and Seismic loads?

I am learning here. We are doing PRELIMINARY Design, so we have to give alternatives.

Columns are spaced almost equally in a recangular grid fashion (top view)

Materials:

1. Outside glass, c/w vertical silicone butt glazing joints, c/w vert spandrels (r=12min), thermally broken curtain wall, c/w metal back pan...

2. 225 mm architectural insulated precast panels, exposed agg finish

3. reinforced poured concrete foundation, retaining walls

4. Pre-finished aluminum panels in some parts

5. for a few columns that are exposed from the building structure on the ground, aluminum covers,

6. On one face of mech pent. Deep ribbed 5000 series, prefinished metal wall insulated system.

We don't know how to design shear walls and we are learning just now how to design X bracing. I don't know what girder-to-column moment connection is???

So the wind loads are applied as Distributed loads in a triangular shape over the columns? What about on the inside? The same force? I'm thinking it's going to be less, no? If so, how do I learn to transfer the wind loads? Portal Analysis I guess? I think this may be too much. How can this Prof expect us to do all this stuff, look for info, learn it and apply it? all within 3 weeks? pfttttt...

I don't get the part that windward and leeward are additive. For the exterior column on the windward face, I calculate positive windward pressure, then subtract internal pressure from it, acting on opposite direction. But then what do I do with leeward? It's sucking the building, so the force is acting in the same direction as the windward. So we add them basically? Is windward considered positive and leeward negative? If so, when we subtract, we would end up adding since - times - = +. And how much of the leeward force do I subtract from resulting windward for this exterior column?

On corners, the wind pressure coefficient is given as negative. So even in the windward face, I design the column using a negative pressure?

Do I design each building face as windward? What if wind is blowing from leeward? I'm lost.

What about beams, are they going to be affected by wind? (load acting axially?). Or do I simply design for top-down loads (live and dead?)

If I have a concrete roof, do I consider wind suction? On the roof there is only wind uplift, right? And I know it's important for flanges of I-beams (top unsupported part), but what about if I have roof slab, do I consider Snow load + Deadload + companion Wind load (-ve?)?

How detailed is a preliminary design? They keep telling us to give them rough dimensions, estimated. He doesn't tell us how though. If I do steel beam/column, I need MOMENT. based on that I choose a type of beam/column from the steel handbook. So how is that rough? it means I don't have to check for stiffeners, type of connection etc?

Do I design assuming simply supported beams? If I do frame, I would have to use SAP to figure out all the loads..

I have to work on this. Any input is appreciated.

Hello albogino,

Wowsers, that's a lot of questions!

First thing first: Find a copy of that Gaylord and Gaylord handbook that I previously mentioned, as it explains how to do the analysis and how to apply the wind loads........A Portal Frame Analysis is actually a Hardy Cross analysis method, that's all. You should be able to find this handbook in your university library....go look for it!

If you're having no luck finding the text then I could scan a few applicable pages of that book and send them to you.....and also a few pages from the AISC Steel Manual that show you what the various types of Moment Connection are.

I don't understand your diagram, so I wish I had a copy of your drawing. I'm using CMS IntelliCAD PE here and can import only older AUTOCAD formats, but not R12 or 13, so that's not much help.

Other than going to a local structural engineer for guidance, can you find a grad student to hire briefly as a tutor? That may be the best way to learn how to do this on short order...

Are your project team mates helping out at all? From this end it sounds like you're doing all of the work by yourself!

See what you can do about finding help on your end. If I still had Skype I'd be willing to help you online, but unfortunately that software really screwed around with my system bios and I ended up dumping it. Telephone is a option, but I normally don't make it a habit doing such things with peeps on the web, plus I'm extremely busy tomorrow and won't be home, plus I have to attend a special meeting for our Village Planning Board late tomorrow afternoon that will end late into the PM hours.

I could try to explain to you here how things are done, but that'd take a few hours of typing away and you may still not know what I'm talking about.

You're an awesome person. Thank you very much for offering to help as much as you have. I didn't expect this kind of response from anyone from the internet. I will look for the book. I also went to a phd student and asked him for help. He said he was really busy and gave me his lecture notes and 2 books on pdf format that talk about everything in Tall Buildings. I also was finally able.to get a hold.of my steel design professor and he explained a few things to me. I spoke to my structural.analysis professor and he explained a few things. I had an appointment with ny advisor today and I drilled him with questions and he FINALLY made it crystal clear what he was looking for with respect to wind load analysis. FINALLY. I had to go through all this stuff in order for me to be able to show him some work and then... So for now Captain, I.know what I have to do for wind load analysis. It doesn't have to be 100% accurate. What is required of me for this semester I can finally do it. What a headache it's been so far! I dread my next semester, when I will have to do detailed wind load analysis! I'm gonna have to hit te books next semester....only now I am finding out more about what a detailed design entails! So for the time being I am good on wind loads. I'm sure I will have other questions and I will come back here and ask more. I do have team members, but they need to be led! We had one group leader and because of him, and him not pressuring the advisor for "more", we are in this state right now. It got to me so I kindt of took the reins and made the group realize the scope of our project. (I didnt know either). Thanks and I will keep you updated. Right now, by next wednesday he wants everything to be almost done. He wants us to focus more .on methodology, literature review, what's done in practise for design and building, cost analysis etc. So I now need to read up on information on all this stuff. Let the fun begin.. I am typing this from my phone as my internet does bot work. I try to start a new line, and it shows here a new line but when i submit the reply, I see it all with no paragraphs. Sorry for the mistake, I know it's a heqdache to read. I will post back soon.

Albogino, I'm glad that you finally made some important headway with the PhD candidate, your Structural Analysis Prof, and finally your adviser! AWESOME, and great leg-work! It's too bad that things got to the point where they ended up....and shame on your classmates for being absolute slouches (errr leeches?) and letting you do everything to get the ball rolling!!!!!

Frankly, since you were in such a "pickle" I was becoming very concerned that the University staff was dropping the ball (so to speak) that I was prepared to call you in order to help you out more than I can ever hope to online....

Here's some preliminary help for you regarding the Portal Frame Analysis (if you choose to go that route):

A. Calculate the wind force, and then the wind force at each roof and floor level. You will have to do this for each building axis...call them X-axis and Y-axis, or whatever tag you choose, say N-S Axis and E-W Axis, etc etc etc ......

B. Whether you have steel or concrete frame, you can go with either moment connections at each end of the roof and floor girders where they frame into the exterior and interior columns....OR you can incorporate X-Bracing or K-Bracing in either or both the exterior walls and/or the core. Since you only have a short building (5 stories?), you can most likely use what the AISC refers to as Type 2 moment connects (usually a top and bottom clip angle at the end of the girder) or plates T & B at the end of the girders that in most cases are welded to the face of the column. If you can obtain a copy of the AISC Steel Manual (either the ASD or LRDF manual) that will help you figure out what is what if you're not familiar with these types of connections. Personally, for such a short building the X-bracing in the exterior walls may be the easiest and most economical, but you may have to ALSO incorporate interior x-bracing along an interior column gridline if one axis is too long.

C. Preliminary size the columns due to gravity design loads (LL + DL) for all columns, without wind loading, then "bump-up" the size one or two notches to account for wind forces to be added later on. You can refine the actual final column sizes later on with more refined numbers and data. Anyhow, you need to establish RELATIVE column stiffnesses for all columns along all gridlines in order to do the Portal Frame Analysis......the column stiffness are used to calculate the wind load distributions. Obviously exterior columns will be less stiff then interior columns. Q: Have you done any work relating to INDETERMANENT STRUCTURAL ANALYSIS and the application of the HARDY-CROSS METHOD yet? I sure hope so if you are going to do a Portal Frame analysis! If not, you better off just using X-Bracing secondary frames floor-to-floor and casing the wind loads down to the foundations.

D. With any type multi-story frame building the wind loads applied at the exterior floor-to-column node (at roof and each floor) will be distributed (or carried) throughout the frame's plane to other columns (interior and leeward exterior columns) via the roof and floor girders. With the Portal Frame Analysis the wind forces are transformed into wind-induced moments at both the columns and the girders. I can't explain it more than that without a black board or having Gaylord & Gaylord's handbook under your nose so you can see how those moments and derived.

Each column segment (between floors) will have applied design gravity loads and wind-inducted moments T & B. Each girder segment between columns will have applied design gravity loads and "externally" applied wind moments applied at the ends of the girder (you will end up with positive and negative internal moments throughout the span).....you then analyze the girder 1). with gravity loads only, and then 2). with the gravity loads and wind moments....to find the worse case shear, moment and deflection magnitudes.

If you need the applicable section from that handbook I can email them to you after I scan them from my copy....OR you most likely will find a copy (1968 Copyright) of it in your University Library [Try to find an older copy of it as it's much better and more concise than the newer editions].

IF permitted by the Canadian NBC: In sizing both your columns and girders you can take Design Live load Reductions pursuant to building code provisions + allowable stress increases due to wind loadings.

Good luck with that project! And give your teammates a swift kick in the arses and get them to stat helping you more than they have done so far!

HTH!!!!

Sorry, that should have been "Portal Frame Wind Analysis Procedure"....

Albogino, I forgot to mention something+ ask a pertinent question:

Have you done any Finite Element Method (FEM) analysis work yet in any of your structural engineering courses, and/or have access to any FEM software?

If not, here's a very good (and free) downloadable FEM program that you could use to perform the Portal Frame Analysis:

www.engilab.com

Download the EngiLab Beam.2D ML version 1.20 program (Linear Static Analysis of Plane Frames Program [2D Frames])+ tutorial + user's manual. I use this program a lot for my 2D frame analysis. It does have some limitations though in regard to the number of nodes and segments. The learning curve is short and sweet if you're ever done FEM analysis before.....thoroughly read the manual, especially the parts in regard to inputting degrees of stiffness etc. Also, make sure you do the tutorials!

Have fun with this program.....it'll teach you a thing or two along the way!

===signed,

CaptMoosie, PhD, PE

In Florida we build everything to 130 mph if it is within 100 miles of a coastline or 140 mph if it is directly on the beach so it seems to me the starting point would be to find out how high a wind load they are requiring.

In North Carolina high up in the mountains I have had to build to 90 mph because the building was in a valley that funneled the lower average wind speeds into higher gusts.

My experience is that you design to some pre-determined maximum wind load.

The OP already has the Canadian National Building Code to determine Basic Wind Speed and pressures. One wind speed and pressure doesn't fit all elevations, as it will increase with a corresponding increase in elevation (AGL).

The wind speed up in Canada will be most certainly less that that found along the NC, SC, GA and FL coasts.

The building is surrounded by rough terrain on north and east. West side - highway and then rough terrain. South side - 2 small warehouses located about 500 m away and that's it, the rest is open terrain.

SO do I design for open terrain?

Here is where you find a difference between the academic solution and typical practice. Technically you should design each of the four principal wind directions based on the upwind exposure condition for each direction. In practice you usually design all directions based on the most extreme exposure condition. Therefore, if one side faces open terrain I would use open terrain for design in all directions.

I am having a hard time figuring out coefficients.
Do we calculate wind load at different heights? like at every floor? Do we take each face and calculate the windward pressure? Do we calculate wind pressure for the whole building, and then for main structural elements? What is the difference in calculation?

There are two parts to wind load design. You have to design the lateral load resisting system of the building as a whole for strength and stability and you must also design the individual elements on the exterior of the building supporting the cladding to resist the wind pressures. The pressure on the leeward side is constant, calculated with Ce at mid-height. The windward pressure varies with height based on value of Ce.

The gust factor Cg is 2.0 for the whole building, +-2.5 for small cladding and small elements. The internal gust factor, Cgi, is 2.0. Where and how do I make use of this internal factor?

The internal wind pressures are only used for the design of the cladding and its supporting structural members. Internal pressures cancel out when you look at the building as a whole.

For the pressure coefficient, Cp, I can make an assumption and get the coefficient for the windward and leeward sides. How does the INTERNAL pressure coefficient fit into this? My building falls into category 1. Cpi= -0.15 to 0. Why is it a range?

Internal pressures can vary due to a number of factors. Again, they are only used to design the cladding and its supports. You would use the highest positive number in combination with the leeward external pressure and the highest negative number with the external windward pressure.

Do I take each building face and calculate the windward, leeward and sideface pressures? Then do I design for the maximum?

You design for one principal wind direction at a time. Sideface pressures are only for cladding design.

What about the leeward face? when I take that face as a windward, what do I do in the very end? subtract leeward from windward to get overall effect on that face?

You are not calculating net pressures on a face when you are designing the building as a whole, just the net force acting on the building. You have to check your building for four wind load cases, one positive and one negative in each principal direction.

How does internal pressure fit into all this?
The code gives specific gust factors and pressure coefficients for small elements or cladding. Isn't the whole building cover cladding? Glass and concrete/aluminum cladding? Why then am I given the "other" coefficients for whole building and main structural members? How do they relate, interact?

Whole building and main structural members refers to design of the building's lateral load resisting system such as the bracing or shear walls.

What if I have concrete roof? Do I use only 1.25 dead load + 1.5 Snow load?

You should check for 0.9D+1.4W but you will quickly discover that it never governs in a concrete roof.

How do I know if I should design for wind or seismic load?

You have to design for both.

How do wind lateral loads transfer into the interior members? axially on beams all the way to the shear wall inside?

Yes, or through the floor and roof diaphragms. It depends on your layout and the magnitude of the loads. Try to use the diaphragms first.

Can someone explain to me (1), (2) and (3) in relation to each other?

(1) is the formula for calculating external pressures
(2) tells you how to combine the external pressures to design the lateral load resisting system of the building
(3) tells you how to calculate net pressures on cladding and its supporting members.

The algebraic difference to me means: Pwindward - Pleeward, but Pleeward is -ve, so we would end up actually adding the absolute value of Pleeward to Pwindward?

Yes. Wind pushes on the windward side and creates suction on the leeward side. Therefore these two forces are in the same direction and should add together.

Pleeward is on the other side of the building, so how does that help me with the windload on the windward face? What if wind blows on the leeward face?

Wind cannot blow on a leeward face. If it did then the wind would have changed direction. Remember your are creating different load cases by looking at one wind direction at a time. You have to check your building for four wind load cases, one positive and one negative in each principal direction.