Comments for How To Engineer

Comment on ASD Stress vs ASD Strength vs LRFD All are LSD by Ryan Freund

Ryan Freund — Sat, 02 Jul 2016 01:10:05 +0000

@Edgar – That is a somewhat reasonable explanation but I would not say that ASD never goes beyond yield and would also not say that LSD is plastic design. Here are a couple examples – A compact beam that is continuously braced is allowed to be designed using the plastic section modulus under ASD design. And conversely a slender column or unbraced beam will buckle (or laterally torsionally buckle) elastically in ASD or LRFD or LSD. Limit States Design just means you are checking the capacity of different failure modes. Having said that I do associate an LRFD approach to be more of a “plastic design” or “ultimate” design method.

Comment on ASD Stress vs ASD Strength vs LRFD All are LSD by Edgar Peetam

Edgar Peetam — Wed, 29 Jun 2016 17:16:54 +0000

In my perception there is a difference between ASD and LSD, since Allowable Stress Design is an elastic design method while Limit State Design is a plastic design method. In ASD the allowable is determined by an not to exceed elastic stress, which is never beyond yield, while in LSD the capacity of the structure is calculated. This can include yield stresses as long as the structure doesn’t collapse. Also in my perception LRFD is the same as LSD. LRFD is the American name for it. I feel AISC is (and was) confusing in this since the 1989 version is an ASD standard.

Comment on Stability – AISC’s Approximate Second-Order Analysis B1 B2 Method by Edgar Peetam

Edgar Peetam — Wed, 29 Jun 2016 14:40:33 +0000

Dear Ryan,

I have two questions for you. I wonder your view on how to deal with it.
1) To run a second order analysis for ASD it is directed by AISC to multiply the loads by 1.6. But how to deal with it if for these loadcases a 1/3 increase is applicable. Should this factor by reduced to 1.2?
2) If the structure is braced with braces designed to take the lateral loads and the ratio of max second order drift to max first order drift is (say) <1.5, is a first order analysis not always adequate enough?

Thanks.

Comment on Seismic Design ASCE7 Part 1 by ali

ali — Sat, 26 Dec 2015 17:22:37 +0000

can you help me with this
In the below figure you may find the plan view of a building which consists of two identical special moment frames (SMF) in x direction and two identical special concentrically braced frames (SCBF) in y direction for resistance against lateral loads. The whole beam-to-column connections expect for the ones of SMF are simple, so the inner columns are gravity columns. Actually, you will not deal with the overall building, you will focus on the design of SMFs during your studies. Briefly, you will assign section properties to these two identical perimeter SMFs, only. You will use the plan area just for the determination of seismic effective weight during the equivalent lateral load procedure. SMF will resist against the earthquake load affecting in x direction. You may neglect the torsional effects (including accidental torsion) and carry on analyses with 2D simplified systems. The building is 8-story. Initially, you shall determine the appropriate locations for column splices. (Assume that the standard profile length is 30-40 ft.)
Total dead load value assigned to the normal stories is 3.83 kN/m2 (80 psf) including the structural steel elements. Live load applied to the normal stories is 3.11 kN/m2 (65 psf) including the weight of partition walls. Dead and live load values assigned to the roof used for promenade purposes are 3.11 kN/m2 (65 psf) and 2.87 kN/m2 (60 psf), respectively. This structure will be an office building so occupancy category is II and importance factor is 1.0. Please consider that the site class is D. The mapped maximum considered earthquake spectral response acceleration values at short and 1 second periods are 1.5g and 0.6g, respectively. Wind load shall be neglected and the earthquake load is assumed to be the dominant lateral load affecting the building. During the whole structural analyses for design, rigid-end offsets due to the fully restraint beam-to-column connections shall be taken into consideration. Furthermore, the presence of composite slab yields the rigid diaphragm behavior which shall not be overlooked during structural modeling. The whole structural elements including the braces of SCBF shall be assigned as wide flange sections with the structural steel type of A992 (Fy = 34.5 kN/cm2, 50 ksi). The assigned sections for columns shall be from W14 series.
For pre-design: For SMF, you may start with W14x342 for the bottom parts of the columns (from ground to the first column splice), and you may gradually decrease the column section for the upper parts (ie. W14x283 for the middle part and W14x257 for the top). Similarly, you may decrease the height of the cross section of beams gradually starting from the bottom and starting with W21x101 or W24x84, etc. for the first story. But do not forget 1-) The cross section elements shall be seismically compact, 2-) The height of the beams and columns shall be consistent with the qualified connections. (Please revisit the limitations for beam and column sections for prequalified beam-to-column connections and pick up one type, be consistent with it.)
The final cross section properties can be very different from the above initial trial cross sections !
Do not forget that for SMF drift limitations will most probably dominate the design!

Comment on Masonry Subject to Compression and Flexure – Stability – ASD by RauneyR

RauneyR — Mon, 04 May 2015 16:51:22 +0000

How can i obtain a copy of excel
Spreadsheet for cantilever soldier pile
Design, any help Would
Be appreciated , thanks Ray h,
Email;rthillis@comcast.net

Comment on ASD Stress vs ASD Strength vs LRFD All are LSD by ryanfreund

ryanfreund — Sun, 08 Mar 2015 21:25:18 +0000

Well there are many design types so that is a difficult question to answer. However I think the main thing to understand is that there are generally two different levels of “design loads”. There are “service level” loads which are generally used to check deflection. This is too make sure the building is comfortable. Then there are “strength level” loads. These are also called “ultimate loads”. We generally check the strength of the members verse these loads and are not so concerned about serviceability, we just want to make sure the structure will not collapse/fail.

Comment on ASD Stress vs ASD Strength vs LRFD All are LSD by omid

omid — Sun, 08 Mar 2015 16:53:26 +0000

I am confused about types of design methods???
how many types of design methods we have?
plz answer it if any one know?

Comment on General Diaphragm Design by Ryan Freund

Ryan Freund — Tue, 27 Jan 2015 02:15:24 +0000

Good catch! It should be 300plf applied along the diaphragm.

Comment on General Diaphragm Design by Colby L

Colby L — Tue, 27 Jan 2015 01:32:41 +0000

In your moment calc for the flexible diaphragm did you mean to use 450 plf or should it have been the 300 plf applied along the diaphragm?

Comment on Spreadsheets by YourSpreadsheets

YourSpreadsheets — Fri, 23 Jan 2015 01:29:28 +0000

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