Need a book? Engineering books recommendations...

# RE: Structural Engineering Rules of Thumb

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Structural Engineering Rules of Thumb
• From: "Rich Lewis" <seaint05(--nospam--at)lewisengineering.com>
• Date: Sun, 7 Feb 2010 21:55:06 -0600

Several years ago, make that many years ago (red book), I worked out the formulas in the AISC beam diagrams to calculate deflection based on moment.  I worked it out for moment to be in k-ft and length to be in feet.  These are easy formulas to remember.  I copy them over every time I get a new AISC book.

Delta=5wl^4/(384EI)  For steel this is Delta=(M*L^2)/(161*I)  For any other material it is delta=(180*M*L^2)/(E*I)

Delta=Pl^3/(48EI)   For steel this is Delta=(M*L^2)/(201*I)

For a center peak triangular load

Delta=(W8l^3)/(60*E*I)  For steel this is Delta=(M*L^@)/(168*I)

And I did this for several more load cases.  What is interesting is the Uniform load produces the highest deflection.  It has the smallest denominator constant.  So, if you want a quick conservative deflection check, not matter what the actual loading is, the Delta=(M*L^2)/(161*I)  will give you the largest deflection possible.

Reverse Delta and ‘I’ and you get quick equations to calculate minimum ‘I’ required for a deflection ratio limit.

I usually have my results from analysis in k-ft & ft so these formulas are very handy.

Rich

From: G Vishwanath [mailto:gvshwnth(--nospam--at)yahoo.com]
Sent: Friday, February 05, 2010 5:30 AM
To: seaint(--nospam--at)seaint.org; steel steel
Subject: RE: Structural Engineering Rules of Thumb

 Thanks for this initiative in the seaint list Let me also make a humble contribution.   Here is one rule of Thumb I have found useful during may years as a designer of steel structures.   If you need to do a quick check of the maximum deflection of a steel beam subjected to some udl and some  point loads, you can use the following easy formula   defln = M x L^2 / (10 x E x I)   M = max BM L = span E = Elastic modulus I = Moment of Inertia. Take care to use a consistent set of units. You will be surprised to note how close you are to the exact value if you do a rigourous calculation. This is for beams with a mix of udl and a few point loads scattered here and there along the span. Don't use this for a standard beam with a standard udl only or beams with just one point load. You can use it with beams with three or more point loads and some udl of partial udl.