What is the best book/reference to learn how to determine loads using IBC/ASCE 7 6

[jm_eng]

I recently started my first job as an entry level structural engineer and I am having confidence issues when it comes to calculating loads (dead, live, wind, etc.). I have no problem with the design and analysis tasks I am assigned since I have had a lot of practice however I have done only a few load determination calculations.

I was wondering what is the best book/reference to use for practicing load calculations using the IBC code/ASCE 7? I have seen the “Structural Loads 2012 IBC and ASCE 7-10” by Fanella as well, what are your thoughts on this book?

 

[Aesur]

I cannot provide any feedback regarding a reference book other than the codes themselves as I never used reference books, but would suggest having a more senior engineer in the company walk you through it. There are few (if any) colleges that have students do load calculations in courses, and many engineers know and understand this and would be willing to help a newbie working with them. Additionally, when you and your employer eventually get a survey from the school you went to, make sure to add a comment about making load calculations mandatory.

Also, don't be afraid to post more specific information here and usually someone will pop in and help you out to understand it.

I'll try to give a quick explanation of loading:

Dead load = weights of materials, finishes, self weight of structure, etc..
Roof Example: (NOTE: Weights can vary and the below weights may not be applicable to all projects of similar construction)
Asphalt Singles = 2.4 psf
1/2" plywood sheathing = 1.5 psf
Prefab wood trusses at 24" o.c. = 3.5 psf
12" batt insulation = 3.6 psf
Suspended acoustical ceiling 1.5 psf
Sprinklers = 1.5 psf
MPE (Mechanical, Plumbing and Electrical) = 1.5 psf
Misc. = 1.7 psf
Total = 17 psf
For this the dead load at the roof structure would be 17 psf. Prefab wood trusses are usually a deferred submittal (these are bid out and multiple manufacturers will provide a design for their specific system) so because of this we will typically give a superimposed dead load for them to design the trusses for, which would be total dead - trusses = 13.5 psf. This applies to many other deferred submittals. When working with a joist and girder system you will not want to design your joists with the girder dead load as the girder supports the joists, so you could have varying dead loads on the structure. It is also important to look at mechanical unit loading and sprinkler loading. In most buildings with sprinklers you will have a PSF loading in the dead load, but you will also need to look at how the pipes/mains run, these can get very heavy and can place large point loads on members, or line loads when parallel to a member. Make sure you look at the architectural sections and callouts for the assembly, and don't be afraid to ask for more specifics when needed, especially with insulation as the weight can very significantly based on the type used.

Roof Live Load:
This is the loading imparted by workers, tools, temporary materials, and movable objects (decorative items) that are no occupancy related (a roof is not typically occupied by persons). Most roofs are designed for 20 psf roof live load. Exceptions are vegetative and landscaped roofs, awnings and canopies, solar structures, etc.

Live Load (typically floor live loading):
This is the load imparted on the structure by occupancy. This varies significantly, see ASCE 7 chapter 4 and IBC chapter 16 for a list of loads based on the room use. These often vary within a building, as you may have offices, conference rooms, assembly rooms, storage, corridors, stairs and other uses in the same structure. Also read 4.3.2 of ASCE, Provisions for Partitions; partitions are non-bearing walls that typically are not shown on the structure plans and are subject to being moved during renovations, etc. You will also notice there are concentrated loads in the tables, these are typically not applied at the same time as the spread loading, and you will design for the greater load effects. There are other types of loads defined in the code text, not all will be in the table, ie cranes and guardrails.

Reduction in Live Loads:
There are reductions in live loads. The reductions are described in the code a quick overview is this: Roof live load can be reduced based on the tributary area a member supports and the pitch of the roof, but not past the lower limit. Floor live load can be reduced based on area, but also includes a factor based on what the member is (K[sub]LL[/sub]).

Wind: This is probably one of the most complicated loads thanks to the many professors who need to justify their jobs (research grant money) by making our lives harder and harder.
Wind has two types of loads, Components and Cladding (C&C) and Main Wind Force Resisting System (MWFRS).
There seems to be some confusion among engineers on what exactly C&C loading is versus MWFRS (based on reading previous discussions on this forum), but my understand is this: C&C is for objects attached to the building and the first few layers of the building. C&C loading accounts of localized pressures that are higher than the average across the whole building, for this reason the objects attached to the building could see higher localized pressures. Other items designed for C&C are Decking and attachments, these are typically shorter spans between attachments and could see higher wind loading as a result. I typically take the C&C loading down to the joists as well for the same reasons as the decking. At the girders (main beam lines) I typically switch to MWFRS and by this point when looking at the controlling load cases the wind typically doesn't control anymore. The MWFRS is the wind loading uses for the major elements carrying high tributary areas & for lateral design of the building. These loads will be lower than C&C loading as it is more of a smeared loading across the entire structure.

Typically you start by calculating the Velocity Pressure (qz). To calculate this you will need the following: Velocity Pressure Exposure Coefficient, Topographic Factor, Wind Directionality Factor, Ground Elevation Factor and Wind Speed. These factors are determined by formulas and or tables in the code. I'll give a quick overview of them here: Velocity Pressure Exposure Coefficients are based on exposure category & building height. The exposure category is another tricky thing to calculate as it has been better defined each code revision. Basically it's sheilding of wind by other structures surrounding the building for a set distance upwind. I suggest reading the commentary in the latest ASCE 7 as it finally gives a great explanation and many engineers would be surprised to find that many buildings that have historically been designed as B are actually exposure C based on open areas. Topographic factors are factors that increase the wind loading when the building is located in terrain that can speed up wind, ie mountains and high hills. Wind Directionality Factor is a factor that is applied based on what the structure is, most of the time a building is 0.85, see the table in the code. Ground elevation factor is a factor that reduces the forces based on elevation above sea level, as the altitude increases the air is thinner, which results in lower forces. Wind speed is based on figures in the code and there are a few different ones based on the Risk Category. The Risk Category is defined based on building use, see chapter 1, this requires some time to go through and understand and will require talking with the owner/architect regarding the use of the building, but the majority of buildings will fall into Risk Category II. Exceptions are defined in the code and include things like hospitals, military facilities, essential facilities and others (things that should remain open and in working condition after a disaster or where occupants require additional time to evacuate, etc.). A higher risk category results in higher loads. I will note that in former codes ASCE 7-05 and earlier, there was an importance factor included as well, this was because wind was designed at ASD level, whereas not it's designed at ultimate level, you may occasionally still see the older codes used or referenced. With these factors you can calculate the velocity pressure which is used with other factors based on the shape of the building and other criteria to calculate the pressures on the surfaces of the building. Take a look at the figures in chapters 26 through 30 for the various figures, you essentially find the one that matches your building and use the tables in the figures to find a pressure coefficient to multiply by the velocity pressure. There are multiple zones on a building defined in the figures (the "a" distance) that will have varying wind loading. Additionally you have internal pressure coefficient (pressure that builds inside the building) that will be applicable many times and needs to be considered. Take a look at the formulas in the chapters for how to apply these.

Seismic is just as complex as wind loading, I'll let someone else step in and provide an explanation.

 

[Ron]

Aesur....good info for the young structural engineer. Kudos.

 

[mmarlow]

ASCE has a wind guide. Its full of examples of both procedures. I found it helpful.


I had free access to it at the school library in grad school. I would say it is worth the purchase for a young engineer.

Wind Loads: Guide to the Wind Load Provisions of ASCE 7-10

However, as stated before, nothing beats a good mentor


-MMARLOW EIT

 

[JLNJ]

Check out the Commentary on the various sections at the back of ASCE 7. It explains some of the derivations, usages, whys and wherefores.

 

[msquared48]

Great post Assur. Keep it up!

Mike McCann, PE, SE (WA, HI)

 

[Lomarandil]

Breyer's design of wood structures, if I recall, also included some explanation of developing loads.

----
just call me Lo.

 

[DrZoidberWoop]

I really liked the Fanella book and highly recommend it. The flowcharts are great. There is a new book of his out for the updated codes.

https://www.amazon.com/Structural-Load-Determination-2018-ASCE/dp/1260135624